Moq has certain behaviors that might appear quite opinionated. Although these behaviors are active by default, they can be customized through special properties exposed by mocks.

Mock behavior

By default, Moq supports developers by allowing them to create unit tests without forcing them to declare every expected call. This is considered a particularly forgiving behavior because mocking frameworks would usually reject any call that was not configured.

Moq's default behavior enables quicker development of unit tests by defining a fallback configuration that accepts any parameter and returns empty sequences or the default value.

Sometimes there is a need for a more controlled environment by requiring Moq to attain a more strict behavior. For this reason, when creating a mock, developers can specify the desired behavior and override the default.

var mock = new Mock<IService>(MockBehavior.Strict);

The MockBehavior enumeration has the following values:

• Strict: an exception is thrown whenever a method or property is invoked without a matching configuration

• Loose: Moq accepts all invocations and attempts to create a valid return value

• Default: equivalent to Loose.

Default value

Another behavior of mocks that can be customized is how values are generated when there is no configuration in Loose mode. Moq supports two behaviors: Empty and Mock.

The first one is the default behavior and was explained earlier: Moq would either return the default value for the specified type or, in case of a sequence of values, an empty sequence.

var mock = new Mock<IService> { DefaultValue = DefaultValue.Empty };

Alternatively, Moq can return a new mock, assuming the return type is somehow mockable.

var mock = new Mock<IService> { DefaultValue = DefaultValue.Mock };

Custom DefaultValueProvider

Rather than relying on the built-in Default Value behaviors presented earlier, developers can customize this aspect by specifying a DefaultValueProvider.

public class MyCustomDefaultValueProvider : DefaultValueProvider { ... }
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var mock = new Mock<IService> { DefaultValueProvider = new MyCustomDefaultValueProvider() };

Whilst publicly available, this extension seam should be used sparingly and mostly when integrating with other libraries.