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    <p>Ooooh, man! You've picked quite a complex case for your first fiddling experience. I recommend doing something simpler first, and then moving on to the real stuff.</p> <p><strong>Firstly</strong>, <code>CharSet=CharSet.Ansi</code> looks suspicious. All your strings and chars are <code>_tuchar</code>, and I gather the <code>u</code> in there means "Unicode", doesn't it? If that's the case, you need <code>CharSet=CharSet.Unicode</code>.</p> <p><strong>Secondly</strong>, (and this is the most likely culprit) why is the <code>ErrorMessages</code> field marshaled as <code>ByValArray</code>? You know that <code>ByVal</code> here means "by value", don't you? As in, <strong>not</strong> by reference. And you know that little asterisk thing in C++ means "reference", don't you? So why does your reference field <code>ErrorMessages</code> marshaled as a by-value array? In case you don't know, an array is generally said to be passed "by value" when all of it's content is being passed, instead of just passing a reference (pointer) to a memory location where all that content is stored. In C++ struct definition, you specify <code>_tuchar*</code>, which means "a reference (pointer) to some memory containing one or more of _tuchars", whereas in C# you specify <code>[MarshalAs(UnmanagedType.ByValArray, SizeConst=500)]</code>, which means "500 _tuchars are supposed to be here, no more and no less". Seeing how a reference (pointer) usually takes 4 bytes (or 8 bytes on 64bit machines), and 500 unicode characters take 1000 bytes, you have an obvious mismatch right here.</p> <p><strong>Thirdly</strong> and <strong>fourthly</strong>, same point goes for <code>result</code> and <code>blocks</code> fields.</p> <p><strong>Fifthly</strong>, the <code>language</code> field is exactly reverse situation: the C++ code says "there are 3 _tuchars here", while C# code says "there is a reference (pointer) to a string here" (in case you don't know, <code>LPStr</code> means "Long Pointer to STRing")</p> <p><strong>And finally</strong>, after you have fixed all those problems, I recommend you execute your program and print out the result of call to <code>Marshal.SizeOf( typeof( MyStruct ) )</code>. That will give you exactly how big your struct is, in .NET's opinion. The go on the C++ side and print out <code>sizeof( MyStruct )</code>. That will give you what C++ thinks about the size.</p> <p>If they turn out different, see what's wrong. Try to remove fields one by one, until they become same. This will give you the culprit field(s). Work with them.</p> <p>Overall, I suggest you need a better understanding of how things work first. This case is waaaay too complex for a beginner.</p> <p>Good luck!</p>
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    1. COWell sir, you are the man : ) I thank you very much for your input. will see if i can solve this mess. One thing I can't really get my mind around are these strings that the c++ will write to... I thought well if i send a string with 500 chars then the c++ dll won't write outside of boundary. Is there a limit as to how big the result can be?
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    2. COCould it be that having writable string fields in a struct is impossible? Or do you know how they should be defined (errormessages and result)?
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