[c#] How to both read and write a file in C#

I want to both read from and write to a file. This doesn't work.

static void Main(string[] args)
{
    StreamReader sr = new StreamReader(@"C:\words.txt");
    StreamWriter sw = new StreamWriter(@"C:\words.txt");
}

How can I both read from and write to a file in C#?

Made an improvement code by @Ipsita - for use asynchronous read\write file I/O

readonly string logPath = @"FilePath";
...
public async Task WriteToLogAsync(string dataToWrite)
{
    TextReader tr = new StreamReader(logPath);
    string data = await tr.ReadLineAsync();
    tr.Close();

    TextWriter tw = new StreamWriter(logPath);
    await tw.WriteLineAsync(data + dataToWrite);
    tw.Close();
}
...
await WriteToLogAsync("Write this to file");

You need a single stream, opened for both reading and writing.

FileStream fileStream = new FileStream(
      @"c:\words.txt", FileMode.OpenOrCreate, 
      FileAccess.ReadWrite, FileShare.None);

you can try this:"Filename.txt" file will be created automatically in the bin->debug folder everytime you run this code or you can specify path of the file like: @"C:/...". you can check ëxistance of "Hello" by going to the bin -->debug folder

P.S dont forget to add Console.Readline() after this code snippet else console will not appear.

TextWriter tw = new StreamWriter("filename.txt");
        String text = "Hello";
        tw.WriteLine(text);
        tw.Close();

        TextReader tr = new StreamReader("filename.txt");
        Console.WriteLine(tr.ReadLine());
        tr.Close();

Don't forget the easy route:

    static void Main(string[] args)
    {
        var text = File.ReadAllText(@"C:\words.txt");
        File.WriteAllText(@"C:\words.txt", text + "DERP");
    }

var fs = File.Open("file.name", FileMode.OpenOrCreate, FileAccess.ReadWrite);
var sw = new StreamWriter(fs);
var sr = new StreamReader(fs);

...

fs.Close();
//or sw.Close();

The key thing is to open the file with the FileAccess.ReadWrite flag. You can then create whatever Stream/String/Binary Reader/Writers you need using the initial FileStream.

This thread seems to answer your question : simultaneous-read-write-a-file

Basically, what you need is to declare two FileStream, one for read operations, the other for write operations. Writer Filestream needs to open your file in 'Append' mode.