"Yield keyword"

Question

I want to know more about the yield keyword?

Vulpes · Answer

Notice that 'yield' is not a true keyword; it's what's called a 'contextual' keyword. This means that it's only a keyword in the context of an iterator, when it's always used in conjunction with 'return' and 'break'. In all other contexts, it can be used as a normal identifier.

Aarti · Answer

Hi Sonakshi, The yield keyword signals to the compiler that the method in which it appears is an iterator block. The compiler generates a class to implement the behavior that is expressed in the iterator block. In the iterator block, the yield keyword is used together with the return keyword to provide a value to the enumerator object. This is the value that is returned, for example, in each loop of a foreach statement. The yield keyword is also used with break to signal the end of iteration. For more information about iterators. 1.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. 2.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 System.Collections.Generic . IEnumerable (Of T ) counterpart) or the Dispose method of the enumerator object. 3.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. 4. A yield statement cannot appear in an anonymous method. 5. 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. Example: 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 PowersOf2 { public static System.Collections.IEnumerable Power( int number, int exponent) { int counter = 0; int result = 1; while (counter++ { 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 */

Mukesh Kumar · Answer

Hi Sonakshi, Read these links : http://blogs.msdn.com/oldnewthing/archive/2008/08/12/8849519.aspx (part 1) http://blogs.msdn.com/oldnewthing/archive/2008/08/13/8854601.aspx (part 2) http://blogs.msdn.com/oldnewthing/archive/2008/08/14/8862242.aspx (part 3) http://blogs.msdn.com/oldnewthing/archive/2008/08/15/8868267.aspx (part 4)

Satyapriya Nayak · Answer

Hi Sonakshi, The yield statement can only appear inside an iterator block, which might be used as a 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. using System; using System.Collections; public class List { public static IEnumerable Power(int number, int exponent) { int counter = 0; int result = 1; while (counter++ { 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); } } } Refer http://msdn.microsoft.com/en-us/library/9k7k7cf0%28v=vs.80%29.aspx Thanks