I keep getting "Unable to find an entry point named 'GetWindowLongPtrA' in DLL 'user32.dll'"

Question

I am trying to use GetWindowLongPtrA but I keep getting the "Unable to find an entry point named 'GetWindowLongPtrA' in DLL 'user32.dll'". (also SetWindowLongPtrA getting the same error). I've tried many solutions found on Google, but they didn't solve it.

Here is the declaration of the function I've written:

[DllImport("user32.dll")]
public static extern IntPtr GetWindowLongPtrA(IntPtr hWnd, int nIndex);

Tried to put EntryPoint = "GetWindowLongPtrA", changed GetWindowLongPtrA to GetWindowLongPtr, put CharSet = CharSet.Ansi, switched to GetWindowLongPtrW with CharSet = CharSet.Unicode etc., They all didn't work.

My computer is exactly "64-bit" (but cannot call that 64-bit WinAPI function?). the OS is Windows 10.

But my system drive is running out of free space. Is this a possible cause?

What is the solution for this problem?

Answer 1

There is no function named GetWindowLongPtr, GetWindowLongPtrA or GetWindowLongPtrW in the 32-bit version of user32.dll:

The reason that using GetWindowLongPtr regardless of target bitness works C and C++ WinAPI code is that in 32-bit code it's a macro that calls GetWindowLong(A|W). It only exists in the 64-bit version of user32.dll:

The documentation for importing GetWindowLongPtr on pinvoke.net includes a code sample for how to make this importation transparent to target bitness (remember, the error is thrown when you actually try to call the imported function that doesn't exist, not on the DllImport line):

[DllImport("user32.dll", EntryPoint="GetWindowLong")]
private static extern IntPtr GetWindowLongPtr32(IntPtr hWnd, int nIndex);

[DllImport("user32.dll", EntryPoint="GetWindowLongPtr")]
private static extern IntPtr GetWindowLongPtr64(IntPtr hWnd, int nIndex);

// This static method is required because Win32 does not support
// GetWindowLongPtr directly
public static IntPtr GetWindowLongPtr(IntPtr hWnd, int nIndex)
{
     if (IntPtr.Size == 8)
     return GetWindowLongPtr64(hWnd, nIndex);
     else
     return GetWindowLongPtr32(hWnd, nIndex);
}