C# Signature:

public static IntPtr SetClassLong(HandleRef hWnd, int nIndex, IntPtr dwNewLong)
{
    if (IntPtr.Size > 4)
        return SetClassLongPtr64(hWnd, nIndex, dwNewLong);
    else
        return new IntPtr(SetClassLongPtr32(hWnd, nIndex, unchecked((uint)dwNewLong.ToInt32())));
}

[DllImport("user32.dll", EntryPoint="SetClassLong")]
public static extern uint SetClassLongPtr32(HandleRef hWnd, int nIndex, uint dwNewLong);

[DllImport("user32.dll", EntryPoint="SetClassLongPtr")]
public static extern IntPtr SetClassLongPtr64(HandleRef hWnd, int nIndex, IntPtr dwNewLong);

User-Defined Types:

None.

Notes:

SetClassLongPtr is 64-bit safe, SetClassLong is not.

Because SetClassLongPtr is not available on older 32-bit operating systems, a special wrapper function is required. The MSDN documentation specifies that SetClassLongPtr is supported on older 32-bit operating systems such as Windows 95. However, the Win32 API actually uses a macro to change SetClassLongPtr to SetClassLong on these older systems.

Testing the size of the IntPtr is the easiest way to determine if the platform is 64-bit.

Tips & Tricks:

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Sample Code:

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Alternative Managed API:

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