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 = ..;
hasher.Initialize();

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.

The System.Security.Cryptography.MD5 class contains a ComputeHash method that takes either a byte[] or Stream. Check out the documentation.

Further to @peter-mourfield 's answer, here is the code that uses ComputeHash():

private static string CalculateMd5(string filePathName) {
   using (var stream = File.OpenRead(filePathName))
   using (var md5 = MD5.Create()) {
      var hash = md5.ComputeHash(stream);
      var base64String = Convert.ToBase64String(hash);
      return base64String;
   }
}

Since both the stream as well as MD5 implement IDisposible, you need to use using(...){...}

The method in the code example returns the same string that is used for the MD5 checksum in Azure Blob Storage.