YIELD

Signal an iterator block

WNSupported in Synergy .NET on Windows

YIELD MRETURN expression

or

YIELD EXIT

Arguments

expression

An expression that is evaluated and returned as a value to the enumerator object.

Discussion

The YIELD statement indicates to the compiler that the current method is an iterator block. The two versions of the YIELD syntax perform different functions:

• YIELD MRETURN expression gives the next value from the iteration (i.e., the value returned in each iteration of a FOREACH statement). It causes an element in the source sequence to be returned immediately to the caller before the next element in the sequence is accessed.
• YIELD EXIT indicates the end of iteration. Control returns to the caller of the iterator block.

Although an iterator is written as a method, the compiler translates it into a nested class that keeps track of the position of the iterator as long as the FOREACH loop on the client code continues. When YIELD MRETURN is reached, the current location is stored. Execution begins at this location the next time the iterator is called.

An iterator method must have a return type of IEnumerable or IEnumerable<>. A class can have multiple iterators.

A YIELD MRETURN statement cannot appear in a CATCH block, a TRY block that contains a CATCH, or a FINALLY block.

Examples

The example below returns powers of 2 using the YIELD statement.

import System.Collections
namespace ns1
    public class PowersOf2
        public static method Power, @IEnumerable
            number, int
            exponent, int
        proc
            data counter, int, 0
            data result, int, 1
            while (counter < exponent)
            begin
              incr counter
              result = result * number
              yield mreturn result
            end
        endmethod
    endclass
endnamespace
main
proc
    data i, int
    foreach i in ns1.PowersOf2.Power(2,8)
      begin
        Console.Write(i)
        Console.Write(" ")
      end
Console.WriteLine()