Why is this an issue?

You cannot assume that any given stream reading call will fill the byte[] passed in to the method with the number of bytes requested. Instead, you must check the value returned by the read method to see how many bytes were read. Fail to do so, and you introduce a bug that is both harmful and difficult to reproduce.

This rule raises an issue when a Stream.Read or a Stream.ReadAsync method is called, but the return value is not checked.

Noncompliant code example

public void DoSomething(string fileName)
{
  using (var stream = File.Open(fileName, FileMode.Open))
  {
    var result = new byte[stream.Length];
    stream.Read(result, 0, (int)stream.Length); // Noncompliant
    // ... do something with result
  }
}

Compliant solution

public void DoSomething(string fileName)
{
  using (var stream = File.Open(fileName, FileMode.Open))
  {
    var buffer = new byte[1024];
    using (var ms = new MemoryStream())
    {
        int read;
        while ((read = stream.Read(buffer, 0, buffer.Length)) > 0)
        {
            ms.Write(buffer, 0, read);
        }
        // ... do something with ms
    }
  }
}