AddTest.cs

[Fact]
public void Add_TestClassObjectPassed_ProperMethodCalled()
{
    // Arrange
    var testObject = new TestClass();

    var context = new Mock<DbContext>();
    var dbSetMock = new Mock<DbSet<TestClass>>();
    context.Setup(x => x.Set<TestClass>()).Returns(dbSetMock.Object);
    dbSetMock.Setup(x => x.Add(It.IsAny<TestClass>())).Returns(testObject);

    // Act
    var repository = new Repository<TestClass>(context.Object);
    repository.Add(testObject);

    //Assert
    context.Verify(x => x.Set<TestClass>());
    dbSetMock.Verify(x => x.Add(It.Is<TestClass>(y => y == testObject)));
}