To send a message and return immediately, use the SendMessageCallback or SendNotifyMessage function. To post a message to a thread's message queue and return immediately, use the PostMessage or PostThreadMessage function.

C# Signature:

[DllImport("user32.dll")]
static extern IntPtr SendMessage(IntPtr hWnd, uint Msg, IntPtr wParam, IntPtr lParam);

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

// Overloads use on toolbar/rebar
[DllImport("user32.dll", CharSet=CharSet.Auto)]
static extern void SendMessage(IntPtr hWnd, int msg, int wParam, ref RECT lParam);

[DllImport("user32.dll", CharSet=CharSet.Auto)]
static extern int SendMessage(IntPtr hWnd, int msg, int wParam, ref POINT lParam);

[DllImport("user32.dll", CharSet=CharSet.Auto)]
static extern void SendMessage(IntPtr hWnd, int msg, int wParam, ref TBBUTTON lParam);

[DllImport("user32.dll", CharSet=CharSet.Auto)]
static extern void SendMessage(IntPtr hWnd, int msg, int wParam, ref TBBUTTONINFO lParam);

[DllImport("user32.dll", CharSet=CharSet.Auto)]
static extern void SendMessage(IntPtr hWnd, int msg, int wParam, ref REBARBANDINFO lParam);

VB Signature:

Declare Function SendMessage Lib "user32.dll" (ByVal hWnd As IntPtr, ByVal Msg As Integer,
   ByVal wParam As UIntPtr, ByVal lParam As IntPtr) As IntPtr

' Overloads use on toolbar/rebar
<DllImport("user32.dll", CharSet=CharSet.Auto)> _
static extern void SendMessage(IntPtr hWnd, Integer msg, Integer wParam, RECT lParam)

<DllImport("user32.dll", CharSet=CharSet.Auto)> _
static extern Integer SendMessage(IntPtr hWnd, Integer msg, Integer wParam, POINT lParam)

<DllImport("user32.dll", CharSet=CharSet.Auto)> _
static extern void SendMessage(IntPtr hWnd, Integer msg, Integer wParam, TBBUTTON lParam)

<DllImport("user32.dll", CharSet=CharSet.Auto)> _
static extern void SendMessage(IntPtr hWnd, Integer msg, Integer wParam, TBBUTTONINFO lParam)

<DllImport("user32.dll", CharSet=CharSet.Auto)> _
static extern void SendMessage(IntPtr hWnd, Integer msg, Integer wParam, REBARBANDINFO lParam)

Private Declare Unicode Function SendMessageW Lib "user32" (ByVal hWnd As HandleRef, ByVal msg As Int32, ByVal wParam As Int32, ByRef lparam As Int32) As Int32

Notes:

1) As the number of messages are varied, just keep overloading the function as you need.

2) Use IntPtr instead of UIntrPtr: The UIntPtr type is not CLS-compliant

Tips & Tricks:

Please add some more!

Sample Code:

//C#
//This samlpe first uses SendMessage and WM_GETTEXTLENGTH
//to get the length of the text and then it uses that value
//and SendMessage with WM_GETTEXT and a StringBuilder to
//extract the text.
public static String GetWindowText(IntPtr hWnd)
{
     IntPtr txtLength;
     IntPtr retValue;
     IntPtr zeroVal = new IntPtr(0); // use the imutable singelton IntPtr.Zero instead

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

     //Setup the size of the sb
     StringBuilder sb = new StringBuilder(txtLength.ToInt32() + 1);

     //Get the text of the window/control t
     retValue =  SendMessage(hWnd,WM_GETTEXT,txtLength,sb); // must be txtLength + 1

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

Alternative Managed API:

Do you know one? Please contribute it!..