string myValue = "Test";

byte[] data = Encoding.Default.GetBytes(myValue);
using (MD5 algorithm = MD5.Create())
{
    byte[] hashInBytes = algorithm.ComputeHash(data);
    string resultInBitFormat = BitConverter.ToString(hashInBytes);
    string resultAsBase64 = Convert.ToBase64String(hashInBytes);

    Console.WriteLine("Plain {0}", myValue);
    Console.WriteLine("Bit {0}", resultInBitFormat);
    Console.WriteLine("Base64 {0}", resultAsBase64);
}

/// <summary>Gibt einen MD5 Hash als String zurück</summary>
/// <param name="textToHash">string der Gehasht werden soll.</param>
/// <returns>Hash als string.</returns>
/// <exampel>
///   string md5 = "abs".GetMD5Hash();
/// </exampel>
[DebuggerStepThrough]
public static string GetMD5Hash(this string textToHash) {
  //-----------------------------------------------------------------------
  // Prüfen ob Daten übergeben wurden.
  //-----------------------------------------------------------------------
  if (string.IsNullOrEmpty(textToHash)) {
    return string.Empty;
  }

  //-----------------------------------------------------------------------
  // MD5 Hash aus dem String berechnen. Dazu muss der string in ein Byte[]
  // zerlegt werden. Danach muss das Resultat wieder zurück in ein String.
  //-----------------------------------------------------------------------
  MD5 md5 = new MD5CryptoServiceProvider();
  byte[] byteTextToHash = Encoding.Default.GetBytes(textToHash);
  byte[] result = md5.ComputeHash(byteTextToHash);
  
  md5.Dispose();

  return BitConverter.ToString(result);
}

using System.Security.Cryptography;

public static String GetMd5Hash(String value)
{
    Byte[] bytes;
    var hashValue = new Byte[1];
    var sb = new StringBuilder();

    var MD5 = new MD5CryptoServiceProvider();
    bytes = Encoding.ASCII.GetBytes(value);
    hashValue = MD5.ComputeHash(bytes);

    for (var i = 0; i <= hashValue.Length - 1; i++)
    {
        sb.Append(hashValue.ToString("X2"));
    }
    return sb.ToString();
}[/code]