C# -- lambda表达式转为where条件sql语句的方法_.NET_编程开发_程序员俱乐部

我们在开发时，一般都会编写自已的数据库帮助类，如：DbHelper。DbHelper类中可能都有类拟有：一个根据条件查询得到一个实体集的方法:

class="brush:csharp;collapse:true;;gutter:true;">/// <summary>
/// 根据条件语句 ，查实体列表集
///  调用：如Users实体：字段：ID、Name  
///  List<Users> list = GetListByWhereString <Users>(“ Id >0 and Name like ‘FengCodes%’ ”);
/// </summary>
/// <typeparam name="T">实体类（实体名与数据表的名相同）</typeparam>
/// <param name="strWhere">条件语句（不含where）</param>
/// <returns></returns>
public List<T> GetListByWhereString<T>(string strWhere) where T : class,new()
{

    //根据T获得数据表名
      T t = new T();   
      string tableName= t.GetType().Name;

    //接着拼接sql语句
    　string sql=string.format(“select * from {0} ”,tableName);

    　If(!string.IsNullOrEmpty(strWhere))
    　{ 
        sql=sql+” where ”+strWhere;
    　}

    //获得sql语句后，就可下一步操作，略

   // …… 

}

                     

实体类：

/// <summary>
///测试的实体类
/// </summary>
public class Users
{

    public int Id{set;get;}

    public string Name{set;get;}

}

至这里，有人会想 strWhere查询语句，又要自已手写拼接条件sql，能不能方便一些的方法呢？如 ：lambda表达式。

/// <summary>
/// 根据条件语句 ，查实体列表集
///  调用：User实体：字段：ID、Name   
///  List<Users> list = GetList<Users>(“ Id >0 and Name like ‘FengCodes%’ ”);
/// </summary>
/// <typeparam name="T">实体类（实体名与数据表的名相同）</typeparam>
/// <param name="strWhere">条件语句（不含where）</param>
/// <returns></returns>
public List<T> GetList<T>(Expression<Func<T, bool>> wherePredicate) where T : class,new()
{
     //有个方法据 wherePredicate的lambda表达式转为 strWhere拼接查询条件
       string strWhere=GetWhereByLambda(wherePredicate);
    //调用上面方法返回 
       return GetListByWhereString<T>(strWhere);

｝
    

说到这里,就要入主题：如何把lambda表达式转为where条件sql? 如何实现 GetWhereByLambda方法？

参考了国外网站的一些方法后，结合本构想：可以重写 ExpressionVisitor类来实现!

上代码：

using System;
using System.Collections;
using System.Collections.Generic;
using System.Collections.ObjectModel;
using System.Linq;
using System.Linq.Expressions;
using System.Reflection;
using System.Text;

namespace LambdaToWhereSqlTest.Common
{
    #region Expression

    public abstract class ExpressionVisitor35
    {
        protected ExpressionVisitor35() { }

        protected virtual Expression Visit(Expression exp)
        {
            if (exp == null)
                return exp;
            switch (exp.NodeType)
            {
                case ExpressionType.Negate:
                case ExpressionType.NegateChecked:
                case ExpressionType.Not:
                case ExpressionType.Convert:
                case ExpressionType.ConvertChecked:
                case ExpressionType.ArrayLength:
                case ExpressionType.Quote:
                case ExpressionType.TypeAs:
                    return this.VisitUnary((UnaryExpression)exp);
                case ExpressionType.Add:
                case ExpressionType.AddChecked:
                case ExpressionType.Subtract:
                case ExpressionType.SubtractChecked:
                case ExpressionType.Multiply:
                case ExpressionType.MultiplyChecked:
                case ExpressionType.Divide:
                case ExpressionType.Modulo:
                case ExpressionType.And:
                case ExpressionType.AndAlso:
                case ExpressionType.Or:
                case ExpressionType.OrElse:
                case ExpressionType.LessThan:
                case ExpressionType.LessThanOrEqual:
                case ExpressionType.GreaterThan:
                case ExpressionType.GreaterThanOrEqual:
                case ExpressionType.Equal:
                case ExpressionType.NotEqual:
                case ExpressionType.Coalesce:
                case ExpressionType.ArrayIndex:
                case ExpressionType.RightShift:
                case ExpressionType.LeftShift:
                case ExpressionType.ExclusiveOr:
                    return this.VisitBinary((BinaryExpression)exp);
                case ExpressionType.TypeIs:
                    return this.VisitTypeIs((TypeBinaryExpression)exp);
                case ExpressionType.Conditional:
                    return this.VisitConditional((ConditionalExpression)exp);
                case ExpressionType.Constant:
                    return this.VisitConstant((ConstantExpression)exp);
                case ExpressionType.Parameter:
                    return this.VisitParameter((ParameterExpression)exp);
                case ExpressionType.MemberAccess:
                    return this.VisitMemberAccess((MemberExpression)exp);
                case ExpressionType.Call:
                    return this.VisitMethodCall((MethodCallExpression)exp);
                case ExpressionType.Lambda:
                    return this.VisitLambda((LambdaExpression)exp);
                case ExpressionType.New:
                    return this.VisitNew((NewExpression)exp);
                case ExpressionType.NewArrayInit:
                case ExpressionType.NewArrayBounds:
                    return this.VisitNewArray((NewArrayExpression)exp);
                case ExpressionType.Invoke:
                    return this.VisitInvocation((InvocationExpression)exp);
                case ExpressionType.MemberInit:
                    return this.VisitMemberInit((MemberInitExpression)exp);
                case ExpressionType.ListInit:
                    return this.VisitListInit((ListInitExpression)exp);
                default:
                    throw new Exception(string.Format("Unhandled expression type: '{0}'", exp.NodeType));
            }
        }

