IsWindow

IsWindow is a function in the Win32 API that tests whether a given window handle identifies an existing window. It is part of the Windows user interface functions and is typically used in C or C++ programs that interact with the Windows graphical subsystem. The function is declared in the WinUser.h header and implemented in user32.dll.

The prototype is BOOL IsWindow(HWND hWnd). It accepts a single parameter: a handle to a window (HWND). The return value is a BOOL, with a nonzero value indicating that hWnd is a valid handle to an existing window, and zero indicating that it is not.

IsWindow checks only the existence of the window associated with the provided handle at the moment of the call. It does not assess visibility, enablement, accessibility, or ownership by a particular process. Because windows can be created and destroyed asynchronously, a handle that IsWindow previously returned as valid can become invalid immediately afterward. Therefore, even after a positive IsWindow result, subsequent operations that use the handle should be prepared to handle failure if the window has been destroyed in the interim.

Common usage involves validating a window handle before sending messages or invoking other window-related APIs. For example, a program might branch on IsWindow(hWnd) before calling functions that require a valid window. IsWindow is one of several window-related checks, with related functions like IsWindowVisible and IsWindowEnabled providing different aspects of window state.