C# Signatures:

[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);

VB Signatures:

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

<DllImport("User32.dll", CharSet:=CharSet.Auto, Entrypoint:="SendMessage")> _
Private Shared Function SendMessageString(ByVal hwnd As IntPtr, _
   ByVal wMsg As Integer, ByVal wparam As IntPtr, ByVal lparam As String) As IntPtr
End Function

User-Defined Types:

None.

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#
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() + 1);

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

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

Alternative Managed API:

Do you know one? Please contribute it!