.NET Community Toolkit

Overview

The .NET Community Toolkit is a collection of libraries maintained by the .NET Foundation that provides MVVM infrastructure, diagnostic helpers, and high-performance utilities. The toolkit is split into three main packages: CommunityToolkit.Mvvm for source-generated MVVM patterns, CommunityToolkit.Diagnostics for guard clauses and type validation, and CommunityToolkit.HighPerformance for memory-efficient data structures and pooling.

The MVVM Toolkit uses C# source generators to eliminate boilerplate for observable properties, commands, and messaging. It is UI-framework agnostic and works with WPF, MAUI, WinUI, Avalonia, and any framework that consumes INotifyPropertyChanged.

Install via NuGet:

dotnet add package CommunityToolkit.Mvvm
dotnet add package CommunityToolkit.Diagnostics
dotnet add package CommunityToolkit.HighPerformance

MVVM Source Generators

The MVVM Toolkit uses attributes to generate boilerplate code at compile time. Annotate fields with [ObservableProperty] to auto-generate properties with change notification, and methods with [RelayCommand] to generate ICommand implementations.

using CommunityToolkit.Mvvm.ComponentModel;
using CommunityToolkit.Mvvm.Input;
using System.Threading;
using System.Threading.Tasks;

public partial class CustomerViewModel : ObservableObject
{
    [ObservableProperty]
    [NotifyPropertyChangedFor(nameof(FullName))]
    private string _firstName = string.Empty;

    [ObservableProperty]
    [NotifyPropertyChangedFor(nameof(FullName))]
    private string _lastName = string.Empty;

    [ObservableProperty]
    [NotifyCanExecuteChangedFor(nameof(SaveCommand))]
    private bool _hasChanges;

    public string FullName => $"{FirstName} {LastName}";

    [RelayCommand(CanExecute = nameof(HasChanges))]
    private async Task SaveAsync(CancellationToken token)
    {
        await _customerService.UpdateAsync(
            new Customer { FirstName = FirstName, LastName = LastName }, token);
        HasChanges = false;
    }

    [RelayCommand]
    private void Reset()
    {
        FirstName = string.Empty;
        LastName = string.Empty;
        HasChanges = false;
    }
}

Messenger Pattern

The WeakReferenceMessenger or StrongReferenceMessenger decouples communication between view models without direct references. Define message types as records for immutability.

using CommunityToolkit.Mvvm.Messaging;
using CommunityToolkit.Mvvm.Messaging.Messages;

// Define messages as records
public record OrderPlacedMessage(int OrderId, decimal Total);

public record StatusRequestMessage : RequestMessage<string>;

// Sender view model
public partial class CheckoutViewModel : ObservableObject
{
    [RelayCommand]
    private void PlaceOrder()
    {
        var orderId = ProcessOrder();
        WeakReferenceMessenger.Default.Send(new OrderPlacedMessage(orderId, 99.99m));
    }
}

// Receiver view model
public partial class DashboardViewModel : ObservableRecipient, IRecipient<OrderPlacedMessage>
{
    public DashboardViewModel()
    {
        IsActive = true; // activates messenger registration
    }

    public void Receive(OrderPlacedMessage message)
    {
        LatestOrderId = message.OrderId;
        TotalRevenue += message.Total;
    }

    [ObservableProperty]
    private int _latestOrderId;

    [ObservableProperty]
    private decimal _totalRevenue;
}

Diagnostics Guard Clauses

CommunityToolkit.Diagnostics provides a fluent Guard class for argument validation that produces clear exception messages and integrates with nullable analysis.

using System;
using System.Collections.Generic;
using CommunityToolkit.Diagnostics;

public class InventoryService
{
    public void AddStock(string sku, int quantity, IReadOnlyList<string> warehouses)
    {
        Guard.IsNotNullOrWhiteSpace(sku);
        Guard.IsGreaterThan(quantity, 0);
        Guard.IsNotNull(warehouses);
        Guard.HasSizeGreaterThan(warehouses, 0);

