What is fact and theory in xuint unit testing ?

In the context of xUnit unit testing frameworks, "fact" and "theory" are terms used to describe different approaches to writing and organizing tests.

1. Fact:

   • A fact is a simple unit test that verifies a specific behavior of a method or function.
   • It is typically represented by a method or function annotated with a specific attribute (e.g., [Fact] in xUnit frameworks).
   • Facts are often used for testing scenarios where there is a single input and a single expected output.
   • They are straightforward and easy to understand, making them suitable for simple test cases.

   Example (C# with xUnit):

   [Fact]
   public void Add_TwoNumbers_ReturnsSum()
   {
       // Arrange
       Calculator calculator = new Calculator();
   
       // Act
       int result = calculator.Add(2, 3);
   
       // Assert
       Assert.Equal(5, result);
   }
   

2. Theory:

   • A theory allows you to write parameterized tests, where the same test logic is executed with multiple sets of input values.
   • Theories are useful when you want to test a function with various inputs and expected outcomes.
   • In xUnit, theories are represented by methods or functions annotated with the [Theory] attribute, and they use data attributes (e.g., [InlineData], [ClassData]) to provide different sets of test data.

   Example (C# with xUnit):

   [Theory]
   [InlineData(2, 3, 5)]
   [InlineData(0, 0, 0)]
   [InlineData(-1, 1, 0)]
   public void Add_TwoNumbers_ReturnsSum(int a, int b, int expectedSum)
   {
       // Arrange
       Calculator calculator = new Calculator();
   
       // Act
       int result = calculator.Add(a, b);
   
       // Assert
       Assert.Equal(expectedSum, result);
   }
   

In summary, a fact is a basic unit test that verifies a specific scenario, while a theory allows you to parameterize your tests and run them with multiple sets of input data. Both facts and theories contribute to creating comprehensive test suites for your software.