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POMapping unmanaged data to a managed structure in .NET
 

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    <p>I have spent many hours working with unmanaged code, and platform invoke in .NET. The code below illustrates something that is puzzling me regarding how unmanaged data is mapped to a managed object in .NET.</p> <p>For this example I am going to use the <a href="http://msdn.microsoft.com/en-gb/library/windows/desktop/dd162897%28v=vs.85%29.aspx">RECT</a> structure:</p> <p><strong>C++ RECT implementation (unmanaged Win32 API)</strong></p> <pre><code>typedef struct _RECT { LONG left; LONG top; LONG right; LONG bottom; } RECT, *PRECT; </code></pre> <p><strong>C# RECT implementation (managed .NET/C#)</strong></p> <pre><code>[StructLayout(LayoutKind.Sequential)] public struct RECT { public int left, top, right, bottom; } </code></pre> <p>Okay, so my C# equivalent should work, right? I mean, all of the variables are in the same order as the C++ struct, and it's using the same variable names.</p> <p>My assumption of <code>LayoutKind.Sequential</code> means that the unmanaged data is mapped to the managed object in the same sequence it appears in the C++ structure. i.e. the data will be mapped, starting with left, then top, then right, then bottom.</p> <p>On this basis I should be able to modify my C# structure...</p> <p><strong>C# RECT implementation (a bit cleaner)</strong></p> <pre><code>[StructLayout(LayoutKind.Sequential)] public struct Rect //I've started by giving it a .NET compliant name { private int _left, _top, _right, _bottom; // variables are no longer directly accessible. /* I can now access the coordinates via properties */ public Int32 Left { get { return _left; } set { this._left = value; } } public Int32 Top { get { return _top; } set { this._top = value; } } public Int32 Right { get { return _right; } set { this._right = value; } } public Int32 Bottom { get { return _bottom; } set { this._bottom = value; } } } </code></pre> <p>So what happens if the variables are declared in the wrong order? Presumably this screws up the coordinates because they will no longer be mapped to the correct thing?</p> <pre><code>public struct RECT { public int top, right, bottom, left; } </code></pre> <p>At a guess, this would map like so:</p> <blockquote> <p>top = left </p> <p>right = top </p> <p>bottom = right </p> <p>left = bottom</p> </blockquote> <p>So my question is simply, Am I correct in my assumption that, I can modify the managed structure in terms of the access specifier of each variable, and even the variable name, but I cannot alter the order of the variables?</p>
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