        protected virtual MemberBinding VisitBinding(MemberBinding binding)
        {
            switch (binding.BindingType)
            {
                case MemberBindingType.Assignment:
                    return this.VisitMemberAssignment((MemberAssignment)binding);
                case MemberBindingType.MemberBinding:
                    return this.VisitMemberMemberBinding((MemberMemberBinding)binding);
                case MemberBindingType.ListBinding:
                    return this.VisitMemberListBinding((MemberListBinding)binding);
                default:
                    throw new Exception(string.Format("Unhandled binding type '{0}'", binding.BindingType));
            }
        }

        protected virtual ElementInit VisitElementInitializer(ElementInit initializer)
        {
            ReadOnlyCollection<Expression> arguments = this.VisitExpressionList(initializer.Arguments);
            if (arguments != initializer.Arguments)
            {
                return Expression.ElementInit(initializer.AddMethod, arguments);
            }
            return initializer;
        }

        protected virtual Expression VisitUnary(UnaryExpression u)
        {
            Expression operand = this.Visit(u.Operand);
            if (operand != u.Operand)
            {
                return Expression.MakeUnary(u.NodeType, operand, u.Type, u.Method);
            }
            return u;
        }

        protected virtual Expression VisitBinary(BinaryExpression b)
        {
            Expression left = this.Visit(b.Left);
            Expression right = this.Visit(b.Right);
            Expression conversion = this.Visit(b.Conversion);
            if (left != b.Left || right != b.Right || conversion != b.Conversion)
            {
                if (b.NodeType == ExpressionType.Coalesce && b.Conversion != null)
                    return Expression.Coalesce(left, right, conversion as LambdaExpression);
                else
                    return Expression.MakeBinary(b.NodeType, left, right, b.IsLiftedToNull, b.Method);
            }
            return b;
        }

        protected virtual Expression VisitTypeIs(TypeBinaryExpression b)
        {
            Expression expr = this.Visit(b.Expression);
            if (expr != b.Expression)
            {
                return Expression.TypeIs(expr, b.TypeOperand);
            }
            return b;
        }

        protected virtual Expression VisitConstant(ConstantExpression c)
        {
            return c;
        }

        protected virtual Expression VisitConditional(ConditionalExpression c)
        {
            Expression test = this.Visit(c.Test);
            Expression ifTrue = this.Visit(c.IfTrue);
            Expression ifFalse = this.Visit(c.IfFalse);
            if (test != c.Test || ifTrue != c.IfTrue || ifFalse != c.IfFalse)
            {
                return Expression.Condition(test, ifTrue, ifFalse);
            }
            return c;
        }

        protected virtual Expression VisitParameter(ParameterExpression p)
        {
            return p;
        }

        protected virtual Expression VisitMemberAccess(MemberExpression m)
        {
            Expression exp = this.Visit(m.Expression);
            if (exp != m.Expression)
            {
                return Expression.MakeMemberAccess(exp, m.Member);
            }
            return m;
        }

        protected virtual Expression VisitMethodCall(MethodCallExpression m)
        {

            //MethodCallExpression mce = m;
            //if (mce.Method.Name == "Like")
            //    return string.Format("({0} like {1})", ExpressionRouter(mce.Arguments[0]), ExpressionRouter(mce.Arguments[1]));
            //else if (mce.Method.Name == "NotLike")
            //    return string.Format("({0} Not like {1})", ExpressionRouter(mce.Arguments[0]), ExpressionRouter(mce.Arguments[1]));
            //else if (mce.Method.Name == "In")
            //    return string.Format("{0} In ({1})", ExpressionRouter(mce.Arguments[0]), ExpressionRouter(mce.Arguments[1]));
            //else if (mce.Method.Name == "NotIn")
            //    return string.Format("{0} Not In ({1})", ExpressionRouter(mce.Arguments[0]), ExpressionRouter(mce.Arguments[1]));
            //MethodCallExpression mce = m;
          

