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The following example verifies that the Foo() method throws an InvalidOperationException which Message property has a specific value.

subject.Invoking(y => y.Foo("Hello"))
    .WithMessage("Hello is not allowed at this moment");

But if you prefer the arrange-act-assert syntax, you can also use an action in your act part.

Action act = () => subject.Foo2("Hello");


Notice that the example also verifies that the exception has a particular inner exception with a specific message. In fact, you can even check the individual properties of the exception instance using the And property.

Action act = () => subject.Foo(null);


An alternative syntax for doing the same is by chaining one or more calls to the Where() method:

Action act = () => subject.Foo(null);
act.Should().Throw<ArgumentNullException>().Where(e => e.Message.StartsWith("did"));

However, we discovered that testing the exception message for a substring is so common, that we changed the default behavior of WithMessage to support wildcard expressions and match in a case-insensitive way.

Action act = () => subject.Foo(null);

On the other hand, you may want to verify that no exceptions were thrown.

Action act = () => subject.Foo("Hello");

We know that a unit test will fail anyhow if an exception was thrown, but this syntax returns a clearer description of the exception that was thrown and fits better to the AAA syntax.

If you want to verify that a specific exception is not thrown, and want to ignore others, you can do that using an overload:

Action act = () => subject.Foo("Hello");

Sometimes you may want to retry an assertion until it either succeeds or a given time elapses. For instance, you could be testing a network service which should become available after a certain time, say, 10 seconds:

Action act = () => service.IsReady().Should().BeTrue();
act.Should().NotThrowAfter(10.Seconds(), 100.Milliseconds());

The second argument of NotThrowAfter specifies the time that should pass before act is executed again after an execution which threw an exception.

If the method you are testing returns an IEnumerable or IEnumerable<T> and it uses the yield keyword to construct that collection, just calling the method will not cause the effect you expected because the real work is not done until you actually iterate over that collection. You can use the Enumerating() extension method to force enumerating the collection like this.

Func<IEnumerable<char>> func = () => obj.SomeMethodThatUsesYield("blah");

You do have to use the Func<T> type instead of Action<T> then.

The exception throwing API follows the same rules as the trycatch…construction does. In other words, if you’re expecting a certain exception to be (not) thrown, and a more specific exception is thrown instead, it would still satisfy the assertion. So throwing an ApplicationException when an Exception was expected will not fail the assertion. However, if you really want to be explicit about the exact type of exception, you can use ThrowExactly and WithInnerExceptionExactly.

Talking about the async keyword, you can also verify that an asynchronously executed method throws or doesn’t throw an exception:

Func<Task> act = async () => { await asyncObject.ThrowAsync<ArgumentException>(); };
await act.Should().ThrowAsync<InvalidOperationException>();
await act.Should().NotThrowAsync();

Alternatively, you can use the Awaiting method like this:

Func<Task> act = () => asyncObject.Awaiting(async x => await x.ThrowAsync<ArgumentException>());

Both give you the same results, so it’s just a matter of personal preference.

As for synchronous methods, you can also check that an asynchronously executed method executes successfully after a given wait time using NotThrowAfter:

Stopwatch watch = Stopwatch.StartNew();
Func<Task> act = async () =>
    if (watch.ElapsedMilliseconds <= 1000)
        throw new ArgumentException("The wait time has not yet elapsed.");

    await Task.CompletedTask;

await act.Should().NotThrowAfterAsync(2.Seconds(), 100.Milliseconds());
act.Should().NotThrowAfter(2.Seconds(), 100.Milliseconds());

If you prefer single-statement assertions, consider using the FluentActions static class, which has Invoking, Awaiting, and Enumerating methods:

FluentActions.Invoking(() => MyClass.Create(null)).Should().Throw<ArgumentNullException>();

To make it even more concise:

using static FluentAssertions.FluentActions;


Invoking(() => MyClass.Create(null)).Should().Throw<ArgumentNullException>();

Automatic AggregateException unwrapping

.NET 4.0 and later includes the AggregateException type. This exception type is typically thrown by methods which return either Task or Task<TResult> and are executed synchronously, instead of using async and await. This type contains a collection of inner exceptions which are aggregated.

Methods such as Throw<TException>, ThrowAsync<TException>, NotThrow<TException> and NotThrowAsync<TException> described above will also work for exceptions that are aggregated, whether or not you are asserting on the actual AggregateException or any of its (nested) aggregated exceptions.

However, the ThrowExactly<TException> and ThrowExactlyAsync<TException> methods will only work for exceptions that aren’t aggregated. If you are asserting that an exception type other than AggregateException is thrown, an AggregateException must not be thrown, even if it contains an inner exception of the asserted type.