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copied!<p>If you want to stick to binary files then <strong>don't use <code>StreamWriter</code></strong>. Just use a <code>FileStream</code> and <code>Write</code>/<code>WriteByte</code>. StreamWriters (and TextWriters in generally) are expressly designed for <em>text</em>. Whether you want an encoding or not, one will be applied - because when you're calling <code>StreamWriter.Write</code>, that's writing a <code>char</code>, not a <code>byte</code>.</p> <p>Don't create arrays of <code>wchar_t</code> values either - again, those are for <em>characters</em>, i.e. text.</p> <p><code>BinaryWriter.Write</code> should have worked for you unless it was promoting the values to <code>char</code> in which case you'd have exactly the same problem.</p> <p>By the way, without specifying <em>any</em> encoding, I'd expect you to get non-0x3F values, but instead the bytes representing the UTF-8 encoded values for those characters.</p> <p>When you specified <code>Encoding.Default</code>, you'd have seen 0x3F for any Unicode values not in that encoding.</p> <p>Anyway, the basic lesson is to stick to <code>Stream</code> when you want to deal with binary data rather than text.</p> <p>EDIT: Okay, it would be something like:</p> <pre><code>public static void ConvertHex(TextReader input, Stream output) { while (true) { int firstNybble = input.Read(); if (firstNybble == -1) { return; } int secondNybble = input.Read(); if (secondNybble == -1) { throw new IOException("Reader finished half way through a byte"); } int value = (ParseNybble(firstNybble) << 4) + ParseNybble(secondNybble); output.WriteByte((byte) value); } } // value would actually be a char, but as we've got an int in the above code, // it just makes things a bit easier private static int ParseNybble(int value) { if (value >= '0' && value <= '9') return value - '0'; if (value >= 'A' && value <= 'F') return value - 'A' + 10; if (value >= 'a' && value <= 'f') return value - 'a' + 10; throw new ArgumentException("Invalid nybble: " + (char) value); } </code></pre> <p>This is very inefficient in terms of buffering etc, but should get you started.</p>

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