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Yield Keyword In C#

This article describes the yield keyword in C#.

Purpose

The yield keyword is used to return elements one at a time from a collection, typically in methods that implement the IEnumerable or IEnumerator interface.

Consider the following example: -

using System;
using System.Collections.Generic;

namespace Yield_Sample {
  class Program {
    static void Main(string[] args) {
      Program prog = new Program();
      foreach(int i in prog.ReturnList())
      Console.WriteLine(i);

      Console.ReadLine();
    }

    public IEnumerable < int > ReturnList() {
      int count = 10;
      int[] coll = new int[count];

      for (int i = 1; i <= count; i++) {
        coll[i - 1] = i;
      }

      return coll;
    }
  }
}

In the above example, we are returning a collection. However, to do so, we need to create one more temporary variable. Once you run the code above, you will get the following output.

Picture showing loop through the collection without using the yield function

Modify the above code to use the yield keyword. The updated version of the code will be: -

using System;
using System.Collections.Generic;

namespace Yield_Sample {
  class Program {
    static void Main(string[] args) {
      Program prog = new Program();
      foreach(int i in prog.ReturnList())
      Console.WriteLine(i);

      Console.ReadLine();
    }

    public IEnumerable < int > ReturnList() {
      int count = 10;

      for (int i = 1; i <= count; i++) {
        yield
        return i;
      }
    }
  }
}

The above program produces the same output, but with two key differences:

It avoids using a temporary list to store intermediate values.
It simultaneously returns one value at a time instead of the entire collection.

Important Point Regarding The Yield Keyword

You cannot return a yield statement in a try-catch block.
You cannot return out or ref parameters using the yield keyword.
You cannot use the yield statement inside an anonymous method.
You cannot use the yield statement inside the unsafe method.

Posted By -	Karan Gupta

Posted On -	Tuesday, August 28, 2018

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