Parallel.ForEach in VB.Net

Parallel.ForEach is a method in the System.Threading.Tasks.Parallel class that allows you to iterate over a collection in parallel. This can improve the performance of your application, especially if the collection is large or if the operation that you are performing on each item in the collection is computationally expensive.


To use Parallel.ForEach, you must first create a delegate that represents the operation that you want to perform on each item in the collection. The delegate must take one parameter, which is the item from the collection.

Once you have created the delegate, you can call the Parallel.ForEach method, passing in the collection and the delegate. The Parallel.ForEach method will then iterate over the collection in parallel and execute the delegate on each item.

The Parallel.ForEach method is part of the System.Threading.Tasks.Parallel class. You can use it to process items in a collection concurrently. The basic structure looks like this:

Parallel.ForEach(collection, Sub(item) ' Process the item End Sub) Here's an example that uses Parallel.ForEach to process a list of numbers concurrently: Dim numbers As List(Of Integer) = Enumerable.Range(1, 100).ToList() Parallel.ForEach(numbers, Sub(number) Console.WriteLine($"Processed number: {number}") End Sub)

Local State

You can capture variables from the outer scope within the delegate. Be cautious with shared state and ensure proper synchronization if necessary.

Dim sum As Integer = 0 Parallel.ForEach(numbers, Sub(number) Interlocked.Add(sum, number) End Sub) Console.WriteLine($"Sum of numbers: {sum}")

In this example, we're using Interlocked.Add to safely increment the shared sum variable.

Breaking Early

You can use the ParallelLoopState to request an early exit from the loop when a certain condition is met. For example, you can stop processing when a specific item is found:

Dim targetNumber As Integer = 42 Dim found As Boolean = False Parallel.ForEach(numbers, Function(number, loopState) If number = targetNumber Then loopState.Break() found = True End If End Function) If found Then Console.WriteLine($"Found the target number: {targetNumber}") Else Console.WriteLine($"Target number not found: {targetNumber}") End If

Exception Handling

You should handle exceptions properly within the delegate, as exceptions in parallel loops may not propagate as expected. For example:

Try Parallel.ForEach(numbers, Sub(number) If number = 42 Then Throw New Exception("Something went wrong.") End If End Sub) Catch ex As AggregateException For Each innerEx As Exception In ex.InnerExceptions Console.WriteLine($"Exception: {innerEx.Message}") Next End Try

Why does Parallel.ForEach code freeze the program?

There are a few possible reasons why your Parallel.ForEach code might be freezing the program up:

  1. The collection is too large. If the collection that you are iterating over is too large, the Parallel.ForEach method may create too many threads and overwhelm the operating system. This can cause the program to freeze.
  2. The operation that you are performing on each item in the collection is too computationally expensive. If the operation that you are performing on each item in the collection is too computationally expensive, it can cause the program to freeze.
  3. There is a deadlock. A deadlock is a situation where two or more threads are waiting for each other to finish, and neither thread can finish because it is waiting for the other thread to finish. This can cause the program to freeze.
  4. There is a bug in your code. It is also possible that there is a bug in your code that is causing the program to freeze.


Parallel.ForEach can significantly improve the performance of operations that can be executed concurrently. However, be cautious when dealing with shared state, and consider thread safety to prevent race conditions and other synchronization issues.