NSubstitute

Uses natural language patterns, as shown in examples like _calculator.Add(1, 2).Returns(3), making test code less cluttered and more expressive than traditional mocking libraries.

Supports Arg.Any<T>() and Arg.Is<T>(predicate) for precise control, allowing tests to verify calls with dynamic or conditional arguments without complex setup.

Provides Received() and DidNotReceive() methods with descriptive error messages, as demonstrated in the call verification example that lists non-matching arguments.

Allows setting return values, sequences, and behaviors using Returns(), including lambdas for computed returns, reducing boilerplate in test setups.