        // Business logic proceeds with validated inputs
    }

    public decimal CalculateDiscount(decimal price, double percentage)
    {
        Guard.IsGreaterThanOrEqualTo(price, 0m);
        Guard.IsInRange(percentage, 0.0, 1.0);

        return price * (decimal)percentage;
    }
}

HighPerformance Utilities

The HighPerformance package provides memory-efficient types like StringPool, MemoryOwner<T>, and Ref<T> for scenarios where allocation pressure matters.

using System;
using System.Buffers;
using CommunityToolkit.HighPerformance;
using CommunityToolkit.HighPerformance.Buffers;

public class DataProcessor
{
    private static readonly StringPool _pool = new StringPool();

    public string InternString(ReadOnlySpan<char> input)
    {
        // Returns a cached string instance, reducing allocations
        return _pool.GetOrAdd(input);
    }

    public void ProcessLargeBuffer(int size)
    {
        // Rent a buffer from the pool and return it on dispose
        using var owner = MemoryOwner<byte>.Allocate(size);
        Span<byte> span = owner.Span;

        // Fill and process the span without allocating a new array
        span.Fill(0xFF);
        ProcessSpan(span);
    }

    public void TwoDimensionalAccess(int[] flat, int rows, int cols)
    {
        // View a flat array as a 2D span without copying
        var span2D = new Span2D<int>(flat, rows, cols);
        for (int r = 0; r < span2D.Height; r++)
        {
            for (int c = 0; c < span2D.Width; c++)
            {
                span2D[r, c] = r * cols + c;
            }
        }
    }

    private void ProcessSpan(Span<byte> data) { }
}

Observable Validator

For view models that need validation, inherit from ObservableValidator to integrate System.ComponentModel.DataAnnotations with MVVM property change notification.

using System.ComponentModel.DataAnnotations;
using CommunityToolkit.Mvvm.ComponentModel;
using CommunityToolkit.Mvvm.Input;

public partial class RegistrationViewModel : ObservableValidator
{
    [ObservableProperty]
    [NotifyDataErrorInfo]
    [Required(ErrorMessage = "Email is required")]
    [EmailAddress(ErrorMessage = "Invalid email format")]
    private string _email = string.Empty;

    [ObservableProperty]
    [NotifyDataErrorInfo]
    [Required]
    [MinLength(8, ErrorMessage = "Password must be at least 8 characters")]
    private string _password = string.Empty;

    [RelayCommand]
    private void Submit()
    {
        ValidateAllProperties();

        if (!HasErrors)
        {
            // Proceed with registration
        }
    }
}

Package Comparison

Best Practices

1. Mark view model classes as partial -- the source generators require partial classes to emit generated code alongside your declarations.
2. Use [NotifyPropertyChangedFor] on fields to trigger dependent property change notifications (e.g., a FullName computed from FirstName and LastName).
3. Use [NotifyCanExecuteChangedFor] on fields that affect command availability so the UI automatically re-evaluates CanExecute when those fields change.
4. Prefer WeakReferenceMessenger over StrongReferenceMessenger to avoid memory leaks from subscribers that are never explicitly unregistered.
5. Use Guard methods at public API boundaries and inside type constructors rather than scattering null checks throughout call sites.
6. Activate ObservableRecipient via IsActive = true to begin receiving messages -- forgetting this causes silent message loss.
7. Use ObservableValidator instead of ObservableObject when the view model needs input validation with DataAnnotations support.
8. Pool strings with StringPool in parsing or deserialization code where the same string values recur frequently to reduce GC pressure.
9. Use MemoryOwner<T> instead of ArrayPool<T> directly because it implements IDisposable and automatically returns the buffer on dispose, preventing pool exhaustion.
10. Keep all generated code inspection in IDE -- use "Go to Definition" on generated properties and commands to verify the emitted code matches your intent.