StreamWriter sw = new StreamWriter("d:\\a.txt", false, System.Text.Encoding.Default);
        sw.Write("哈哈");
        sw.Flush();
        sw.Close();

------解决方案--------------------
要引入
//写入
using System.IO;
protect string file =@"C:\test.txt";
private void btnWriteFile_Click(object sender, System.EventArgs e)
 {  
    if (File.Exists(file))  
   {   
     sw.WriteLine(“这是你要输入的汉字”);
     sw.Close();
   }
   else
   {   
     StreamWriter sr = File.CreateText(file);
     sw.WriteLine(“这是你要输入的汉字”);
     sr.Close();
     sw.Close();
   }  
 } 


------解决方案--------------------

//写入

//要引入 以下命名空间
  
using System.IO;  
protect const string file =@"C:\test.txt";  
private void btnWriteFile_Click(object sender, System.EventArgs e)  
 { 
    if (File.Exists(file)) 
   {    
     StreamWriter sw = new StreamWriter(file, false, Encoding.GetEncoding("gb2312"));//***
     sw.WriteLine(“这是你要输入的汉字”);  
     sw.Close();  
   }  
   else  
   { 
     StreamWriter sr = File.CreateText(file);   
     StreamWriter sw = new StreamWriter(file, false, Encoding.GetEncoding("gb2312"));
     sw.WriteLine(“这是你要输入的汉字”);  
     sw.Close();    
   }   
 }