CommunityToolkit Guard

Overview

CommunityToolkit.Diagnostics provides the Guard class, a set of static helper methods for writing concise and performant guard clauses. Guard clauses validate method arguments and invariants at the boundaries of public APIs, constructors, and factory methods. The Guard class throws standard .NET exceptions (ArgumentNullException, ArgumentOutOfRangeException, ArgumentException) with descriptive messages generated at compile time via CallerArgumentExpression. It is part of the .NET Community Toolkit and works with .NET Standard 2.0+, .NET 6+, and all modern .NET platforms.

Basic Guard Clauses

Use Guard.IsNotNull, Guard.IsNotNullOrEmpty, and Guard.IsNotNullOrWhiteSpace for null and string validation.

using CommunityToolkit.Diagnostics;

namespace MyApp.Domain;

public class Customer
{
    public int Id { get; }
    public string Name { get; }
    public string Email { get; }
    public int Age { get; }

    public Customer(int id, string name, string email, int age)
    {
        Guard.IsGreaterThan(id, 0);
        Guard.IsNotNullOrWhiteSpace(name);
        Guard.IsNotNullOrWhiteSpace(email);
        Guard.IsInRange(age, 18, 120);

        Id = id;
        Name = name;
        Email = email;
        Age = age;
    }
}

Numeric and Collection Guards

Guard methods cover numeric ranges, collection sizes, and enum values.

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

namespace MyApp.Services;

public class OrderService
{
    public void PlaceOrder(
        IReadOnlyList<OrderItem> items,
        decimal discount,
        int quantity)
    {
        Guard.IsNotNull(items);
        Guard.HasSizeGreaterThan(items, 0);
        Guard.HasSizeLessThanOrEqualTo(items, 100);

        Guard.IsGreaterThanOrEqualTo(discount, 0m);
        Guard.IsLessThanOrEqualTo(discount, 1m);

        Guard.IsInRange(quantity, 1, 10_000);

        // All arguments are validated, proceed with order logic
        var total = CalculateTotal(items, discount, quantity);
    }

    public void SetStatus(Order order, OrderStatus newStatus)
    {
        Guard.IsNotNull(order);
        Guard.IsDefined(newStatus);

        order.Status = newStatus;
    }

    private decimal CalculateTotal(
        IReadOnlyList<OrderItem> items, decimal discount, int quantity)
    {
        var subtotal = items.Sum(i => i.Price * i.Quantity);
        return subtotal * (1 - discount);
    }
}

public enum OrderStatus { Pending, Processing, Shipped, Delivered, Cancelled }
public record OrderItem(string Name, decimal Price, int Quantity);
public class Order { public OrderStatus Status { get; set; } }

Guard Clauses in Value Objects and Domain Types

Combine guards with private constructors to enforce invariants on domain value types.

using CommunityToolkit.Diagnostics;
using System.Text.RegularExpressions;

namespace MyApp.Domain.ValueObjects;

public sealed partial class EmailAddress
{
    private static readonly Regex EmailPattern = EmailRegex();

    public string Value { get; }

    private EmailAddress(string value) => Value = value;

    public static EmailAddress Create(string value)
    {
        Guard.IsNotNullOrWhiteSpace(value);
        Guard.HasSizeLessThanOrEqualTo(value, 254);

        if (!EmailPattern.IsMatch(value))
        {
            ThrowHelper.ThrowArgumentException(nameof(value),
                $"'{value}' is not a valid email address.");
        }

        return new EmailAddress(value.ToLowerInvariant());
    }

    public override string ToString() => Value;

    [GeneratedRegex(@"^[^@\s]+@[^@\s]+\.[^@\s]+$", RegexOptions.Compiled)]
    private static partial Regex EmailRegex();
}

public readonly struct Money
{
    public decimal Amount { get; }
    public string Currency { get; }

    public Money(decimal amount, string currency)
    {
        Guard.IsGreaterThanOrEqualTo(amount, 0m);
        Guard.IsNotNullOrWhiteSpace(currency);
        Guard.HasSizeEqualTo(currency, 3);

