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ParallelLoopState.Break Method

.NET Framework 4.6 and 4.5

Communicates that the Parallel loop should cease execution of iterations beyond the current iteration at the system's earliest convenience.

Namespace:  System.Threading.Tasks
Assemblies:   System.Threading.Tasks.Parallel (in System.Threading.Tasks.Parallel.dll)
  mscorlib (in mscorlib.dll)

public void Break()

ExceptionCondition
InvalidOperationException

The Stop method was previously called. Break and Stop may not be used in combination by iterations of the same loop.

Break indicates that no iterations after the current iteration should be run. It effectively cancels any additional iterations of the loop. However, it does not stop any iterations that have already begun execution. For example, if Break is called from the 100th iteration of a parallel loop iterating from 0 to 1,000, all iterations less than 100 should still be run, but the iterations from 101 through to 1000 that have not yet started are not executed.

For long-running iterations that may already be executing, Break sets the LowestBreakIteration property to the current iteration's index if the current index is less than the current value of LowestBreakIteration. To stop iterations whose index is greater than the lowest break iteration from competing execution, you should do the following:

  1. Check whether the ShouldExitCurrentIteration property is true.

  2. Exit from the iteration if its index is greater than the LowestBreakIteration property value.

The example provides an illustration.

Break is typically employed in search-based algorithms where an ordering is present in the data source.

The following example executes up to 100 iterations of a loop in parallel. Each iteration pauses for a random interval from 1 to 1,000 milliseconds. A randomly generated value determines on which iteration of the loop the Break method is called. As the output from the example shows, no iterations whose index is greater than the LowestBreakIteration property value start after the call to the Break method.

using System;
using System.Threading;
using System.Threading.Tasks;

public class Example
{
   public static void Main()
   {
      var rnd = new Random();
      int breakIndex = rnd.Next(1, 11);
      Nullable<long> lowest = new Nullable<long>();

      Console.WriteLine("Will call Break at iteration {0}\n",
                        breakIndex);

      var result = Parallel.For(1, 101, (i, state) => {
                                            Console.WriteLine("Beginning iteration {0}", i);
                                            int delay;
                                            Monitor.Enter(rnd);
                                               delay = rnd.Next(1, 1001);
                                            Monitor.Exit(rnd);
                                            Thread.Sleep(delay);

                                            if (state.ShouldExitCurrentIteration) {
                                               if (state.LowestBreakIteration < i)
                                                  return;
                                            }

                                            if (i == breakIndex) {
                                               Console.WriteLine("Break in iteration {0}", i);
                                               state.Break();
                                               if (state.LowestBreakIteration.HasValue)
                                                  if (lowest < state.LowestBreakIteration)
                                                     lowest = state.LowestBreakIteration;
                                                  else
                                                     lowest = state.LowestBreakIteration;
                                            }

                                            Console.WriteLine("Completed iteration {0}", i);
                                       });
         if (lowest.HasValue)
            Console.WriteLine("\nLowest Break Iteration: {0}", lowest);
         else
            Console.WriteLine("\nNo lowest break iteration.");
   }
}
// The example displays output like the following: 
//       Will call Break at iteration 8 
// 
//       Beginning iteration 1 
//       Beginning iteration 13 
//       Beginning iteration 97 
//       Beginning iteration 25 
//       Beginning iteration 49 
//       Beginning iteration 37 
//       Beginning iteration 85 
//       Beginning iteration 73 
//       Beginning iteration 61 
//       Completed iteration 85 
//       Beginning iteration 86 
//       Completed iteration 61 
//       Beginning iteration 62 
//       Completed iteration 86 
//       Beginning iteration 87 
//       Completed iteration 37 
//       Beginning iteration 38 
//       Completed iteration 38 
//       Beginning iteration 39 
//       Completed iteration 25 
//       Beginning iteration 26 
//       Completed iteration 26 
//       Beginning iteration 27 
//       Completed iteration 73 
//       Beginning iteration 74 
//       Completed iteration 62 
//       Beginning iteration 63 
//       Completed iteration 39 
//       Beginning iteration 40 
//       Completed iteration 40 
//       Beginning iteration 41 
//       Completed iteration 13 
//       Beginning iteration 14 
//       Completed iteration 1 
//       Beginning iteration 2 
//       Completed iteration 97 
//       Beginning iteration 98 
//       Completed iteration 49 
//       Beginning iteration 50 
//       Completed iteration 87 
//       Completed iteration 27 
//       Beginning iteration 28 
//       Completed iteration 50 
//       Beginning iteration 51 
//       Beginning iteration 88 
//       Completed iteration 14 
//       Beginning iteration 15 
//       Completed iteration 15 
//       Completed iteration 2 
//       Beginning iteration 3 
//       Beginning iteration 16 
//       Completed iteration 63 
//       Beginning iteration 64 
//       Completed iteration 74 
//       Beginning iteration 75 
//       Completed iteration 41 
//       Beginning iteration 42 
//       Completed iteration 28 
//       Beginning iteration 29 
//       Completed iteration 29 
//       Beginning iteration 30 
//       Completed iteration 98 
//       Beginning iteration 99 
//       Completed iteration 64 
//       Beginning iteration 65 
//       Completed iteration 42 
//       Beginning iteration 43 
//       Completed iteration 88 
//       Beginning iteration 89 
//       Completed iteration 51 
//       Beginning iteration 52 
//       Completed iteration 16 
//       Beginning iteration 17 
//       Completed iteration 43 
//       Beginning iteration 44 
//       Completed iteration 44 
//       Beginning iteration 45 
//       Completed iteration 99 
//       Beginning iteration 4 
//       Completed iteration 3 
//       Beginning iteration 8 
//       Completed iteration 4 
//       Beginning iteration 5 
//       Completed iteration 52 
//       Beginning iteration 53 
//       Completed iteration 75 
//       Beginning iteration 76 
//       Completed iteration 76 
//       Beginning iteration 77 
//       Completed iteration 65 
//       Beginning iteration 66 
//       Completed iteration 5 
//       Beginning iteration 6 
//       Completed iteration 89 
//       Beginning iteration 90 
//       Completed iteration 30 
//       Beginning iteration 31 
//       Break in iteration 8 
//       Completed iteration 8 
//       Completed iteration 6 
//       Beginning iteration 7 
//       Completed iteration 7 
// 
//       Lowest Break Iteration: 8

Because iterations of the loop are still likely to be executing when the Break method is called, each iteration calls the ShouldExitCurrentIteration property to check whether another iteration has called the Break method. If the property value is true, the iteration checks the value of the LowestBreakIteration property and, if it is greater than the current iteration's index value, returns immediately.

.NET Framework

Supported in: 4.6, 4.5, 4

.NET Framework Client Profile

Supported in: 4

Portable Class Library

Supported in: Portable Class Library

Supported in: Windows Phone 8.1
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