C#入門編（27）自動テスト入門 ～xUnitで「壊れたら気づける」仕組みを作る～

[Fact]
public void Add_WithTwoNumbers_ReturnsSum()
{
    // Arrange（準備）
    var calculator = new Calculator();

    // Act（実行）
    var result = calculator.Add(2, 3);

    // Assert（検証）
    Assert.Equal(5, result);
}

namespace CalcApp;

public interface ITaxRateLoader
{
    int GetTaxRatePercent();
}

namespace CalcApp;

public class TaxRateLoader : ITaxRateLoader
{
    private readonly string _filePath;

    public TaxRateLoader(string filePath = "taxrate.txt")
    {
        _filePath = filePath;
    }

    public int GetTaxRatePercent()
    {
        var text = File.ReadAllText(_filePath);
        return int.Parse(text.Trim());
    }
}

namespace CalcApp;

public class Calculator
{
    private readonly ITaxRateLoader _taxRateLoader;

    public Calculator(ITaxRateLoader taxRateLoader)
    {
        _taxRateLoader = taxRateLoader;
    }

    public int Add(int a, int b)
    {
        return a + b;
    }

    public int Divide(int a, int b)
    {
        if (b == 0)
            throw new DivideByZeroException();
        return a / b;
    }

    public decimal CalculateWithTax(decimal price)
    {
        var taxRate = _taxRateLoader.GetTaxRatePercent();
        return price * (100 + taxRate) / 100;
    }
}

10

...
<ItemGroup>
  <None Update="taxrate.txt">
    <CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
  </None>
</ItemGroup>
...

namespace CalcApp.Tests;

using CalcApp;

public class CalculatorTests
{
    [Fact]
    public void Add_WithTwoNumbers_ReturnsSum()
    {
        // Arrange（準備）
        var taxRateLoader = new TaxRateLoader();
        var calculator = new Calculator(taxRateLoader);

        // Act（実行）
        var result = calculator.Add(2, 3);

        // Assert（検証）
        Assert.Equal(5, result);
    }

    [Fact]
    public void Divide_WithValidInputs_ReturnsQuotient()
    {
        // Arrange
        var taxRateLoader = new TaxRateLoader();
        var calculator = new Calculator(taxRateLoader);

        // Act
        var result = calculator.Divide(10, 2);

        // Assert
        Assert.Equal(5, result);
    }

    [Fact]
    public void Divide_WithZeroDivisor_ThrowsDivideByZeroException()
    {
        // Arrange
        var taxRateLoader = new TaxRateLoader();
        var calculator = new Calculator(taxRateLoader);

        // Act & Assert
        Assert.Throws<DivideByZeroException>(() => calculator.Divide(10, 0));
    }

    [Fact]
    public void CalculateWithTax_WithValidPrice_ReturnsPriceWithTax()
    {
        // Arrange
        var taxRateLoader = new TaxRateLoader();
        var calculator = new Calculator(taxRateLoader);

        // Act
        var result = calculator.CalculateWithTax(1000m);

        // Assert
        Assert.Equal(1100m, result);  // 税率10%: 1000 * 1.1 = 1100
    }
}

...
[Theory]
[InlineData(2, 3, 5)]
[InlineData(0, 0, 0)]
[InlineData(-1, 1, 0)]
[InlineData(100, 200, 300)]
public void Add_WithVariousInputs_ReturnsExpectedSum(
    int a, int b, int expected)
{
    // Arrange
    var taxRateLoader = new TaxRateLoader();
    var calculator = new Calculator(taxRateLoader);

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

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

...

[Theory]
[InlineData(1000, 1100)]   // 1000円 → 1100円
[InlineData(500, 550)]     // 500円 → 550円
[InlineData(0, 0)]         // 0円 → 0円
public void CalculateWithTax_WithVariousPrices_ReturnsExpected(
    decimal price, decimal expected)
{
    // Arrange
    var taxRateLoader = new TaxRateLoader();
    var calculator = new Calculator(taxRateLoader);

    // Act
    var result = calculator.CalculateWithTax(price);

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

public int Add(int a, int b)
{
    return a - b;  // + を - に変更（バグ）
}