------------------------写txt文件---------------------------------
string fileName = Path.Combine(Application.StartupPath, @"History.txt");
StreamWriter writer = new StreamWriter(fileName,false,Encoding.Default);
foreach (string name in this.userName.AutoCompleteCustomSource)
{
  writer.WriteLine(name);
}
writer.Flush();
writer.Close();
------------------------读txt文件---------------------------------
  string fileName = Path.Combine(Application.StartupPath, @"History.txt");
  if (File.Exists(fileName))
  {
       StreamReader reader = new StreamReader(fileName,Encoding.Default);
        string name = reader.ReadLine();
        while (name != null)
        {
            this.userName.AutoCompleteCustomSource.Add(name);
             this.userName.Items.Add(name);
              name = reader.ReadLine();
         }
           reader.Close();
  }