Compute a hash from a stream of unknown length in C#

MD5, like other hash functions, does not require two passes.

To start:

HashAlgorithm hasher = ..;

As each block of data arrives:

byte[] buffer = ..;
int bytesReceived = ..;
hasher.TransformBlock(buffer, 0, bytesReceived, null, 0);

To finish and retrieve the hash:

hasher.TransformFinalBlock(new byte[0], 0, 0);
byte[] hash = hasher.Hash;

This pattern works for any type derived from HashAlgorithm, including MD5CryptoServiceProvider and SHA1Managed.

HashAlgorithm also defines a method ComputeHash which takes a Stream object; however, this method will block the thread until the stream is consumed. Using the TransformBlock approach allows an “asynchronous hash” that is computed as data arrives without using up a thread.

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