C# Signature:

public static IntPtr GetClassLongPtr(HandleRef hWnd, int nIndex)
{
    if (IntPtr.Size > 4)
        return GetClassLongPtr64(hWnd, nIndex);
    else
        return new IntPtr(GetClassLongPtr32(hWnd, nIndex));
}

[DllImport("user32.dll", EntryPoint="GetClassLong")]
public static extern uint GetClassLongPtr32(HandleRef hWnd, int nIndex);

[DllImport("user32.dll", EntryPoint="GetClassLongPtr")]
public static extern IntPtr GetClassLongPtr64(HandleRef hWnd, int nIndex);

VB.NET Signature:

Public Shared Function GetClassLongPtr(ByVal hWnd As HandleRef, ByVal nIndex As Integer) As IntPtr
    If IntPtr.Size > 4 Then
        Return GetClassLongPtr64(hWnd, nIndex)
    Else
        Return New IntPtr(GetClassLongPtr32(hWnd, nIndex))
    End If
End Function

<DllImport("user32.dll", EntryPoint:="GetClassLong")> _
Public Shared Function GetClassLongPtr32(ByVal hWnd As HandleRef, <MarshalAs(UnmanagedType.I4)>nIndex As ClassLongFlags) As UInteger
End Function

<DllImport("user32.dll", EntryPoint:="GetClassLongPtr")> _
Public Shared Function GetClassLongPtr64(ByVal hWnd As HandleRef, <MarshalAs(UnmanagedType.I4)>nIndex As ClassLongFlags) As IntPtr
End Function

VB Signature:

Public Declare Function GetClassLongPtr Lib "user32"
        (ByVal hWnd As Long, _
         ByVal nIndex As ClassLongFlags) As Long

User-Defined Types:

ClassLongFlags

Notes:

GetClassLongPtr is 64-bit safe, GetClassLong is not.

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

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

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