Clone or download
Fetching latest commit…
Cannot retrieve the latest commit at this time.
Failed to load latest commit information.



A lightweight .NET library for expressive Guard Clauses.

Video introduction to Light.GuardClauses

Why do I need it?

When you write methods with parameters in C# (including constructors), you for sure are used to Guard Clauses that check the validity of parameter values. The most common one is probably the null check where you throw an ArgumentNullException if an object reference does not point to an actual object. Light.GuardClauses provides a set of extension methods simplifying this task for you:

public class Foo
    private readonly IBar _bar;
    public Foo(IBar bar)
        // Perform a simple not-null-check using the MustNotBeNull extension method
        _bar = bar;

Since v2.0.0, you can also directly use the return value of assertion methods to chain multiple assertions or, as in the following example, directly set the parameter to a field:

public class Foo
    private readonly IBar _bar;
    public Foo(IBar bar)
        // You can also use the return value of the assertion methods to e.g. directly set a field
        _bar = bar.MustNotBeNull();

There is a vast variety of scenarios where Light.GuardClauses can support you, e.g. with GUIDs and numeric values:

public void SetMovieRating(Guid movieId, int numberOfStars)
    var movie = _movieRepo.GetById(movieId);

In addition to methods that throw exceptions, Light.GuardClauses also provides a lot of assertions the simply tell you that a statement is true or false - this way you can easily perform complex condition checks in a simple statement:

private Expression[] ResolveDependenciesRecursivelyIfNecessary(IReadOnlyList<Dependency> dependencies, ResolveExpressionContext context)
    if (dependencies.IsNullOrEmpty())
        return null;

    var resolvedDependencyExpressions = new Expression[dependencies.Count];
    for (var i = 0; i < dependencies.Count; i++)
        var dependency = dependencies[i];
        var dependencyType = dependency.DependencyType;
        if (dependencyType.IsGenericTypeParameter())
            dependencyType = context.ResolveGenericTypeParameter(dependencyType);
        resolvedDependencyExpressions[i] = CreateResolveExpressionRecursively(new TypeKey(dependencyType, dependency.TargetRegistrationName), context.Container, dependency.ResolveAll);

    return resolvedDependencyExpressions;

Inspired by FluentAssertions, there are many more methods tailored for strings, URIs, DateTime, Type and TypeInfo, IComparable<T>, IEnumerable<T>, IEquatable<T>, and IDictionary<T>. See a list of all of them in the release notes or discover them on the fly via IntelliSense - all the methods are fully documented. Just be sure to add the following using statement at the top of your code files to see the extension methods: using Light.GuardClauses;.

Where do I get it?

Download the assembly via NuGet: Install-Package Light.GuardClauses - Or use the code from this repo.

Light.GuardClauses is a .NET Standard 1.0 library since v2.0.0. This means it is compatible with e.g. .NET 4.5 or later, .NET Core 1.0 and UWP, Mono 4.6, Xamarin.iOS 10.0, Xamarin.Android 7.0, Windows 8 / 8.1 Store Apps, and Windows Phone 8.1 / Windows Phone 8 Silverlight.

And what's the difference to other assertion libraries?

Light.GuardClauses is specifically tailored for the scenario of creating precondition checks in production code. While the purpose of many other libraries is to provide a fluent syntax for assertions in automated tests, Light.GuardClauses achieves the following two goals:

  1. Light.GuardClauses is high-performant as all checks involve only static method calls which create as less objects as possible (to keep the pressure on the Garbage Collector low)
  2. Light.GuardClauses provides meaningful exception messages that you would expect from production code. Additionally, you can easily customize the message or the exception being thrown for every assertion call.

Check out the following section to learn how you can customize the resulting exceptions when precondition checks fail.

Customizing messages and exceptions

Every exception-throwing assertion of Light.GuardClauses has three optional parameters: parameterName, message and exception. With these, you can customize the outcome of an assertion:

  • parameterName lets you inject the name of the parameter being checked into the resulting exception message. By default, the standard exception messages use e.g. "The value", "The string", or "The URI" to talk about the subject - these parts are exchanged when you specify the name of the parameter. My advise is to not use paramterName when developing application-specific code, but if you create framework, library, or any other type of reusable code, it is better to use parameterName so that exception messages immediately point to the erroneous parameter.
  • message lets you exchange the entire exception message if you are not satisfied with the default message in your current context.
  • exception lets you specify a delegate creating an exception object that is thrown instead of the default exception.
public class ConsoleWriter
    private readonly ConsoleColor _foregroundColor;

    public ConsoleWriter(ConsoleColor foregroundColor = ConsoleColor.Black)
        foregroundColor.MustBeValidEnumValue(parameterName: nameof(foregroundColor));
        _foregroundColor = foregroundColor;
public class Entity
    public Guid Id { get; }
    public Entity(Guid id)
        id.MustNotBeEmpty(message: "You cannot create an entity with an empty GUID.");
        Id = id;
public class CustomerController : ApiController
    private readonly ICustomerRepository _repo;
    public CustomerController(ICustomerRepository repo)
        repo.MustNotBeNull(exception: () => new StupidTeamMembersException("Who is the idiot that forgot to register ICustomerRepository with the DI container?"));
        _repo = repo;

I can't find a suitable assertion method

If you can't find an assertion method that suits your current needs, you can always fall back to the Check.That and Check.Against methods that can be used in all circumstances:

public void CompleteOrder()
               () => new InvalidOperationException("You cannot complete the order because some customer information is missing.");
    // Implemetation omitted to keep the example small

Of course, you can write your own extension methods, too.