            Expression obj = this.Visit(m.Object);
            IEnumerable<Expression> args = this.VisitExpressionList(m.Arguments);
            if (obj != m.Object || args != m.Arguments)
            {
                return Expression.Call(obj, m.Method, args);
            }
            return m;
        }

        protected virtual ReadOnlyCollection<Expression> VisitExpressionList(ReadOnlyCollection<Expression> original)
        {
            List<Expression> list = null;
            for (int i = 0, n = original.Count; i < n; i++)
            {
                Expression p = this.Visit(original[i]);
                if (list != null)
                {
                    list.Add(p);
                }
                else if (p != original[i])
                {
                    list = new List<Expression>(n);
                    for (int j = 0; j < i; j++)
                    {
                        list.Add(original[j]);
                    }
                    list.Add(p);
                }
            }
            if (list != null)
            {
                return list.AsReadOnly();
            }
            return original;
        }

        protected virtual MemberAssignment VisitMemberAssignment(MemberAssignment assignment)
        {
            Expression e = this.Visit(assignment.Expression);
            if (e != assignment.Expression)
            {
                return Expression.Bind(assignment.Member, e);
            }
            return assignment;
        }

        protected virtual MemberMemberBinding VisitMemberMemberBinding(MemberMemberBinding binding)
        {
            IEnumerable<MemberBinding> bindings = this.VisitBindingList(binding.Bindings);
            if (bindings != binding.Bindings)
            {
                return Expression.MemberBind(binding.Member, bindings);
            }
            return binding;
        }

        protected virtual MemberListBinding VisitMemberListBinding(MemberListBinding binding)
        {
            IEnumerable<ElementInit> initializers = this.VisitElementInitializerList(binding.Initializers);
            if (initializers != binding.Initializers)
            {
                return Expression.ListBind(binding.Member, initializers);
            }
            return binding;
        }

        protected virtual IEnumerable<MemberBinding> VisitBindingList(ReadOnlyCollection<MemberBinding> original)
        {
            List<MemberBinding> list = null;
            for (int i = 0, n = original.Count; i < n; i++)
            {
                MemberBinding b = this.VisitBinding(original[i]);
                if (list != null)
                {
                    list.Add(b);
                }
                else if (b != original[i])
                {
                    list = new List<MemberBinding>(n);
                    for (int j = 0; j < i; j++)
                    {
                        list.Add(original[j]);
                    }
                    list.Add(b);
                }
            }
            if (list != null)
                return list;
            return original;
        }

        protected virtual IEnumerable<ElementInit> VisitElementInitializerList(ReadOnlyCollection<ElementInit> original)
        {
            List<ElementInit> list = null;
            for (int i = 0, n = original.Count; i < n; i++)
            {
                ElementInit init = this.VisitElementInitializer(original[i]);
                if (list != null)
                {
                    list.Add(init);
                }
                else if (init != original[i])
                {
                    list = new List<ElementInit>(n);
                    for (int j = 0; j < i; j++)
                    {
                        list.Add(original[j]);
                    }
                    list.Add(init);
                }
            }
            if (list != null)
                return list;
            return original;
        }

        protected virtual Expression VisitLambda(LambdaExpression lambda)
        {
            Expression body = this.Visit(lambda.Body);
            if (body != lambda.Body)
            {
                return Expression.Lambda(lambda.Type, body, lambda.Parameters);
            }
            return lambda;
        }

        protected virtual NewExpression VisitNew(NewExpression nex)
        {
            IEnumerable<Expression> args = this.VisitExpressionList(nex.Arguments);
            if (args != nex.Arguments)
            {
                if (nex.Members != null)
                    return Expression.New(nex.Constructor, args, nex.Members);
                else
                    return Expression.New(nex.Constructor, args);
            }
            return nex;
        }

        protected virtual Expression VisitMemberInit(MemberInitExpression init)
        {
            NewExpression n = this.VisitNew(init.NewExpression);
            IEnumerable<MemberBinding> bindings = this.VisitBindingList(init.Bindings);
            if (n != init.NewExpression || bindings != init.Bindings)
            {
                return Expression.MemberInit(n, bindings);
            }
            return init;
        }

        protected virtual Expression VisitListInit(ListInitExpression init)
        {
            NewExpression n = this.VisitNew(init.NewExpression);
            IEnumerable<ElementInit> initializers = this.VisitElementInitializerList(init.Initializers);
            if (n != init.NewExpression || initializers != init.Initializers)
            {
                return Expression.ListInit(n, initializers);
            }
            return init;
        }

        protected virtual Expression VisitNewArray(NewArrayExpression na)
        {
            IEnumerable<Expression> exprs = this.VisitExpressionList(na.Expressions);
            if (exprs != na.Expressions)
            {
                if (na.NodeType == ExpressionType.NewArrayInit)
                {
                    return Expression.NewArrayInit(na.Type.GetElementType(), exprs);
                }
                else
                {
                    return Expression.NewArrayBounds(na.Type.GetElementType(), exprs);
                }
            }
            return na;
        }

