PInvoke: Getting all child handles of window
If you don’t know I have a new job in Bulgaria. I went away form Web Development and now I’m working as Winforms Developer. However, we had a client that exceed the number of window handles (more than 10 000) due to bad design of application. While diagnosing his application, we needed to understand how many handles are created and when they are born. 
Reading MSDN I figure out that Windows API provide the availability to get all child handles for specified handle. We should use unmanaged EnumChildWindows function provided by user32.dll.
MSDN Definition
EnumChildWindows function enumerates the child windows that belong to the specified parent window by passing the handle to each child window, in turn, to an application-defined callback function. EnumChildWindows continues until the last child window is enumerated or the callback function returns FALSE.
Syntax
BOOL EnumChildWindows(
HWND hWndParent,
WNDENUMPROC lpEnumFunc,
LPARAM lParam
);
- hWndParent
- [in] Handle to the parent window whose child windows are to be enumerated.
- lpEnumFunc
- [in] Pointer to an application-defined callback function.
- lParam
- [in] Specifies an application-defined value to be passed to the callback function.
Note: If a child window has created child windows of its own, EnumChildWindows enumerates those windows as well.
How is used from .NET Framework
public class WindowHandleInfo
{
private delegate bool EnumWindowProc(IntPtr hwnd, IntPtr lParam);
[DllImport("user32")]
[return: MarshalAs(UnmanagedType.Bool)]
private static extern bool EnumChildWindows(IntPtr window, EnumWindowProc callback, IntPtr lParam);
private IntPtr _MainHandle;
public WindowHandleInfo(IntPtr handle)
{
this._MainHandle = handle;
}
public List<IntPtr> GetAllChildHandles()
{
List<IntPtr> childHandles = new List<IntPtr>();
GCHandle gcChildhandlesList = GCHandle.Alloc(childHandles);
IntPtr pointerChildHandlesList = GCHandle.ToIntPtr(gcChildhandlesList);
try
{
EnumWindowProc childProc = new EnumWindowProc(EnumWindow);
EnumChildWindows(this._MainHandle, childProc, pointerChildHandlesList);
}
finally
{
gcChildhandlesList.Free();
}
return childHandles;
}
private bool EnumWindow(IntPtr hWnd, IntPtr lParam)
{
GCHandle gcChildhandlesList = GCHandle.FromIntPtr(lParam);
if (gcChildhandlesList == null || gcChildhandlesList.Target == null)
{
return false;
}
List<IntPtr> childHandles = gcChildhandlesList.Target as List<IntPtr>;
childHandles.Add(hWnd);
return true;
}
}
We are using DLLImport attribute to expose the method in .NET Environment. The specific point is that we create a handle to the list where are collected all pointers, due to availability to access it from the callback function where we know which is the child handle.
References
