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POMarshalling nested structs to C#
 

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    <p>We have COM Server (as dll, we have only dll file), that implements two COM interfaces. One of interfaces allows us to get messages (as structure) from some device. Depending on message we need to process corresponding structure. Every structure is pointer to a structure <strong>VARIANT</strong>. This structure is of type byte array <strong>(VT_ARRAY | VT_UI1)</strong>. </p> <p>All structures contains structure <code>Header</code> and some other information. Structure <code>Header</code> contains field <code>MsgId</code>. Depending on <code>MsgId</code> we need to process other structures.</p> <p>In order to get structures from dll to c# we use reflection. </p> <p>Now, it's time for example:</p> <pre><code>// Header (sub-struct of main struct) - [StructLayout(LayoutKind.Sequential)] public struct Header { public int MsgId; } // main struct [StructLayout(LayoutKind.Sequential)] public struct Main { [MarshalAs(UnmanagedType.ByValArray, ArraySubType = UnmanagedType.Struct)] public Header h; public int count; } </code></pre> <p>Every 100 ms we get data from device via reflection:</p> <pre><code> private bool ReadMessage(ref object msg) { object _msg = null; object[] args = new Object[] { _msg }; ParameterModifier byRefParamMod = new ParameterModifier(1); byRefParamMod[0] = true; ParameterModifier[] pmArray = { byRefParamMod }; var value = SomeWrapper.CallMethod(ClientInstance, "ReadMessage", args, pmArray); msg = args[0]; return ((int)value == S_OK) ? true : false; } </code></pre> <p>Then we cast our <code>msg</code> as <code>byte[]</code> and then we need to convert array of bytes into our struct. At this place we have some problem with marshaling.</p> <p>As i have wrote in order to define which structure we need to process (another words <strong>we need to marshaling</strong>) first of all we need to marshal <code>Header</code> structure that is contained in all structures (another words <strong>in all messages from device</strong>).</p> <p>In order to marshal <code>Header</code> struct we use the method proposed in <a href="https://stackoverflow.com/questions/8704161/c-sharp-array-within-a-struct">C# array within a struct</a> (with some editing):</p> <pre><code> public static T DeserializeMsg&lt;T&gt;(byte[] msg) where T : struct { // Pin the managed memory while, copy it out the data, then unpin it GCHandle handle = GCHandle.Alloc(msg, GCHandleType.Pinned); T theStructure = (T)Marshal.PtrToStructure(handle.AddrOfPinnedObject(), typeof(T)); handle.Free(); return theStructure; } </code></pre> <p>How we are using one:</p> <pre><code> private void ProcessMessage(byte[] message) { Header msgHeader = new Header(); msgHeader = DeserializeMsg&lt;Header&gt;(message); } </code></pre> <p>Everything is OK! Here we get <code>msgHeader.MsgId</code> and then we need to get another structure:</p> <pre><code>private void ProcessMessage(byte[] message) { Header msgHeader = new Header(); msgHeader = DeserializeMsg&lt;Header&gt;(message); switch (msgHeader.MsgId) { case START: Main msgMain = new Main(); // Here we get exception (see below) msgMain = DeserializeMsg&lt;Main&gt;(message); break; default: break; } } </code></pre> <p>Here we get exception: <strong>Cannot marshal field 'h' of type 'Main'. Invalid managed/unmanaged type combination (this value type must be paired with Struct)</strong></p> <p>We have tried to change <code>MarshalAsAttribute</code> declaration of inner struct <code>h</code> to </p> <p><code>[MarshalAsAttribute(UnmanagedType.SafeArray, SafeArraySubType=VarEnum.VT_SAFEARRAY)]</code> </p> <p>and</p> <p><code>[MarshalAsAttribute (UnmanagedType.SafeArray, SafeArrayUserDefinedSubType=typeof(Header))]</code></p> <p>but it does not work. How can we get data from <code>Main</code> strucure if it is possible?</p>
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    1. USKonstantin
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    1. COThat cannot possibly work, no way for the pinvoke marshaller to map a struct to an array of structs. Declaring it Header[] h is required. You'll no doubt now have a problem with that array not being a predictable size, that *count* field spells trouble. Which is impossible as well, that field can only reliably be read when you know where it is located. Which depends on the array size, major chicken-and-egg problem. It can only work if you put *count* before the array.
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      1. USHans Passant
    2. COThank you for the answer. If i put `count` before `h` i get the same exception. Hans, thus, if I understand you correctly, we only can do this work via some unmanage wrapper?
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      1. USKonstantin
    3. COI am so sorry, but here http://social.msdn.microsoft.com/Forums/vstudio/en-US/4c71b08e-d549-4b8f-8796-ffe6ad3bef9f/marshalling-nested-struct-from-c-to-c?forum=csharpgeneral problem was solved...
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