Question

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What is the yield keyword used for in C#?

Say I have code that looks like:

(steam is a filestream)

using(BinaryWriter bw = new BinaryWriter(stream))
{
  foreach(byte[] b in BreakBytes(objectOfBytes))
  {
    writer.Write(b);
  }
}

So for BreakBytes to work, it has to do something like:

public static IEnumerable<byte[]> BreakBytes(byte[] b)
{
  ..
  while(..) {

     yield return some_buffer;

  }
  ..
}

What exactly is yield doing? Does it keep track of where it was position wise?

I believe it is return to the calling foreach loop, but continues to the next iteration when called again?

Answer 1

In short, the code in the method is re-written as a state machine that does as you suspect: it keeps track of where it is in the loop, returns to the caller, and continues where it left off.