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POpointer arithmetic and the C# compiler
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copied!<p>For the purpose of learning I recently looked at an existing assembly (using Reflector) that uses Win32 WriteFile. The implementation is:</p> <pre><code>Write(IntPtr handleFile, void* bufferData, uint length){ void* buffer = bufferData while (length > 0) { uint wrtn; if (!WriteFile(handle, buffer, len, out wrtn, IntPtr.Zero)) { // Do some error handling } // This does not compile, because of the cast but also simply because void* does not have += operators (it is unknown size). buffer += (void*)wrtn; len -= wrtn; } </code></pre> <p>}</p> <p>It is actually the last 2 lines that are problematic... For one, the compiler complains that you cannot cast uint to void*. Additionally, it is just not possible to use += or even + on void* because it is not of known size.</p> <pre><code>Write(IntPtr handleFile, void* bufferData, uint length){ byte* buffer = (byte*)bufferData while (length > 0) { uint wrtn; if (!WriteFile(handle, (void*)buffer, len, out wrtn, IntPtr.Zero)) { // Do some error handling } // This works! I can add to a byte* buffer = buffer + wrtn; // I could also have used buffer += wrtn len -= wrtn; } } </code></pre> <p>The above code does work but still the last few lines will compile to:</p> <pre><code>buffer += (byte*)wrtn; </code></pre> <p>I do not understand why and very much would like to know why the compiler behaves in this way:</p> <ol> <li>Why does it generate the cast like this (and why is it not accepted to do this in user written code)?</li> <li>What is up with the += operators on void* in the first example? What original code code have generated buffer += (void*)wrtn where buffer is also void* ????</li> </ol>

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