C# Signature:

[DllImport("user32.dll", SetLastError=true, CharSet=CharSet.Auto)]
static extern uint RegisterWindowMessage(string lpString);

VB.NET Signature:

<DllImport("user32.dll", CharSet:=CharSet.Auto)> _
Private Shared Function RegisterWindowMessage( _
     ByVal lpString As String) As UInteger
End Function

VB Signature:

Declare Function RegisterWindowMessage Lib "user32.dll" (ByVal lpString As String) As Integer

User-Defined Types:

None.

Notes:

Unicode notification

    DllImport("user32.dll", EntryPoint = "RegisterWindowMessageW", SetLastError = true)]
    private static extern int RegisterWindowMessage(string lpString);

If you use the above notation, it will not be the Unicode call, Always add the CharSet=CharSet.Auto or CharSet.Unicode

In VB6 the definition for the API declaration should be:

Declare Function RegisterWindowMessage Lib "user32.dll" Alias "RegisterWindowMessageW" (ByVal lpString As long) As Integer

    strMessageToRegister = "QueryCancelAutoPlay"
    lngWindowMsg = RegisterWindowMessage(StrPtr(strMessageToRegister))

If you use RegisterWindowMessage("QueryCancelAutoPlay") it will NOT work, put the "QueryCancelAutoPlay" in a variable or constant and use that one.

Tips & Tricks:

Please add some!

Sample Code:

VB.NET Sample Code

    Dim MessageID As Integer
    MessageID = RegisterWindowMessage("QueryCancelAutoPlay")

C# Sample Code

//Register the message
lMsg = Win32.RegisterWindowMessage("WM_HTML_GETOBJECT");
//Get the object
Win32.SendMessageTimeout(windowHandle, lMsg, IntPtr.Zero, IntPtr.Zero, SendMessageTimeoutFlags.SMTO_ABORT_IF_HUNG, 1000, out lRes);
if(lRes != IntPtr.Zero)
{
    //Get the object from lRes
    htmlDoc= (mshtml.IHTMLDocument)Win32.ObjectFromLresult(lRes, IID_IHTMLDocument, IntPtr.Zero);
    return htmlDoc;
}

C# Sample Code

//provide a private internal message id
private UInt32 queryCancelAutoPlay = 0;

[DllImport("user32.dll", SetLastError=true, CharSet=CharSet.Auto)]
static extern uint RegisterWindowMessage(string lpString);

/* only needed if your application is using a dialog box and needs to
* respond to a "QueryCancelAutoPlay" message, it cannot simply return TRUE or FALSE.
[DllImport("user32.dll")]
static extern int SetWindowLong(IntPtr hWnd, int nIndex, int dwNewLong);
*/

protected override void WndProc(ref Message m)
{
    //calling the base first is important, otherwise the values you set later will be lost
    base.WndProc (ref m);

    //if the QueryCancelAutoPlay message id has not been registered...
    if (queryCancelAutoPlay == 0)
        queryCancelAutoPlay = RegisterWindowMessage("QueryCancelAutoPlay");

    //if the window message id equals the QueryCancelAutoPlay message id
    if ((UInt32)m.Msg == queryCancelAutoPlay)
    {
        /* only needed if your application is using a dialog box and needs to
        * respond to a "QueryCancelAutoPlay" message, it cannot simply return TRUE or FALSE.
        SetWindowLong(this.Handle, 0, 1);
        */
        m.Result = (IntPtr)1;
    }
} //WndProc

Alternative Managed API:

Do you know one? Please contribute it!