Use FileStream with StreamWriter to provide a greater degree of control in how you open the file : File Access Mode « File Directory Stream « C# / CSharp Tutorial

1. C# / CSharp Tutorial
2. File Directory Stream
3. File Access Mode

using System;
using System.IO;

public class MainClass
{
  static void Main(string[] args)
  {
    FileStream MyFileStream = new FileStream(@"c:\Testing.txt", FileMode.CreateNew, FileAccess.Write, FileShare.None);
    StreamWriter MyStreamWriter = new StreamWriter(MyFileStream);

    MyStreamWriter.Write("a string");

    MyFileStream.Close();
    MyStreamWriter.Close();
    
  }
}