该进程无法访问该文件，因为该文件正在被另一个进程使用（文件已创建但不包含任何内容）

using System.IO;

class test
{
    public static void Main()
    {

        string path=@"c:\mytext.txt";

        if(File.Exists(path))
        {
            File.Delete(path);
        }


        FileStream fs=new FileStream(path,FileMode.OpenOrCreate);
        StreamWriter str=new StreamWriter(fs);
        str.BaseStream.Seek(0,SeekOrigin.End); 
        str.Write("mytext.txt.........................");
        str.WriteLine(DateTime.Now.ToLongTimeString()+" "+DateTime.Now.ToLongDateString());
        string addtext="this line is added"+Environment.NewLine;
        File.AppendAllText(path,addtext);  //Exception occurrs ??????????
        string readtext=File.ReadAllText(path);
        Console.WriteLine(readtext);
        str.Flush();
        str.Close();

        Console.ReadKey();
  //System.IO.IOException: The process cannot access the file 'c:\mytext.txt' because it is //being used by another process.
  // at System.IO.__Error.WinIOError(Int32 errorCode, String maybeFullPath)

    }
}

Try This

string path = @"c:\mytext.txt";

if (File.Exists(path))
{
    File.Delete(path);
}

{ // Consider File Operation 1
    FileStream fs = new FileStream(path, FileMode.OpenOrCreate);
    StreamWriter str = new StreamWriter(fs);
    str.BaseStream.Seek(0, SeekOrigin.End);
    str.Write("mytext.txt.........................");
    str.WriteLine(DateTime.Now.ToLongTimeString() + " " + 
                  DateTime.Now.ToLongDateString());
    string addtext = "this line is added" + Environment.NewLine;
    str.Flush();
    str.Close();
    fs.Close();
    // Close the Stream then Individually you can access the file.
}

File.AppendAllText(path, addtext);  // File Operation 2

string readtext = File.ReadAllText(path); // File Operation 3

Console.WriteLine(readtext);

在每次文件操作中，文件都会被打开，并且必须在打开之前关闭。同样，在操作 1 中，您必须关闭文件流以进行进一步的操作。