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by asveikau 4845 days ago
That wasn't the issue in my case. It was foreach (var b in f()). f did not do any allocations, just a loop with a bunch of yield return statements.

This is why I suspected that it was simply worse machine code after JIT. But I wasn't sure of all the details of the code that was inserted on my behalf.
1 comments
kevingadd 4845 days ago
One of the biggest differences is that if you do:

  foreach (var byte in byteArray)
The compiler transforms that into a normal for-loop that accesses the array directly. So that's already an advantage for the array over your enumerator.

The other difference that will have a major performance impact is the way the IEnumerator pattern works, even with generics. For:

  foreach (var b in f())
The generated code looks roughly like this:

  using (var enumerator = f())
  while (enumerator.MoveNext()) {
    var byte = enumerator.get_Current();
  }
As a result, you've gone from 0 method calls per iteration (direct array access) to 2 virtual method calls per iteration (MoveNext and get_Current). An incredibly smart JIT might be able to figure out that the virtual methods are always the same and turn them into static invocations, or even inline them, but I don't think the CLR can do this for you reliably.
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