The integral types WPARAM, LPARAM, and LRESULT are 32 bits wide on 32-bit systems and 64 bits wide on 64-bit systems. What happens when a 32-bit process sends a message to a 64-bit window or vice versa?

There's really only one choice when converting a 64-bit value to a 32-bit value: Truncation. When a 64-bit process sends a message to a 32-bit window, the 64-bit WPARAM and LPARAM values are truncated to 32 bits. Similarly, when a 64-bit window returns an LRESULT back to a 32-bit sender, the value is truncated.

But converting a 32-bit value to a 64-bit value introduces a choice: Do you zero-extend or sign-extend?

The answer is obvious if you remember the history of WPARAM, LPARAM, and LRESULT, or if you just look at the header file.

The WPARAM is zero-extended, while LPARAM and LRESULT are sign-extended.

If you remember that WPARAM used to be a WORD and LPARAM and LRESULT used to be LONG, then this follows from the fact that WORD is an unsigned type (therefore zero-extended) and LONG is a signed type (therefore sign-extended).

Even if you didn't know that, you could look it up in the header file.

typedef UINT_PTR WPARAM;
typedef LONG_PTR LPARAM;
typedef LONG_PTR LRESULT;

UINT_PTR is an unsigned type (therefore zero-extended) and LONG_PTR is a signed type (therefore sign-extended).