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    <p>It's not very well documented, but using the ampersand (&amp;) symbol in an F# P/Invoke signature is the correct (and best) way to pass a value by reference. It compiles to the same type signature as a C# <code>ref</code> parameter; adding <code>[&lt;Out&gt;]</code> to the parameter gives the same signature as a C# <code>out</code> parameter.</p> <pre><code>[&lt;DllImport("user32.dll")&gt;] extern bool GetClientRect(nativeint hWnd, [&lt;Out&gt;] RECT&amp; rect) let getClientRect hwnd = let mutable r : RECT = Unchecked.defaultOf&lt;_&gt; if GetClientRect(hwnd, &amp;r) = false then raise &lt;| System.Exception("GetClientRect failed") r </code></pre> <p>Notice that once you change the P/Invoke signature to use <code>byref&lt;RECT&gt;</code> instead of <code>nativeptr&lt;RECT&gt;</code>, you also need to change the address-of operator (&amp;&amp;) you used on <code>r</code> to a single ampersand (&amp;).</p> <p>Since you're only getting a value <em>out</em> of <code>GetClientRect</code>, it's a good practice to initialize the mutable result value to <code>Unchecked.defaultOf&lt;_&gt;</code> so it's clear that it's meant to be overwritten by <code>GetClientRect</code>.</p> <p>EDIT: If changing the P/Invoke signature to use byref (&amp;) doesn't work, you could also try explicitly specifying the marshalling behavior on the return type. The MSDN documentation for GetClientRect says it returns a BOOL, which is not quite the same as a .NET bool (BOOL is a 4-byte integer value).</p> <p>If you want to try that:</p> <pre><code>[&lt;DllImport("user32.dll")&gt;] extern [&lt;return: MarshalAs(UnmanagedType.Bool)&gt;] bool GetClientRect( nativeint hWnd, [&lt;Out&gt;] RECT&amp; rect) </code></pre>
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    1. POf# p/invoke on GetClientRect with pointers works but not with OutAttribute
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    1. USJack P.
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    1. POf# p/invoke on GetClientRect with pointers works but not with OutAttribute
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    1. CONoting, of course, that this will only work if `RECT` is a struct.
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    2. CO@ildjarn Yep, it is. Thanks!
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      1. USRei Miyasaka
    3. COFor the record, in this case at least, specifying the marshalling on the return type didn't seem to be necessary.
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