In a yield return statement, expression is evaluated and returned as a value to the enumerator object; expression has to be implicitly convertible to the yield type of the iterator.

In a yield break statement, control is unconditionally returned to the caller of the iterator, which is either the IEnumerator..::.MoveNext method (or its generic [T:System.Collections.Generic.IEnumerable'1] counterpart) or the Dispose method of the enumerator object.

The yield statement can only appear inside an iterator block, which can be implemented as the body of a method, operator, or accessor. The body of such methods, operators, or accessors is controlled by the following restrictions:

• Unsafe blocks are not allowed.

• Parameters to the method, operator, or accessor cannot be ref or out.

• A yield return statement cannot be located anywhere inside a try-catch block. It can be located in a try block if the try block is followed by a finally block.

• A yield break statement may be located in a try block or a catch block but not a finally block.

A yield statement cannot appear in an anonymous method. For more information, see Anonymous Methods (C# Programming Guide).

When used with expression, a yield return statement cannot appear in a catch block or in a try block that has one or more catch clauses. For more information, see Exception Handling Statements (C# Reference).

In the following example, the yield statement is used inside an iterator block, which is the method Power(int number, int power). When the Power method is invoked, it returns an enumerable object that contains the powers of a number. Notice that the return type of the Power method is System.Collections..::.IEnumerable, an iterator interface type.

public class List
{
    //using System.Collections;
    public static IEnumerable Power(int number, int exponent)
    {
        int counter = 0;
        int result = 1;
        while (counter++ < exponent)
        {
            result = result * number;
            yield return result;
        }
    }

    static void Main()
    {
        // Display powers of 2 up to the exponent 8:
        foreach (int i in Power(2, 8))
        {
            Console.Write("{0} ", i);
        }
    }
}
/*
Output:
2 4 8 16 32 64 128 256 
*/

For more information, see the following sections in the C# Language Specification:

• 19.3 Iterators

• 22 Iterators

Concepts

Reference

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