Programming language: C#
License: MIT License
Tags: Misc    
Latest version: v1.12.0

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Guard is a fluent argument validation library that is intuitive, fast and extensible.

NuGet Build Coverage
$ dotnet add package Dawn.Guard / PM> Install-Package Dawn.Guard


Here is a sample constructor that validates its arguments without Guard:

public Person(string name, int age)
    if (name == null)
        throw new ArgumentNullException(nameof(name), "Name cannot be null.");

    if (name.Length == 0)
        throw new ArgumentException("Name cannot be empty.", nameof(name));

    if (age < 0)
        throw new ArgumentOutOfRangeException(nameof(age), age, "Age cannot be negative.");

    Name = name;
    Age = age;

And this is how we write the same constructor with Guard:

using Dawn; // Bring Guard into scope.

public Person(string name, int age)
    Name = Guard.Argument(name, nameof(name)).NotNull().NotEmpty();
    Age = Guard.Argument(age, nameof(age)).NotNegative();

If this looks like too much allocations to you, fear not. The arguments are read-only structs that are passed by reference. See the design decisions for details and an introduction to Guard's more advanced features.

What's Wrong with Vanilla?

There is nothing wrong with writing your own checks but when you have lots of types you need to validate, the task gets very tedious, very quickly.

Let's analyze the string validation in the example without Guard:

  • We have an argument (name) that we need to be a non-null, non-empty string.
  • We check if it's null and throw an ArgumentNullException if it is.
  • We then check if it's empty and throw an ArgumentException if it is.
  • We specify the same parameter name for each validation.
  • We write an error message for each validation.
  • ArgumentNullException accepts the parameter name as its first argument and error message as its second while it's the other way around for the ArgumentException. An inconsistency that many of us sometimes find it hard to remember.

In reality, all we need to express should be the first bullet, that we want our argument non-null and non-empty.

With Guard, if you want to guard an argument against null, you just write NotNull and that's it. If the argument is passed null, you'll get an ArgumentNullException thrown with the correct parameter name and a clear error message out of the box. The standard validations have fully documented, meaningful defaults that get out of your way and let you focus on your project.


C# 7.2 or later is required. Guard takes advantage of almost all the new features introduced in C# 7.2. So in order to use Guard, you need to make sure your Visual Studio is up to date and you have <LangVersion>7.2</LangVersion> or later added in your .csproj file.

.NET Standard 1.0 and above are supported. Microsoft Docs lists the following platform versions as .NET Standard 1.0 compliant but keep in mind that currently, the unit tests are only targeting .NET Core 3.0.

Platform Version
.NET Core 1.0
.NET Framework 4.5
Mono 4.6
Xamarin.iOS 10.0
Xamarin.Mac 3.0
Xamarin.Android 7.0
Universal Windows Platform 10.0
Windows 8.0
Windows Phone 8.1
Windows Phone Silverlight 8.0
Unity 2018.1


The default branch (dev) is the development branch, so it may contain changes/features that are not published to NuGet yet. See the master branch for the latest published version.

Standard Validations

[Click here][3] for a list of the validations that are included in the library.

Design Decisions

[Click here][1] for the document that explains the motives behind the Guard's API design and more advanced features.


[Click here][4] to see how to add custom validations to Guard by writing simple extension methods.

Code Snippets

Code snippets can be found in the [snippets][5] folder. Currently, only the Visual Studio is supported.