NUnit’s Assert.That

NUnit’s Assert.That


Nunit’s Assert.That is a versatile assertion method.

It’s part of NUnit’s constraint-based assert model, which is more flexible and powerful than the traditional assert model. This approach allows for more readable and expressive test code.

Here’s a guide on how to use Assert.That effectively in different scenarios:

Basic Usage

The basic syntax of Assert.That is:

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Assert.That(actual, constraint, message);

  • actual is the value being tested.
  • constraint is the condition you expect actual to meet.
  • message is an optional parameter for a custom error message if the assertion fails.


1. Equality Check

int result = GetResultFromMethod(); 
Assert.That(result, Is.EqualTo(5), "The result should be 5.");

2. Null Check

object myObject = GetObject(); 
Assert.That(myObject, Is.Null"Object should be null.");

3. Collection Checks

var collectionnew List<int> { 123 }; 
Assert.That(collection, Has.Member(2), "Collection should contain the number 2.");

4. String Checks

string message"Hello, World!"; 
Assert.That(message, Does.Contain("World"), "Message should contain 'World'.");

5. Type Checking

object myObject = GetObject(); 
Assert.That(myObject, Is.TypeOf<MyClass>(), "Object should be of type MyClass.");

6. Exception Checking

TestDelegate action = () => myObject.SomeMethod(); 
Assert.That(action, Throws.TypeOf<InvalidOperationException>(), "Method should throw InvalidOperationException.");

7. Property Checking

var personnew Person { Name"John", Age30 }; 
Assert.That(person, Has.Property("Name").EqualTo("John"), "Person's name should be John.");

Advanced Constraints

NUnit provides a range of constraints that can be used with Assert.That, making your tests more expressive and precise:

  • Logical Constraints: And, Or, Not for combining conditions.
  • Range Constraints: InRange, GreaterThan, LessThan for numerical comparisons.
  • Collection Constraints: All, Some, None, Exactly for detailed collection testing.
  • String Constraints: StartsWith, EndsWith, Matches (for regex) for string testing.


NUnit’s Assert.That is a powerful part of the framework that allows for expressive and readable tests.

By leveraging the various constraints, you can create robust and comprehensive tests for your .NET applications.

Remember, the key to effective testing is not only to check for the expected outcomes but also to express your tests in a way that makes them easy to understand and maintain.

You can find the official NUnit documentation here: –

I have written a number of other NUnit posts here: – NUnit’s Coolest Feature – Data-Driven Tests, Asynchronous Testing with NUnit and C#


Hi, my name is Stephen Finchett. I have been a software engineer for over 30 years and worked on complex, business critical, multi-user systems for all of my career. For the last 15 years, I have been concentrating on web based solutions using the Microsoft Stack including ASP.Net, C#, TypeScript, SQL Server and running everything at scale within Kubernetes.