        protected virtual Expression VisitInvocation(InvocationExpression iv)
        {
            IEnumerable<Expression> args = this.VisitExpressionList(iv.Arguments);
            Expression expr = this.Visit(iv.Expression);
            if (args != iv.Arguments || expr != iv.Expression)
            {
                return Expression.Invoke(expr, args);
            }
            return iv;
        }
    }
    public class PartialEvaluator : ExpressionVisitor35
    {
        private Func<Expression, bool> m_fnCanBeEvaluated;
        private HashSet<Expression> m_candidates;

        public PartialEvaluator()
            : this(CanBeEvaluatedLocally)
        { }

        public PartialEvaluator(Func<Expression, bool> fnCanBeEvaluated)
        {
            this.m_fnCanBeEvaluated = fnCanBeEvaluated;
        }

        public Expression Eval(Expression exp)
        {
            this.m_candidates = new Nominator(this.m_fnCanBeEvaluated).Nominate(exp);

            return this.Visit(exp);
        }

        protected override Expression Visit(Expression exp)
        {
            if (exp == null)
            {
                return null;
            }

            if (this.m_candidates.Contains(exp))
            {
                return this.Evaluate(exp);
            }

            return base.Visit(exp);
        }

        private Expression Evaluate(Expression e)
        {
            if (e.NodeType == ExpressionType.Constant)
            {
                return e;
            }

            LambdaExpression lambda = Expression.Lambda(e);
            Delegate fn = lambda.Compile();
            return Expression.Constant(fn.DynamicInvoke(null), e.Type);
        }

        private static bool CanBeEvaluatedLocally(Expression exp)
        {
            return exp.NodeType != ExpressionType.Parameter;
        }

        #region Nominator

        /// <summary>
        /// Performs bottom-up analysis to determine which nodes can possibly
        /// be part of an evaluated sub-tree.
        /// </summary>
        private class Nominator : ExpressionVisitor35
        {
            private Func<Expression, bool> m_fnCanBeEvaluated;
            private HashSet<Expression> m_candidates;
            private bool m_cannotBeEvaluated;

            internal Nominator(Func<Expression, bool> fnCanBeEvaluated)
            {
                this.m_fnCanBeEvaluated = fnCanBeEvaluated;
            }

            internal HashSet<Expression> Nominate(Expression expression)
            {
                this.m_candidates = new HashSet<Expression>();
                this.Visit(expression);
                return this.m_candidates;
            }

            protected override Expression Visit(Expression expression)
            {
                if (expression != null)
                {
                    bool saveCannotBeEvaluated = this.m_cannotBeEvaluated;
                    this.m_cannotBeEvaluated = false;

                    base.Visit(expression);

                    if (!this.m_cannotBeEvaluated)
                    {
                        if (this.m_fnCanBeEvaluated(expression))
                        {
                            this.m_candidates.Add(expression);
                        }
                        else
                        {
                            this.m_cannotBeEvaluated = true;
                        }
                    }

                    this.m_cannotBeEvaluated |= saveCannotBeEvaluated;
                }

                return expression;
            }
        }

        #endregion
    }
    internal class ConditionBuilder : ExpressionVisitor35
    {
        /// <summary>
        /// 数据库类型
        /// </summary>
        private string m_dataBaseType = "";
        /// <summary>
        /// 字段是否加引号
        /// </summary>
        private bool m_ifWithQuotationMarks = false;

        private List<object> m_arguments;
        private Stack<string> m_conditionParts;

        public string Condition { get; private set; }

        public object[] Arguments { get; private set; }


      

        #region 加双引号
        /// <summary>
        /// 加双引号
        /// </summary>
        /// <param name="str">字串</param>
        /// <returns></returns>
        public static string AddQuotationMarks(string str)
        {
            if (str != null)
            {
                return "\"" + str.Trim() + "\"";
            }
            else
            {
                return str;
            }
        }

        #endregion

        #region 设置是否加双引号
        public void SetIfWithQuotationMarks(bool ifwith)
        {
            m_ifWithQuotationMarks = ifwith;
        }
        #endregion

        #region 设置是数据库类型
        public void  SetDataBaseType(string  databaseType)
        {
            if(!string.IsNullOrEmpty(databaseType))
            {
                m_dataBaseType = databaseType;
            }
            
        }
        #endregion
        public void Build(Expression expression)
        {
            PartialEvaluator evaluator = new PartialEvaluator();
            Expression evaluatedExpression = evaluator.Eval(expression);

            this.m_arguments = new List<object>();
            this.m_conditionParts = new Stack<string>();

            this.Visit(evaluatedExpression);

            this.Arguments = this.m_arguments.ToArray();
            this.Condition = this.m_conditionParts.Count > 0 ? this.m_conditionParts.Pop() : null;
        }

        protected override Expression VisitBinary(BinaryExpression b)
        {
            if (b == null) return b;

