The ParallelHelper contains high performance APIs to work with parallel code. It contains performance oriented methods that can be used to quickly setup and execute parallel operations over a given data set or iteration range or area.

Platform APIs: ParallelHelper, IAction, IAction2D, IRefAction<T>, IInAction<T><T>

How it works

ParallelHelper type is built around three main concepts:

  • It performs automatic batching over the target iteration range. This means that it automatically schedules the right number of working units based on the number of available CPU cores. This is done to reduce the overhead of invoking the parallel callback once for every single parallel iteration.
  • It heavily leverages the way generic types are implemented in C#, and uses struct types implementing specific interfaces instead of delegates like Action<T>. This is done so that the JIT compiler will be able to "see" each individual callback type being used, which allows it to inline the callback entirely, when possible. This can greatly reduce the overhead of each parallel iteration, especially when using very small callbacks, which would have a trivial cost with respect to the delegate invocation alone. Additionally, using a struct type as callback requires developers to manually handle variables that are being captured in the closure, which prevents accidental captures of the this pointer from instance methods and other values that could considerably slowdown each callback invocation. This is the same approach that is used in other performance-oriented libraries such as ImageSharp.
  • It exposes 4 types of APIs that represent 4 different types of iterations: 1D and 2D loops, items iteration with side effect and items iteration without side effect. Each type of action has a corresponding interface type that needs to be applied to the struct callbacks being passed to the ParallelHelper APIs: these are IAction, IAction2D, IRefAction<T> and IInAction<T><T>. This helps developers to write code that is clearer regarding its intent, and allows the APIs to perform further optimizations internally.


Let's say we're interested in processing all the items in some float[] array, and to multiply each of them by 2. In this case we don't need to capture any variables: we can just use the IRefAction<T> interface and ParallelHelper will load each item to feed to our callback automatically. All that's needed is to define our callback, that will receive a ref float argument and perform the necessary operation:

// Be sure to include this using at the top of the file:
using Microsoft.Toolkit.HighPerformance.Helpers;

// First declare the struct callback
public readonly struct ByTwoMultiplier : IRefAction<float>
    public void Invoke(ref float x) => x *= 2;

// Create an array and run the callback
float[] array = new float[10000];

ParallelHelper.ForEach<float, ByTwoMultiplier>(array);

With the ForEach API, we don't need to specify the iteration ranges: ParallelHelper will batch the collection and process each input item automatically. Furthermore, in this specific example we didn't even have to pass our struct as an argument: since it didn't contain any fields we needed to initialize, we could just specify its type as a type argument when invoking ParallelHelper.ForEach: that API will then create a new instance of that struct on its own, and use that to process the various items.

To introduce the concept of closures, suppose we want to multiply the array elements by a value that is specified at runtime. To do so, we need to "capture" that value in our callback struct type. We can do that like so:

public readonly struct ItemsMultiplier : IRefAction<float>
    private readonly float factor;
    public ItemsMultiplier(float factor)
        this.factor = factor;

    public void Invoke(ref float x) => x *= this.factor;

// ...

ParallelHelper.ForEach(array, new ItemsMultiplier(3.14f));

We can see that the struct now contains a field that represents the factor we want to use to multiply elements, instead of using a constant. And when invoking ForEach, we're explicitly creating an instance of our callback type, with the factor we're interested in. Furthermore, in this case the C# compiler is also able to automatically recognize the type arguments we're using, so we can omit them together from the method invocation.

This approach of creating fields for values we need to access from a callback lets us explicitly declare what values we want to capture, which helps makes the code more expressive. This is exactly the same thing that the C# compiler does behind the scenes when we declare a lambda function or local function that accesses some local variable as well.

Here is another example, this time using the For API to initialize all the items of an array in parallel. Note how this time we're capturing the target array directly, and we're using the IAction interface for our callback, which gives our method the current parallel iteration index as argument:

public readonly struct ArrayInitializer : IAction
    private int[] array;

    public ArrayInitializer(int[] array)
        this.array = array;

    public void Invoke(int i)
        this.array[i] = i;

// ...

ParallelHelper.For(0, array.Length, new ArrayInitializer(array));


Since the callback types are struct-s, they're passed by copy to each thread running parallel, not by reference. This means that value types being stored as fields in a callback types will be copied as well. A good practice to remember that detail and avoid errors is to mark the callback struct as readonly, so that the C# compiler will not let us modify the values of its fields. This only applies to instance fields of a value type: if a callback struct has a static field of any type, or a reference field, then that value will correctly be shared between parallel threads.


These are the 4 main APIs exposed by ParallelHelper, corresponding to the IAction, IAction2D, IRefAction<T> and IInAction<T> interfaces. The ParallelHelper type also exposes a number of overloads for these methods, that offer a number of ways to specify the iteration range(s), or the type of input callback. For and For2D work on IAction and IAction2D instances, and they are meant to be used when some parallel work needs to be done that doesn't necessary map to an underlying collection that can be directly accessed with the indices of each parallel iteration. The ForEach overloads instead wotk on IRefAction<T> and IInAction<T> instances, and they can be used when the parallel iterations directly map to items in a collection that can be indexed directly. In this case they also abstract away the indexing logic, so that each parallel invocation only has to worry on the input item to work on, and not on how to retrieve that item as well.


You can find more examples in the unit tests.