Theory vs Fact attribute — C# XUnit Testing

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[Fact]

The Fact attribute is used to mark a method as a test method. It signifies that the method represents a fact that should always be true.

• A test marked with the Fact attribute represents a single test case.
• If the test method throws an exception or fails an assertion, the test is considered failed.

using Xunit;

public class MathTests
{
    [Fact]
    public void Add_TwoPlusTwo_ReturnsFour()
    {
        // Arrange
        var calculator = new Calculator();

        // Act
        var result = calculator.Add(2, 2);

        // Assert
        Assert.Equal(4, result);
    }
}

[Theory]

• The Theory attribute is used to define a parametrized test. It allows testing multiple inputs against the same test logic.
• You provide one or more data sources (via attributes like InlineData, MemberData, etc.) to supply the test with different input values.
• Each set of input values is treated as a separate test case.
• If any of the test cases fail, the entire theory is considered failed.

using Xunit;

public class MathTests
{
    [Theory]
    [InlineData(2, 3, 5)]
    [InlineData(0, 0, 0)]
    [InlineData(-1, 1, 0)]
    public void Add_ValidInputs_ReturnsCorrectResult(int a, int b, int expected)
    {
        // Arrange
        var calculator = new Calculator();

        // Act
        var result = calculator.Add(a, b);

        // Assert
        Assert.Equal(expected, result);
    }
}

In summary, Fact is used for individual test cases, while Theory is used for parameterized tests where the same test logic is applied to multiple sets of inputs.