            string opr;
            switch (b.NodeType)
            {
                case ExpressionType.Equal:
                    opr = "=";
                    break;
                case ExpressionType.NotEqual:
                    opr = "<>";
                    break;
                case ExpressionType.GreaterThan:
                    opr = ">";
                    break;
                case ExpressionType.GreaterThanOrEqual:
                    opr = ">=";
                    break;
                case ExpressionType.LessThan:
                    opr = "<";
                    break;
                case ExpressionType.LessThanOrEqual:
                    opr = "<=";
                    break;
                case ExpressionType.AndAlso:
                    opr = "AND";
                    break;
                case ExpressionType.OrElse:
                    opr = "OR";
                    break;
                case ExpressionType.Add:
                    opr = "+";
                    break;
                case ExpressionType.Subtract:
                    opr = "-";
                    break;
                case ExpressionType.Multiply:
                    opr = "*";
                    break;
                case ExpressionType.Divide:
                    opr = "/";
                    break;
                default:
                    throw new NotSupportedException(b.NodeType + "is not supported.");
            }

            this.Visit(b.Left);
            this.Visit(b.Right);

            string right = this.m_conditionParts.Pop();
            string left = this.m_conditionParts.Pop();
         
            string condition = String.Format("({0} {1} {2})", left, opr, right);
            this.m_conditionParts.Push(condition);

            return b;
        }

        protected override Expression VisitConstant(ConstantExpression c)
        {
            if (c == null) return c;

            this.m_arguments.Add(c.Value);
            this.m_conditionParts.Push(String.Format("{{{0}}}", this.m_arguments.Count - 1));

            return c;
        }

        protected override Expression VisitMemberAccess(MemberExpression m)
        {
            if (m == null) return m;

            PropertyInfo propertyInfo = m.Member as PropertyInfo;
            if (propertyInfo == null) return m;

            //   this.m_conditionParts.Push(String.Format("(Where.{0})", propertyInfo.Name));
            //是否添加引号
            if (m_ifWithQuotationMarks)
            {
                this.m_conditionParts.Push(String.Format(" {0} ",AddQuotationMarks( propertyInfo.Name)));
            }
            else
            {
                // this.m_conditionParts.Push(String.Format("[{0}]", propertyInfo.Name));
                this.m_conditionParts.Push(String.Format(" {0} ", propertyInfo.Name));
            }

            return m;
        }

        #region 其他
        static string BinarExpressionProvider(Expression left, Expression right, ExpressionType type)
        {
            string sb = "(";
            //先处理左边
            sb += ExpressionRouter(left);

            sb += ExpressionTypeCast(type);

            //再处理右边
            string tmpStr = ExpressionRouter(right);

            if (tmpStr == "null")
            {
                if (sb.EndsWith(" ="))
                    sb = sb.Substring(0, sb.Length - 1) + " is null";
                else if (sb.EndsWith("<>"))
                    sb = sb.Substring(0, sb.Length - 1) + " is not null";
            }
            else
                sb += tmpStr;
            return sb += ")";
        }

        static string ExpressionRouter(Expression exp)
        {
            string sb = string.Empty;
            if (exp is BinaryExpression)
            {

                BinaryExpression be = ((BinaryExpression)exp);
                return BinarExpressionProvider(be.Left, be.Right, be.NodeType);
            }
            else if (exp is MemberExpression)
            {

                MemberExpression me = ((MemberExpression)exp);
                return me.Member.Name;
            }
            else if (exp is NewArrayExpression)
            {
                NewArrayExpression ae = ((NewArrayExpression)exp);
                StringBuilder tmpstr = new StringBuilder();
                foreach (Expression ex in ae.Expressions)
                {
                    tmpstr.Append(ExpressionRouter(ex));
                    tmpstr.Append(",");
                }
                return tmpstr.ToString(0, tmpstr.Length - 1);
            }
            else if (exp is MethodCallExpression)
            {
                MethodCallExpression mce = (MethodCallExpression)exp;
                if (mce.Method.Name == "Like")
                    return string.Format("({0} like {1})", ExpressionRouter(mce.Arguments[0]), ExpressionRouter(mce.Arguments[1]));
                else if (mce.Method.Name == "NotLike")
                    return string.Format("({0} Not like {1})", ExpressionRouter(mce.Arguments[0]), ExpressionRouter(mce.Arguments[1]));
                else if (mce.Method.Name == "In")
                    return string.Format("{0} In ({1})", ExpressionRouter(mce.Arguments[0]), ExpressionRouter(mce.Arguments[1]));
                else if (mce.Method.Name == "NotIn")
                    return string.Format("{0} Not In ({1})", ExpressionRouter(mce.Arguments[0]), ExpressionRouter(mce.Arguments[1]));
                else if (mce.Method.Name == "StartWith")
                    return string.Format("{0} like '{1}%'", ExpressionRouter(mce.Arguments[0]), ExpressionRouter(mce.Arguments[1]));
            }
            else if (exp is ConstantExpression)
            {
                ConstantExpression ce = ((ConstantExpression)exp);
                if (ce.Value == null)
                    return "null";
                else if (ce.Value is ValueType)
                    return ce.Value.ToString();
                else if (ce.Value is string || ce.Value is DateTime || ce.Value is char)
                {
                    