        Amount = amount;
        Currency = currency.ToUpperInvariant();
    }

    public static Money USD(decimal amount) => new(amount, "USD");
    public static Money EUR(decimal amount) => new(amount, "EUR");

    public override string ToString() => $"{Amount:F2} {Currency}";
}

ThrowHelper for Custom Validation Logic

When Guard does not provide a built-in method, use ThrowHelper for consistent exception formatting.

using CommunityToolkit.Diagnostics;

namespace MyApp.Services;

public class UserService
{
    private readonly IUserRepository _repository;

    public UserService(IUserRepository repository)
    {
        Guard.IsNotNull(repository);
        _repository = repository;
    }

    public async Task<User> GetUserAsync(int id)
    {
        Guard.IsGreaterThan(id, 0);

        var user = await _repository.FindByIdAsync(id);

        if (user is null)
        {
            ThrowHelper.ThrowInvalidOperationException(
                $"User with ID {id} was not found.");
        }

        return user;
    }

    public async Task UpdatePasswordAsync(int userId, string currentPassword, string newPassword)
    {
        Guard.IsGreaterThan(userId, 0);
        Guard.IsNotNullOrWhiteSpace(currentPassword);
        Guard.IsNotNullOrWhiteSpace(newPassword);
        Guard.HasSizeGreaterThanOrEqualTo(newPassword, 8);
        Guard.HasSizeLessThanOrEqualTo(newPassword, 128);

        if (currentPassword == newPassword)
        {
            ThrowHelper.ThrowArgumentException(nameof(newPassword),
                "New password must differ from the current password.");
        }

        await _repository.UpdatePasswordAsync(userId, newPassword);
    }
}

Guard vs Other Validation Approaches

Best Practices

1. Place guard clauses at the top of public and protected methods, before any logic so that invalid arguments are rejected immediately with descriptive exceptions; never scatter guards throughout the method body where they can be skipped or overlooked during code review.

2. Use Guard.IsNotNullOrWhiteSpace instead of Guard.IsNotNull for string parameters because a non-null empty or whitespace-only string almost always represents an invalid input; IsNotNull alone lets "" and " " pass through, causing downstream errors in database queries or API calls.

3. Prefer Guard.IsInRange(value, min, max) over separate IsGreaterThan and IsLessThan calls when both bounds are known, because IsInRange performs both checks atomically and generates a single clear exception message including the expected range, reducing debugging time.

4. Combine guards with private constructors on value objects (e.g., EmailAddress.Create(string)) so that the type system guarantees all instances are valid; this moves validation to the creation boundary and eliminates the need to re-validate the same data in every consuming method.

5. Use Guard.IsDefined(enumValue) on every public method that accepts an enum parameter because C# allows casting arbitrary integers to enum types; without IsDefined, a caller can pass (OrderStatus)999 and bypass switch expressions or pattern matches that assume valid values.

6. Use Guard.HasSizeGreaterThan(collection, 0) instead of checking .Count > 0 manually and throwing because the Guard method generates a standardized exception message that includes the collection name (via CallerArgumentExpression) and the expected size constraint.

7. Do not use Guard for business-rule validation that should return error messages to users (e.g., "Password must contain a special character"); Guard throws exceptions that terminate the call stack, which is appropriate for programming errors but not for user input validation that should be handled with result objects or FluentValidation.

8. Use ThrowHelper.ThrowArgumentException(nameof(param), message) for custom validation logic that Guard does not cover (regex matching, cross-parameter checks), keeping the exception type and message format consistent with the auto-generated Guard exceptions.

9. Apply Guard.IsNotNull on injected dependencies in constructor bodies rather than relying on nullable reference type warnings because NRT is a compile-time-only check that does not prevent null at runtime; third-party callers, reflection-based DI containers, and serializers can still pass null.

10. Avoid wrapping Guard calls in try-catch blocks in the same method because ArgumentNullException and ArgumentOutOfRangeException are not recoverable errors -- they indicate a bug in the calling code; catching them masks the root cause and produces confusing behavior in production.