I want to extend Light.GuardClauses

If you want to write your own assertion method throwing an exception, you should follow these recommendations (which of course can be ignored when you only want to use them in your own solution):

  • Create an extension method that should return the value that it is checking, i.e. the this parameter type should also be the return type to provide a fluent API.
  • Apart from the parameters you need, add the three optional parameters parameterName, message, and exception. They should behave as mentioned above in the Customizing messages and exceptions section.
  • Using the Null-Coalescing-Operator (??) is recommended to check if the optional parameters are specified.

Check out the existing methods in the source code of this repository and the following template:

public static *YourType* *YourMethodName*(this *YourType* parameter, *Your other necessary parameters*, string parameterName = null, string message = null, Func<Exception> exception = null)
    if (*check something here*) return parameter;
    throw exception?.Invoke() ?? new *YourExceptionType*(message ?? $"{parameterName ?? "The value"} must not be ...");

Your extension method is cool and you think other developers can benefit from it? Then send me a pull request and I'll check if it is useful to incorporate your method. Please provide tests and XML comments for your method, too.

Writing tests for new extension methods

Light.GuardClauses uses xunit.net as the framework for automated tests and FluentAssertions for assertions within test methods. Please do not use any other frameworks when you want to integrate your own extension methods within Light.GuardClauses.Tests. Also, I would advise you to create one test class per assertion method.

Furthermore, the test project contains two checks that test all assertion methods using reflection:

  • In the test class FluentApiTests, there is a Theory that ensures that all extension method that start with "Must*" in the Light.GuardClauses namespace return the value that they check. This way you cannot forget to provide a fluent API with your new extension method. If for some reason your method cannot provide a fluent API, you can update the omittedMethods variable in the constructor of that class.
  • Because writing tests for custom messages and custom exceptions for all extension methods is tedious, I created a little bit of infrastructure that does most of the heavy lifting for you. This infrastructure is implemented in CustomMessagesAndCustomExceptionsTests and the types in the sub-namespace CustomMessagesAndExceptions. All you have to do is implement the ICustomMessageAndExceptionTestDataProvider interface in your own test class. In the only method PopulateTestDataForCustomExceptionAndCustomMessageTestsof this interface, you get a CustomMessageAndExceptionTestData object as a parameter that you can use to populate as many CustomExceptionTest and CustomMessageTest instances as you like. Check the existing test classes or the example below to see how it's done. Just be aware that CustomMessagesAndCustomExceptionsTests also checks if your test class implements the said interface, and if not will tell you so in the test runs. If you do not implement an assertion extension method, you can whitelist your test class in the OmmitedTestClasses field.

An example of a test class looks like this:

public sealed class MustBeSameAsTests : ICustomMessageAndExceptionTestDataProvider
    [Theory(DisplayName = "MustBeSameAs must throw an ArgumentException when the two specified references do not point to the same instance.")]
    [InlineData("Hello", "World")]
    [InlineData("1", "2")]
    [InlineData(new object[] { }, new object[] { "Foo" })]
    public void ReferencesDifferent<T>(T first, T second) where T : class
        Action act = () => first.MustBeSameAs(second, nameof(first));

            .And.Message.Should().Contain($"{nameof(first)} must point to the object instance \"{second}\", but it does not.");

    [Theory(DisplayName = "MustBeSameAs must not throw an exception when the two specified references point to the same instance.")]
    public void ReferencesEqual(string reference)
        Action act = () => reference.MustBeSameAs(reference);


    public void PopulateTestDataForCustomExceptionAndCustomMessageTests(CustomMessageAndExceptionTestData testData)
        testData.Add(new CustomExceptionTest(exception => "foo".MustBeSameAs("bar", exception: exception)));

        testData.Add(new CustomMessageTest<ArgumentException>(message => "foo".MustBeSameAs("bar", message: message)));

Is it ready for production?

Since the beginning of June 2016, the library is in v1.x and stable. Light.GuardClauses is thoroughly covered with automated tests and I actively use it in my daily work.

In the end, what do I get?

Light.GuardClauses is a lightweight, high-performance .NET solution for precondition checks, providing you with sensible default exception messages and the ability to easily customize them. This removes the clutter at the beginning of your parameterized methods. Additionally, you can easily extend Light.GuardClauses with your own assertions.


Light.GuardClauses was initially developed as part of the iRescYou research project, conducted at the University Hospital of Regensburg and the University of Applied Sciences Regensburg, funded by the Bavarian State Ministry of Health and Care.

Thanks to xunit.net and FluentAssertions - I use these frameworks every day! And Visual Studio in combination with R# is awesome!

Let there be... Light

Light Libraries Logo