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copied!<p>The order of bytes in a GUID are not the same as the order in their <code>ToString()</code> representation on little-endian systems.</p> <p>You should use guid.ToByteArray() rather than using ToString().</p> <p>And, you should use <code>new Guid(byte[] b)</code> to construct it, rather than <code>$str</code>.</p> <p>To express this in pure C#:</p> <pre><code>public string GuidToBase64(Guid guid) { return System.Convert.ToBase64String(guid.ToByteArray()); // Very similar to what you have. } public Guid Base64Toguid(string base64) { var bytes = System.Convert.FromBase64String(base64); return new Guid(bytes); // Not that I'm not building up a string to represent the GUID. } </code></pre> <hr /> <p>Take a look at the <a href="http://en.wikipedia.org/wiki/Globally_unique_identifier#Basic_structure" rel="noreferrer">"Basic Structure" section of the GUID article</a> on Wikipedia for more details.</p> <p>You will see that most of the data is stored in "Native" endianness... which is where the confusion is coming from.</p> <p>To quote:</p> <blockquote> <p>Data4 stores the bytes in the same order as displayed in the GUID text encoding (see below), but the other three fields are reversed on little-endian systems (for example Intel CPUs).</p> </blockquote> <p><hr/></p> <h1>Edit:</h1> <p>Powershell version:</p> <pre><code>function base64toguid { param($str); $b = [System.Convert]::FromBase64String($str); $g = new-object -TypeName System.Guid -ArgumentList (,$b); return $g; } </code></pre> <p>As an additional caveat, you can optionally trim the "==" off of the end of your string, since it is just padding (which may help if you are trying to save space).</p>

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