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Yield Keyword In C#

In this article, we will discuss about yield keyword in C#.

Purpose

Yield keyword is used to return each element (one at a time) from the collection (Implements IEnumerable interface).

Consider an 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. But for doing that, we must create one more temporary variable. Once you run the above code, you will get following output

Picture showing loop through the collection without using the yield function

Click to Enlarge

Now change the above code and add ‘yield’ keyword. So, the above code will be changed to

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;
      }
    }
  }
}

Above program will give the same output but

We are not using any temporary list variable to store the values.
One value is returned at a time instead of the whole collection.

Important Point Regarding Yield Keyword

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

Posted By -	Karan Gupta

Posted On -	Tuesday, August 28, 2018

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