C#

[DllImport("NetApi32.dll",CharSet=CharSet.Unicode)] private static extern int NetUseGetInfo(string SrvrName,string MappedDrive,int Level,ref structUSE_INFO_2 MapInfo);
[DllImport("NetApi32.dll",CharSet=CharSet.Unicode)] private static extern int NetApiBufferFree(ref structUSE_INFO_2 MapInfo);


[StructLayout(LayoutKind.Sequential,CharSet=CharSet.Unicode)]
private struct structUSE_INFO_2
{
	public string sLocal;
	public string sRemote;
	public string sPassword;
	public int iType;
	public int iRefCount;
	public int iUseCount;
	public string sUsername;
	public string sDomain;
}
...
...
...

structUSE_INFO_2 MappInfo = new structUSE_INFO_2();
int Err=NetUseGetInfo(null,DriveLetter,2,ref MappInfo);
Username=MappInfo.sUsername;
Err=NetApiBufferFree(ref MappInfo);

using System;
using System.ComponentModel;
using System.Runtime.InteropServices;

internal class GetInfo
{
  static void Main(string[] args)
  {
    if (args.Length != 1)
    {
      Console.Error.WriteLine("You must specify a drive letter.");
      Environment.Exit(1);
    }

    if (Environment.OSVersion.Platform != PlatformID.Win32NT)
    {
      Console.Error.WriteLine("Supported only on Windows NT.");
      Environment.Exit(2);
    }
 
    // Pay attention here
    IntPtr ptr = IntPtr.Zero;
    if (0 == NetUseGetInfo(null, args[0], 2, out ptr))
    {
      UseInfo2 info = (UseInfo2)Marshal.PtrToStructure(ptr,
        typeof(UseInfo2));

      Console.WriteLine("Username: " + info.username);
      Console.WriteLine("Domain: " + info.domainname);

      NetApiBufferFree(ptr);
    }
 
    else
    {
      int error = Marshal.GetLastWin32Error();
      Console.Error.WriteLine("Error " + error);
      Environment.Exit(error);
    }
  }

  [DllImport("netapi32.dll", CharSet=CharSet.Unicode, SetLastError=true)]
  public static extern uint NetUseGetInfo
  (
    string server,
    string drive,
    uint level,
    out IntPtr info
  );

  [DllImport("netapi32.dll")]
  public static extern uint NetApiBufferFree(IntPtr ptr);
}

[StructLayout(LayoutKind.Sequential, CharSet=CharSet.Unicode)]
internal struct UseInfo2
{
  public string local;
  public string remote;
  public string password;
  public uint status;
  public uint type;
  public uint refcount;
  public uint usecount;
  public string username;
  public string domainname;
}

public class Win32
	{

		public const uint WM_SETTEXT    = 0x000C;

				
		[DllImport("user32.dll")]
		public static extern int FindWindow(
			string lpClassName,  // class name
			string lpWindowName  // window name
			);
		[DllImport("user32.dll")]
		public static extern long SendMessage(
                       int hWnd, //handle to destination
                       uint msg, //message
                       uint wparam, //message parameter
                       string text // what i want to display as text
                       );

private void button1_Click(object sender, System.EventArgs e)
		{
		// Determine the handle to the NotePad window.
		int i=Win32.FindWindow(txtClsNm.Text ,txtWndNm.Text);
                // Post a message to Notepad
		Win32.SendMessage(i,Win32.WM_SETTEXT,0,"Testing my program");
		}

public const int WM_SETTEXT = 0x000C;

		[DllImport("user32.dll")]
		static extern int FindWindow(string lpClassName, string lpWindowName);

		[DllImport("user32.dll")]
		static extern int FindWindowEx(int hwndParent, int hwndChildAfter, string lpszClass, string lpszWindow);
		
		[DllImport("user32.dll")]
		static extern int SendMessage(int hWnd, int Msg, int wParam, string lParam);

		int ParenthWnd;
		int hWnd;

		private void button1_Click(object sender, System.EventArgs e)
		{
			string lpszParentClass = "Notepad";
			string lpszParentWindow = "Untitled - Notepad";
			string lpszClass = "Edit";

			ParenthWnd = FindWindow(lpszParentClass, lpszParentWindow);

			if (ParenthWnd==0)  
				Console.WriteLine("Notepad Not Running");
			else
			{
				hWnd = FindWindowEx(ParenthWnd,hWnd,lpszClass,"");
				if (hWnd.Equals(IntPtr.Zero))  
					Console.WriteLine("What the F??? Notepad doesn't have an edit component ?");
				else
				{
					MessageBox.Show("Notepad Window: " + ParenthWnd.ToString());
					MessageBox.Show("Edit Control: " + hWnd.ToString());

					string hi="hello";
					SendMessage(hWnd, WM_SETTEXT, hi.Length, hi);
				}
			}
		}

//Overload for string lParam (e.g. WM_GETTEXT)
[DllImport("user32.dll")]
static extern IntPtr SendMessage(IntPtr hWnd, uint Msg, IntPtr wParam,
    [Out] StringBuilder lParam);

private void button1_Click(object sender, System.EventArgs e)
{
    char [] arr = {'0'};

    string handle=this.textBox1.Text;
    handle=this.textBox1.Text.TrimStart(arr);

    IntPtr hWnd = new IntPtr(Convert.ToInt32(handle, 16));
    MessageBox.Show(GetWindowText(hWnd));
}

public static String GetWindowText(IntPtr hWnd)
{
    IntPtr txtLength;
    IntPtr retValue;
    IntPtr zeroVal = new IntPtr(0);


    //Get the length of the text
    retValue = SendMessage(hWnd,WM_GETTEXTLENGTH,zeroVal,zeroVal);


    //Setup the size of the sb
    txtLength = retValue;
    StringBuilder sb = new StringBuilder(txtLength.ToInt32() + 2);


    //Get the text of the window/control
    retValue =  SendMessage(hWnd, WM_GETTEXT, txtLength, sb);


    //Return a string
    return sb.ToString();
}

c:\test\notepad writer\Form1.cs(183): Argument '4': cannot convert from 'System.IntPtr' to 'System.Text.StringBuilder'

//Get the length of the text			retValue = SendMessage(hWnd,WM_GETTEXTLENGTH,zeroVal,zeroVal);

public static String GetWindowText(IntPtr hWnd)
{
    IntPtr txtLength;
    IntPtr retValue;
    IntPtr zeroVal = new IntPtr(0);


    //Get the length of the text
    retValue = SendMessage(hWnd,WM_GETTEXTLENGTH,zeroVal,zeroVal);


    //Setup the size of the sb
    txtLength = retValue;
    StringBuilder sb = new StringBuilder(txtLength.ToInt32() + 2);


    //Get the text of the window/control
    retValue =  SendMessage(hWnd, WM_GETTEXT, txtLength, sb);


    //Return a string
    return sb.ToString();
}