                    return string.Format("'{0}'", ce.Value.ToString());
                }
            }
            else if (exp is UnaryExpression)
            {
                UnaryExpression ue = ((UnaryExpression)exp);
                return ExpressionRouter(ue.Operand);
            }
            return null;
        }

        static string ExpressionTypeCast(ExpressionType type)
        {
            switch (type)
            {
                case ExpressionType.And:
                case ExpressionType.AndAlso:
                    return " AND ";
                case ExpressionType.Equal:
                    return " =";
                case ExpressionType.GreaterThan:
                    return " >";
                case ExpressionType.GreaterThanOrEqual:
                    return ">=";
                case ExpressionType.LessThan:
                    return "<";
                case ExpressionType.LessThanOrEqual:
                    return "<=";
                case ExpressionType.NotEqual:
                    return "<>";
                case ExpressionType.Or:
                case ExpressionType.OrElse:
                    return " Or ";
                case ExpressionType.Add:
                case ExpressionType.AddChecked:
                    return "+";
                case ExpressionType.Subtract:
                case ExpressionType.SubtractChecked:
                    return "-";
                case ExpressionType.Divide:
                    return "/";
                case ExpressionType.Multiply:
                case ExpressionType.MultiplyChecked:
                    return "*";
                default:
                    return null;
            }
        }
        #endregion


        /// <summary>
        /// ConditionBuilder 并不支持生成Like操作，如 字符串的 StartsWith，Contains，EndsWith 并不能生成这样的SQL： Like ‘xxx%’, Like ‘%xxx%’ , Like ‘%xxx’ . 只要override VisitMethodCall 这个方法即可实现上述功能。
        /// </summary>
        /// <param name="m"></param>
        /// <returns></returns>
        protected override Expression VisitMethodCall(MethodCallExpression m)
        {
            string connectorWords = GetLikeConnectorWords(m_dataBaseType); //获取like链接符

            if (m == null) return m;
            string format;
            switch (m.Method.Name)
            {
                case "StartsWith":
                    format = "({0} LIKE ''"+ connectorWords+ "{1}"+ connectorWords+"'%')";
                    break;
                case "Contains":
                    format = "({0} LIKE '%'" + connectorWords + "{1}" + connectorWords + "'%')";
                    break;
                case "EndsWith":
                    format = "({0} LIKE '%'" + connectorWords + "{1}" + connectorWords + "'')";
                    break;

                case "Equals":
                    // not in 或者  in 或 not like
                    format = "({0} {1} )";
                    break;
                   
                default:
                    throw new NotSupportedException(m.NodeType + " is not supported!");
            }
            this.Visit(m.Object);
            this.Visit(m.Arguments[0]);
            string right = this.m_conditionParts.Pop();
            string left = this.m_conditionParts.Pop();         
           
            this.m_conditionParts.Push(String.Format(format, left, right));
            return m;
        }

        /// <summary>
        /// 获得like语句链接符
        /// </summary>
        /// <param name="databaseType"></param>
        /// <returns></returns>
        public static string GetLikeConnectorWords(string databaseType)
        {
            string result = "+";
            switch (databaseType.ToLower())
            {

                case DataBaseType.PostGreSql:
                case DataBaseType.Oracle:
                case DataBaseType.MySql:
                case DataBaseType.Sqlite:
                    result = "||";
                    break;
            }

            return result;
        }

    }




    #endregion
}

 转化类 方法：

sing System;
using System.Collections.Generic;
using System.Linq;
using System.Linq.Expressions;
using System.Text;

namespace LambdaToWhereSqlTest.Common
{
    /// <summary>
    /// lambda表达式转为where条件sql
    /// </summary>
    public class SqlSugor
    {
        #region Expression 转成 where
        /// <summary>
        /// Expression 转成 Where String
        /// </summary>
        /// <typeparam name="T"></typeparam>
        /// <param name="predicate"></param>
        /// <param name="databaseType">数据类型（用于字段是否加引号）</param>
        /// <returns></returns>
        public static string GetWhereByLambda<T>(Expression<Func<T, bool>> predicate, string databaseType)
        {
            bool withQuotationMarks = GetWithQuotationMarks(databaseType);

            ConditionBuilder conditionBuilder = new ConditionBuilder();
            conditionBuilder.SetIfWithQuotationMarks(withQuotationMarks); //字段是否加引号（PostGreSql,Oracle）
            conditionBuilder.SetDataBaseType(databaseType);
            conditionBuilder.Build(predicate);

            for (int i = 0; i < conditionBuilder.Arguments.Length; i++)
            {
                object ce = conditionBuilder.Arguments[i];
                if (ce == null)
                {
                    conditionBuilder.Arguments[i] = DBNull.Value;
                }
                else if (ce is string || ce is char)
                {
                    if (ce.ToString().ToLower().Trim().IndexOf(@"in(") == 0 ||
                        ce.ToString().ToLower().Trim().IndexOf(@"not in(") == 0 ||
                         ce.ToString().ToLower().Trim().IndexOf(@" like '") == 0 ||
                        ce.ToString().ToLower().Trim().IndexOf(@"not like") == 0)
                    {
                        conditionBuilder.Arguments[i] = string.Format(" {0} ", ce.ToString());
                    }
                    else
                    {


