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POBoolean Marshalling with LayoutKind.Explicit, Is this broken or failing as designed?
 

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  1. POBoolean Marshalling with LayoutKind.Explicit, Is this broken or failing as designed?
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    <p>First of all the Boolean type is said to have a default marshal type of a four-byte value. So the following code works:</p> <pre><code> struct A { public bool bValue1; public int iValue2; } struct B { public int iValue1; public bool bValue2; } public static void Main() { int[] rawvalues = new int[] { 2, 4 }; A a = (A)Marshal.PtrToStructure(GCHandle.Alloc(rawvalues, GCHandleType.Pinned).AddrOfPinnedObject(), typeof(A)); Assert.IsTrue(a.bValue1 == true); Assert.IsTrue(a.iValue2 == 4); B b = (B)Marshal.PtrToStructure(GCHandle.Alloc(rawvalues, GCHandleType.Pinned).AddrOfPinnedObject(), typeof(B)); Assert.IsTrue(b.iValue1 == 2); Assert.IsTrue(b.bValue2 == true); } </code></pre> <p>Clearly these structures marshal independently just fine. The values are translated as expected. However, when we combine these structures into a "union" by declaring LayoutKind.Explicit like this:</p> <pre><code> [StructLayout(LayoutKind.Explicit)] struct Broken { [FieldOffset(0)] public A a; [FieldOffset(0)] public B b; } </code></pre> <p>We suddenly find ourselves unable to correctly marshal these types. Here is the test code for the above structure and how it fails:</p> <pre><code> int[] rawvalues = new int[] { 2, 4 }; Broken broken = (Broken)Marshal.PtrToStructure(GCHandle.Alloc(rawvalues, GCHandleType.Pinned).AddrOfPinnedObject(), typeof(Broken)); Assert.IsTrue(broken.a.bValue1 != false);// pass, not false Assert.IsTrue(broken.a.bValue1 == true);// pass, must be true? Assert.IsTrue(true.Equals(broken.a.bValue1));// FAILS, WOW, WTF? Assert.IsTrue(broken.a.iValue2 == 4);// FAILS, a.iValue1 == 1, What happened to 4? Assert.IsTrue(broken.b.iValue1 == 2);// pass Assert.IsTrue(broken.b.bValue2 == true);// pass </code></pre> <p>It's very humorous to see this express as true: (a.bValue1 != false &amp;&amp; a.bValue1 == true &amp;&amp; !true.Equals(a.bValue1))</p> <p>Of course the bigger problem here is that a.iValue2 != 4, rather the 4 has been changed to 1 (presumably by the overlapped bool). </p> <p><strong>So the question: Is this a bug, or just failed as designed?</strong></p> <p>Background: this came from <a href="https://stackoverflow.com/questions/1602899"> <a href="https://stackoverflow.com/questions/1602899">What is the difference between structures containing bool vs uint when using PInvoke?</a></a></p> <p>Update: This is even stranger when you use large integer values (> 255) as only the byte that is used for the boolean is being modified to a 1, thus changing 0x0f00 to 0x0f01 for the b.bValue2. For a.bValue1 above it's not translated at all and 0x0f00 provides a false value for a.bValue1.</p> <p>Update #2:</p> <p>The most obvious and reasonable solution to the above issue(s) is to use a uint for the marshalling and expose boolean properties instead. Really solving the issue with a 'workaround' is not at question. I'm mostly wondering is this a bug or is this the behavior you would expect?</p> <pre><code> struct A { private uint _bValue1; public bool bValue1 { get { return _bValue1 != 0; } } public int iValue2; } struct B { public int iValue1; private uint _bValue2; public bool bValue2 { get { return _bValue2 != 0; } } } </code></pre>
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