C#入門編（28）自動テストは何から始める？モックとスモークテストの使いどころ

[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
}

public interface ITaxRateLoader
{
    int GetTaxRatePercent();
}

// 手動で作成したモック（テスト用の偽実装）
public class FakeTaxRateLoader : ITaxRateLoader
{
    private readonly int _taxRate;

    public FakeTaxRateLoader(int taxRate)
    {
        _taxRate = taxRate;
    }

    public int GetTaxRatePercent()
    {
        return _taxRate;
    }
}

[Fact]
public void CalculateWithTax_WithTaxRate5Percent_ReturnsCorrectPrice()
{
    // Arrange - 税率5%を返すモックを使用
    var fakeTaxRateLoader = new FakeTaxRateLoader(5);
    var calculator = new Calculator(fakeTaxRateLoader);

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

    // Assert
    Assert.Equal(1050m, result);  // 1000 * 1.05 = 1050
}

// NSubstituteを使ったモック作成
var taxRateLoader = Substitute.For<ITaxRateLoader>();
taxRateLoader.GetTaxRatePercent().Returns(5);  // 5%を返すように設定

namespace CalcApp.Tests;

using CalcApp;
using NSubstitute;

public class CalculatorTests
{
    ...   

    [Fact]
    public void CalculateWithTax_WithMockedTaxRate10Percent_ReturnsCorrectPrice()
    {
        // Arrange - モックを作成し、税率10%を返すように設定
        var taxRateLoader = Substitute.For<ITaxRateLoader>();
        taxRateLoader.GetTaxRatePercent().Returns(10);

        var calculator = new Calculator(taxRateLoader);

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

        // Assert
        Assert.Equal(1100m, result);
    }

}

[Theory]
[InlineData(1000, 5, 1050)]    // 税率5%: 1000 → 1050
[InlineData(1000, 8, 1080)]    // 税率8%: 1000 → 1080
[InlineData(1000, 10, 1100)]   // 税率10%: 1000 → 1100
[InlineData(500, 10, 550)]     // 税率10%: 500 → 550
[InlineData(0, 10, 0)]         // 税率10%: 0 → 0
public void CalculateWithTax_WithVariousTaxRates_ReturnsExpected(
    decimal price, int taxRate, decimal expected)
{
    // Arrange
    var taxRateLoader = Substitute.For<ITaxRateLoader>();
    taxRateLoader.GetTaxRatePercent().Returns(taxRate);
    
    var calculator = new Calculator(taxRateLoader);

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

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

 [Fact]
 public void CalculateWithTax_WhenTaxRateLoadingFails_ThrowsException()
 {
     // Arrange - 例外をスローするモックを設定
     var taxRateLoader = Substitute.For<ITaxRateLoader>();
     taxRateLoader.GetTaxRatePercent().Returns(x => throw new IOException("ファイル読み込みエラー"));

     var calculator = new Calculator(taxRateLoader);

     // Act & Assert
     Assert.Throws<IOException>(() => calculator.CalculateWithTax(1000m));
 }