C# Signature:

[DllImport("ntdll.dll")]
public static extern void RtlInitUnicodeString(
  ref UNICODE_STRING DestinationString,
  [MarshalAs(UnmanagedType.LPWStr)] string SourceString);

User-Defined Types:

public struct UNICODE_STRING
{
     public ushort Length;
     public ushort MaximumLength;
     [MarshalAs(UnmanagedType.LPWStr)]
     public string Buffer;
}

Sample Code:

http://www.rootkit.com/vault/hoglund/migbot.zip