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", CharSet = CharSet.Auto, SetLastError = false)]
static extern IntPtr SendMessage(HandleRef hWnd, uint Msg, IntPtr wParam, IntPtr lParam);

Common C# Overloads:

[DllImport("user32.dll", CharSet = CharSet.Auto, SetLastError = false)]
static extern IntPtr SendMessage(HandleRef hWnd, uint Msg, IntPtr wParam, StringBuilder lParam);

[DllImport("user32.dll", CharSet = CharSet.Auto, SetLastError = false)]
static extern IntPtr SendMessage(HandleRef hWnd, uint Msg, IntPtr wParam,  String lParam);

//Also can add 'ref' or 'out' ahead 'String lParam'

[DllImport("user32.dll", CharSet = CharSet.Auto, SetLastError = false)]
static extern void SendMessage(HandleRef hWnd, uint msg, IntPtr wParam, ref RECT lParam);

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

VB Signature:

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

Common VB Overloads:

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

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

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

Notes:

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

2) NEVER use "int" or "integer" as wParam or lParam. Your code WILL crash on 64-bit windows. ONLY use IntPtr, a "ref" structure, or an "out" structure.

3) NEVER use "bool", "int", or "integer" as the return value. Your core WILL crash on 64-bit windows. ONLY use IntPtr. It's not safe to use bool - pInvoke cannot marshal an IntPtr to a boolean.

4) Use "IntPtr" as the return value, EVEN if the message says it doesn't return any useful information.

5) "msg" is always a 4-bit integer (do not use IntPtr). It is safe to use both "int" and "uint" (32-bit only), but Microsoft technically defines "msg" as a "double word" - which is the same as "Uint32"

6) You can replace "hWnd" with "IntPtr" instead of "HandleRef". However, you are taking a risk in doing this - it may cause your code to crash with race conditions - .NET can and will dispose your window handles out from under your message causing all sorts of nasty problems!

::: Question, for someone more knowledgable than me: Can't this last issue be addressed with marshaling, specifically pinning?

Tips & Tricks:

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

Alternative Managed API:

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