                        //****************************************
                        conditionBuilder.Arguments[i] = string.Format("'{0}'", ce.ToString());
                    }
                }
                else if (ce is DateTime)
                {
                    conditionBuilder.Arguments[i] = string.Format("'{0}'", ce.ToString());
                }
                else if (ce is int || ce is long || ce is short || ce is decimal || ce is double || ce is float || ce is bool || ce is byte || ce is sbyte)
                {
                    conditionBuilder.Arguments[i] = ce.ToString();
                }
                else if (ce is ValueType)
                {
                    conditionBuilder.Arguments[i] = ce.ToString();
                }
                else
                {

                    conditionBuilder.Arguments[i] = string.Format("'{0}'", ce.ToString());
                }

            }
            string strWhere = string.Format(conditionBuilder.Condition, conditionBuilder.Arguments);
            return strWhere;
        }

        /// <summary>
        /// 获取是否字段加双引号
        /// </summary>
        /// <param name="databaseType"></param>
        /// <returns></returns>
        public static bool GetWithQuotationMarks(string databaseType)
        {
            bool result = false;
            switch (databaseType.ToLower())
            {

                case DataBaseType.PostGreSql:
                case DataBaseType.Oracle:

                    result = true;
                    break;
            }

            return result;


        }


        #endregion
    }
}　　

 数据库类型:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace LambdaToWhereSqlTest.Common
{
    #region 数据库类型
    /// <summary>
    ///类据库类型
    /// </summary>
    public class DataBaseType
    {

        /// <summary>
        /// sql server
        /// </summary>
        public const string SqlServer = "sqlserver";
      
        /// <summary>
        ///  OleDb
        /// </summary>
        public const string Access = "access";
        /// <summary>
        ///MySql
        /// </summary>
        public const string MySql = "mysql";
        /// <summary>
        /// Oracle
        /// </summary>
        public const string Oracle = "oracle";

        /// <summary>
        /// Postgresql
        /// </summary>
        public const string PostGreSql = "postgresql";

        /// <summary>
        /// Sqlite
        /// </summary>
        public const string Sqlite = "sqlite";
    }
    #endregion
}

注：

Lambda表式中的

StartsWith表示 like ‘字串%’ 如： SqlSugor.GetWhereByLambda<Users>(x=>x. StartsWith(“test”)&&(x.Id>2))

Contains表示 like ‘%字串%’ 如： SqlSugor.GetWhereByLambda<Users>(x=>x. Contains (“test”) ||x.Id==2)

EndsWith表示 like ‘字串%’ 如：SqlSugor.GetWhereByLambda<Users>( x=>x. Contains (“test”) &&(x.Id>4||x.Id==2));

建一个测试项目来演示：【LambdaToWhereSqlTest】

选择数据库类型，然后按生成按键

//生成按键事件:
private void btnCreate_Click(object sender, EventArgs e)
        {
            string dataType = cbxDataType.SelectedItem.ToString();

            string sql1 = SqlSugor.GetWhereByLambda<Users>(x => x.Name.StartsWith("test") && x.Id > 2, dataType);
            string sql2 = SqlSugor.GetWhereByLambda<Users>(x => x.Name.EndsWith("test") && (x.Id >4||x.Id==3), dataType);
            string sql3 = SqlSugor.GetWhereByLambda<Users>(x => x.Name.Contains("test") && (x.Id > 4 && x.Id <=8), dataType);
            string sql4 = SqlSugor.GetWhereByLambda<Users>(x => x.Name=="FengCode" && (x.Id >=1), dataType);


            richTextBoxWhere.Text = sql1 + "\n\r" + sql2 + "\n\r" + sql3 + "\n\r" + sql4;

        }

winform代码：

using LambdaToWhereSqlTest.Common;
using LambdaToWhereSqlTest.Model;
using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;

