C# Signature:

[DllImport("user32.dll")]
static extern sbyte GetMessage(out MSG lpMsg, IntPtr hWnd, uint wMsgFilterMin,
   uint wMsgFilterMax);

VB Signature:

<DllImport("user32.dll")> _
Public Shared Function GetMessage( _
     ByRef lpMsg As MSG, _
     ByVal hWnd As IntPtr, _
     ByVal wMsgFilterMin As UInteger, _
     ByVal wMsgFilterMax As UInteger) As <MarshalAs(UnmanagedType.Bool)> Boolean
End Function

User-Defined Types:

None.

Notes:

The GetMessage function retrieves a message from the calling thread's message queue and places it in the specified structure. This function can retrieve both messages associated with a specified window and thread messages posted via the PostThreadMessage function. The function retrieves messages that lie within a specified range of message values. GetMessage does not retrieve messages for windows that belong to other threads or applications.

BOOL GetMessage(
     LPMSG lpMsg,    // address of structure with message
     HWND hWnd,      // handle of window
     UINT wMsgFilterMin, // first message
     UINT wMsgFilterMax  // last message
);

Parameters

lpMsg

Points to an MSG structure that receives message information from the thread's message queue.

hWnd

Identifies the window whose messages are to be retrieved. One value has a special meaning:

Value Meaning

NULL GetMessage retrieves messages for any window that belongs to the calling thread and thread messages posted to the calling thread via PostThreadMessage.

wMsgFilterMin

Specifies the integer value of the lowest message value to be retrieved.

wMsgFilterMax

Specifies the integer value of the highest message value to be retrieved.

Return Values

If the function retrieves a message other than WM_QUIT, the return value is nonzero.

If the function retrieves the WM_QUIT message, the return value is zero.

If there is an error, the return value is -1. For example, the function fails if hWnd is an invalid window handle.

Remarks

An application typically uses the return value to determine whether to end the main message loop and exit the program.

The GetMessage function only retrieves messages associated with the window identified by the hWnd parameter or any of its children as specified by the IsChild function, and within the range of message values given by the wMsgFilterMin and wMsgFilterMax parameters. If hWnd is NULL, GetMessage retrieves messages for any window that belongs to the calling thread and thread messages posted to the calling thread via PostThreadMessage. GetMessage does not retrieve messages for windows that belong to other threads nor for threads other than the calling thread. Thread messages, posted by the PostThreadmessage function, have a message hWnd value of NULL. If wMsgFilterMin and wMsgFilterMax are both zero, GetMessage returns all available messages (that is, no range filtering is performed).

The WM_KEYFIRST and WM_KEYLAST constants can be used as filter values to retrieve all messages related to keyboard input; the WM_MOUSEFIRST and WM_MOUSELAST constants can be used to retrieve all mouse messages. If the wMsgFilterMin and wMsgFilterMax parameters are both zero, the GetMessage function returns all available messages (that is, without performing any filtering).

GetMessage does not remove WM_PAINT messages from the queue. The messages remain in the queue until processed.

Note that the function return value can be TRUE, FALSE, or -1. Thus, you should avoid code like this:

while (GetMessage( lpMsg, hWnd, 0, 0)) ...

The possibility of a -1 return value means that such code can lead to fatal application errors.

Tips & Tricks:

Please add some!

Sample Code:

MSG msg;
sbyte ret;
while((ret = GetMessage(out msg, IntPtr.Zero, 0, 0)) != -1)
{
    if (ret == -1)
    {
       //-1 indicates an error
    }
    else
    {
       TranslateMessage(ref msg);
       DispatchMessage(ref msg);
    }
}

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