private static HandleRef GetHandleRef(Control control) {
return new HandleRef(control, control.Handle);
}

You saw a code sample with new IntPtr(show ? 1 : 0) but your SendMessage accepts a string, so you used ToString. That is not the right thing to do – SetShieldIcon(false) will still set it to true. That is because the string is sent as a pointer and since both “1″ and “0″ are non-null pointers, they are both treated as TRUE. What you should do is call the SendMessage overload that accepts an IntPtr:

private static extern IntPtr SendMessage(HandleRef hWnd, UInt32 Msg, IntPtr wParam, IntPtr lParam);

I used the following utility function, and it works:
public static IntPtr SendMessage_Bool(Control control, uint Msg, IntPtr wParam, bool lParam) {
return SendMessage(GetHandleRef(control), Msg, wParam, new IntPtr(lParam ? 1 : 0));
}

Why not use the following instead?
show ? “1″ : “0″

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