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).