namespace LambdaToWhereSqlTest
{
  
    public partial class Form1 : Form
    {
　　　　private System.Windows.Forms.Button btnCreate;
        private System.Windows.Forms.ComboBox cbxDataType;
        private System.Windows.Forms.Label label1;
        private System.Windows.Forms.RichTextBox richTextBoxWhere;
   　　#region Windows 窗体设计器生成的代码
        /// <summary>
        /// 设计器支持所需的方法 - 不要修改
        /// 使用代码编辑器修改此方法的内容。
        /// </summary>
        private void InitializeComponent()
        {
            this.btnCreate = new System.Windows.Forms.Button();
            this.cbxDataType = new System.Windows.Forms.ComboBox();
            this.label1 = new System.Windows.Forms.Label();
            this.richTextBoxWhere = new System.Windows.Forms.RichTextBox();
            this.SuspendLayout();
            // 
            // btnCreate
            // 
            this.btnCreate.Location = new System.Drawing.Point(510, 31);
            this.btnCreate.Name = "btnCreate";
            this.btnCreate.Size = new System.Drawing.Size(201, 23);
            this.btnCreate.TabIndex = 0;
            this.btnCreate.Text = "生成";
            this.btnCreate.UseVisualStyleBackColor = true;
            this.btnCreate.Click += new System.EventHandler(this.btnCreate_Click);
            // 
            // cbxDataType
            // 
            this.cbxDataType.FormattingEnabled = true;
            this.cbxDataType.Items.AddRange(new object[] {
            "SqlServer"});
            this.cbxDataType.Location = new System.Drawing.Point(130, 34);
            this.cbxDataType.Name = "cbxDataType";
            this.cbxDataType.Size = new System.Drawing.Size(374, 20);
            this.cbxDataType.TabIndex = 1;
            // 
            // label1
            // 
            this.label1.AutoSize = true;
            this.label1.Location = new System.Drawing.Point(12, 37);
            this.label1.Name = "label1";
            this.label1.Size = new System.Drawing.Size(101, 12);
            this.label1.TabIndex = 2;
            this.label1.Text = "选取数据库类型：";
            // 
            // richTextBoxWhere
            // 
            this.richTextBoxWhere.Location = new System.Drawing.Point(14, 78);
            this.richTextBoxWhere.Name = "richTextBoxWhere";
            this.richTextBoxWhere.Size = new System.Drawing.Size(697, 221);
            this.richTextBoxWhere.TabIndex = 3;
            this.richTextBoxWhere.Text = "";
            // 
            // Form1
            // 
            this.AutoScaleDimensions = new System.Drawing.SizeF(6F, 12F);
            this.AutoScaleMode = System.Windows.Forms.AutoScaleMode.Font;
            this.ClientSize = new System.Drawing.Size(764, 346);
            this.Controls.Add(this.richTextBoxWhere);
            this.Controls.Add(this.label1);
            this.Controls.Add(this.cbxDataType);
            this.Controls.Add(this.btnCreate);
            this.Name = "Form1";
            this.Text = "C#--lambda表达式转为where条件sql字串的方法;如有问题，可以通过756535235@qq.com 联系我，非常感谢";
            this.Load += new System.EventHandler(this.Form1_Load);
            this.ResumeLayout(false);
            this.PerformLayout();

        }

        #endregion

        public class ComboxItem
        {
            public string Text = "";

            public string Value = "";
            public ComboxItem(string _Text, string _Value)
            {
                Text = _Text;
                Value = _Value;
            }

            public override string ToString()
            {
                return Text;
            }
        }
        public Form1()
        {
            InitializeComponent();
            //初始化
            Init();
        }

        /// <summary>
        /// 初始化选择数据类型
        /// </summary>
        private void Init()
        {
            //初始化选择数据类型
            cbxDataType.Items.Add(new ComboxItem( DataBaseType.SqlServer, DataBaseType.SqlServer) );
            cbxDataType.Items.Add(new ComboxItem(DataBaseType.Access, DataBaseType.Access));
            cbxDataType.Items.Add(new ComboxItem(DataBaseType.MySql, DataBaseType.MySql));
            cbxDataType.Items.Add(new ComboxItem(DataBaseType.Oracle, DataBaseType.Oracle));
            cbxDataType.Items.Add(new ComboxItem(DataBaseType.PostGreSql, DataBaseType.PostGreSql));
            cbxDataType.Items.Add(new ComboxItem(DataBaseType.Sqlite, DataBaseType.Sqlite));
            cbxDataType.SelectedIndex = 0;
        }
　　　　　//生成按键事件:
        private void btnCreate_Click(object sender, EventArgs e)
        {
            string dataType = cbxDataType.SelectedItem.ToString();

            string sql1 = SqlSugor.GetWhereByLambda<Users>(x => x.Name.StartsWith("test") && x.Id > 2, dataType);
            string sql2 = SqlSugor.GetWhereByLambda<Users>(x => x.Name.EndsWith("test") && (x.Id >4||x.Id==3), dataType);
            string sql3 = SqlSugor.GetWhereByLambda<Users>(x => x.Name.Contains("test") && (x.Id > 4 && x.Id <=8), dataType);
            string sql4 = SqlSugor.GetWhereByLambda<Users>(x => x.Name=="FengCode" && (x.Id >=1), dataType);


            richTextBoxWhere.Text = sql1 + "\n\r" + sql2 + "\n\r" + sql3 + "\n\r" + sql4;

        }

        private void Form1_Load(object sender, EventArgs e)
        {

        }
    }
}

至此，实现lambda表达式转为where条件sql字串的方法！

版权声明：

本文由FengCodes原创并发布于博客园，欢迎转载，未经本人同意必须保留此段声明，且在文章页面明显位置给出原文链接 ，否则保留追究法律责任的权利。如有问题，可以通过756535235@qq.com 联系我，非常感谢。

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