靜態元素 (Static Elements) 的單元測試, 以 DateTime.Now 為例

Unit Test for Static Elements (DateTime.Now) in ASP.NET Core 6 MVC

前言

接續前一篇樂透開獎的例子, 假設有一個新的需求: "樂透開奬不是每天開, 而是固定每個月的 5 日才會開獎".

因此, 程式要加一個當前日期時間的判斷.

若為 5 日才可開獎, 並回傳開獎的結果.
若為其它日期, 就不可開獎, 並回傳警告訊息.
這個邏輯要如何測試呢? 總不能等到每個月 5 日, 或者調整筆電的系統時間, 才來測試吧.

以下係採參考文件2.. "An interface that wraps the DateTime.Now" 方式進行演練及實作.

完整範例可由 GitHub 下載.

演練細節

步驟_1: 加入 IDateTimeProvider 介面及 DateTimeProvider 類別

public interface IDateTimeProvider
{
    DateTime GetCurrentTime();
}

public class DateTimeProvider : IDateTimeProvider
{
    public DateTime GetCurrentTime()
    {
        return DateTime.Now;
    }
}

步驟_2: 將 IDateTimeProvider 註冊至 DI container

#region 註冊相關的服務
builder.Services.AddSingleton<IRandomGenerator, RandomGenerator>();
builder.Services.AddScoped<ILottoService, LottoService>();
builder.Services.AddSingleton<IDateTimeProvider, DateTimeProvider>();
#endregion

步驟_3: 修改 LottoService 的處理邏輯

1.. 修改建構子, 加入 IDateTimeProvider 物件的注入.

private readonly IRandomGenerator _randomGenerator;
private readonly IDateTimeProvider _dateTimeProvider;

public LottoService(IRandomGenerator randomGenerator, IDateTimeProvider dateTimeProvider) 
{
    _randomGenerator = randomGenerator;
    _dateTimeProvider = dateTimeProvider;
}

2.. 修改 Lottoing() 方法, 加入是否為每個月 5 日的判斷.

public LottoViewModel Lottoing(int min, int max)
{

    var result = new LottoViewModel();
    var now = _dateTimeProvider.GetCurrentTime();

    if (now.Day != 5)
    {
        result.YourNumber = -1;
        result.Message = "非每個月5日, 不開獎";
        return result;
    }

    // Random(min, max): 含下界, 不含上界
    var yourNumber = _randomGenerator.Next(min, max);
    // 只要餘數是 9, 就代表中獎
    var message = (yourNumber % 10 == 9) ? "恭喜中獎" : "再接再厲";

    result.YourNumber = yourNumber;
    result.Message = message;

    return result;
}

步驟_4: 修改原有的測試案例

1.. 因為 LottoService 的建構子增加了 IDataTimeProvider 這個參數, 所以, 原有的測試案例, 也要跟著改, 不然會編譯失敗.

[TestMethod()]
public void Test_Lottoing_今天是20240105_輸入亂數範圍_0_10_預期回傳_9_恭喜中獎()
{
    // Arrange
    var expected = new LottoViewModel()
    { YourNumber = 9, Message = "恭喜中獎" }
                .ToExpectedObject();

    int fixedValue = 9;
    DateTime today = new(2024, 01, 05);
    var mockRandomGenerator = new Mock<IRandomGenerator>();
    var mockDateTimeProvider = new Mock<IDateTimeProvider>();
    mockRandomGenerator.Setup(r => r.Next(It.IsAny<int>(), It.IsAny<int>())).Returns(fixedValue);
    mockDateTimeProvider.Setup(d => d.GetCurrentTime()).Returns(today);

    // Act
    var target = new LottoService(mockRandomGenerator.Object, mockDateTimeProvider.Object);
    var actual = target.Lottoing(0, 10);

    // Assert
    expected.ShouldEqual(actual);
}

[TestMethod()]
public void Test_Lottoing_今天是20240105_輸入亂數範圍_0_10_預期回傳_1_再接再厲()
{
    // Arrange
    var expected = new LottoViewModel()
    { YourNumber = 1, Message = "再接再厲" }
                .ToExpectedObject();

    int fixedValue = 1;
    DateTime today = new(2024, 01, 05);
    var mockRandomGenerator = new Mock<IRandomGenerator>();
    var mockDateTimeProvider = new Mock<IDateTimeProvider>();
    mockRandomGenerator.Setup(r => r.Next(It.IsAny<int>(), It.IsAny<int>())).Returns(fixedValue);
    mockDateTimeProvider.Setup(d => d.GetCurrentTime()).Returns(today);

    // Act
    var target = new LottoService(mockRandomGenerator.Object, mockDateTimeProvider.Object);
    var actual = target.Lottoing(0, 10);

    // Assert
    expected.ShouldEqual(actual);
}

步驟_5: 針對不開獎的日期, 建立測試案例

[TestMethod()]
public void Test_Lottoing_今天是20240122_輸入亂數範圍_0_10_預期回傳_負1_非每個月5日_不開獎()
{
    // Arrange
    var expected = new LottoViewModel()
    { YourNumber = -1, Message = "非每個月5日, 不開獎" }
                .ToExpectedObject();

    int fixedValue = 9;
    DateTime today = new(2024, 01, 22);
    var mockRandomGenerator = new Mock<IRandomGenerator>();
    var mockDateTimeProvider = new Mock<IDateTimeProvider>();
    mockRandomGenerator.Setup(r => r.Next(It.IsAny<int>(), It.IsAny<int>())).Returns(fixedValue);
    mockDateTimeProvider.Setup(d => d.GetCurrentTime()).Returns(today);

    // Act
    var target = new LottoService(mockRandomGenerator.Object, mockDateTimeProvider.Object);
    var actual = target.Lottoing(0, 10);

    // Assert
    expected.ShouldEqual(actual);
}

步驟_6: 檢查測試的結果

TestResult

結論

DateTime 為 struct, DateTime.Now 為 static property, 在需要依當前日期時間作判斷的狀況下, 若直接呼叫 DateTime.Now, 將導致相依於 DateTime, 無法快速進行程式邏輯的測試.
故需以一個 interface 進行打包 (Wrap), 讓外界得以操作物件實體, 及其回傳值.

只是, 如同前一篇亂數範例所述的, 也會造成開發人員要習慣使用打包後的介面及類別, 這是比較美中不足的地方.

或許有更好的處理方式, 只是受限筆者的能力, 無法作到盡善盡美, 但至少可以供參考.

參考文件

The most popular strategies for unit testing DateTime.Now in C# are:

An interface that wraps the DateTime.Now

SystemTime static class

Ambient context approach

DateTime property on a class

傑士伯的IT學習之路

2024年1月22日星期一