Basic Assertions

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The following assertions are available to all types of objects.

object theObject = null;
theObject.Should().BeNull("because the value is null");

theObject = "whatever";
theObject.Should().BeOfType<string>("because a {0} is set", typeof(string));
theObject.Should().BeOfType(typeof(string), "because a {0} is set", typeof(string));

Sometimes you might like to first assert that an object is of a certain type using BeOfType and then continue with additional assertions on the result of casting that object to the specified type. You can do that by chaining those assertions onto the Which property like this.

  .Which.Message.Should().Be("Other Message");

To assert that two objects are equal (through their implementation of Object.Equals), use

string otherObject = "whatever";
theObject.Should().Be(otherObject, "because they have the same values");

To assert that an object is equal to one of the provided objects, you can use

theObject.Should().BeOneOf(obj1, obj2, obj3);

To assert object equality using a custom equality comparer, you can use

theObject.Should().Be(otherObject, new ObjEqualityComparer());
theObject.Should().NotBe(otherObject, new ObjEqualityComparer());
theObject.Should().BeOneOf(new[] { obj1, obj2, obj3 }, new ObjEqualityComparer());

If you want to make sure two objects are not just functionally equal but refer to the exact same object in memory, use the following two methods.

theObject = otherObject;

Other examples of some general purpose assertions include

var ex = new ArgumentException();
ex.Should().BeAssignableTo<Exception>("because it is an exception");
ex.Should().NotBeAssignableTo<DateTime>("because it is an exception");

var dummy = new Object();
dummy.Should().Match(d => (d.ToString() == "System.Object"));
dummy.Should().Match<string>(d => (d == "System.Object"));
dummy.Should().Match((string d) => (d == "System.Object"));

Some users requested the ability to easily downcast an object to one of its derived classes in a fluent way.


We’ve also added the possibility to assert that an object can be serialized and deserialized using the XML, binary or data contract formatters.


Internally, BeBinarySerializable uses the Object graph comparison API, so if you are in need of excluding certain properties from the comparison (for instance, because its backing field is [NonSerializable], you can do this:

    options => options.Excluding(s => s.SomeNonSerializableProperty));