c# - 单元测试仅在构建服务器上运行时失败

Question

为了帮助进行单元测试，我们将 DateTime 类包装在一个委托(delegate)中，以便可以在单元测试中覆盖 DateTime.Now。

public static class SystemTime
{
    #region Static Fields

    public static Func<DateTime> Now = () => DateTime.Now;

    #endregion
}

这是它在 xunit 单元测试中的使用示例:

[Fact]
public void it_should_update_the_last_accessed_timestamp_on_an_entry()
{
    // Arrange
    var service = this.CreateClassUnderTest();

    var expectedTimestamp = SystemTime.Now();
    SystemTime.Now = () => expectedTimestamp;

    // Act
    service.UpdateLastAccessedTimestamp(this._testEntry); 

    // Assert
    Assert.Equal(expectedTimestamp, this._testEntry.LastAccessedOn);
}   

测试在本地运行良好，但在我们的构建服务器上失败，因为 Assert 语句中的日期时间不同。

鉴于 DateTime 是通过上述委托(delegate)包装器模拟的，我正在努力想出它失败的原因。我已验证 UpdateLastAccessedTimestamp 方法的实现没有问题，并且测试在本地运行时通过。

不幸的是，我无法在我们的构建服务器上调试它。知道为什么只有在构建服务器上运行时它才会失败吗？

注意UpdateLastAccessedTimestamp的实现如下:

public void UpdateLastAccessedTimestamp(Entry entry)
{
    entry.LastAccessedOn = SystemTime.Now();
    this._unitOfWork.Entries.Update(entry);
    this._unitOfWork.Save();
}

Entry 类只是一个简单的 POCO 类，它有许多字段，包括 LastAccessedOn 字段:

public class Entry
{
   public DateTime LastAccessedOn { get; set; }

   //other fields that have left out to keep the example concise
}

Answer 1

您的问题可能是由于多个单元测试使用了 static SystemTime。例如，如果你有类似的东西:

单元测试 1

[Fact]
public void it_should_update_the_last_accessed_timestamp_on_an_entry()
{
    // Arrange
    var service = this.CreateClassUnderTest();

    var expectedTimestamp = SystemTime.Now();
    SystemTime.Now = () => expectedTimestamp;

    // Act
    service.UpdateLastAccessedTimestamp(this._testEntry); 

    // Assert
    Assert.Equal(expectedTimestamp, this._testEntry.LastAccessedOn);
}   

public void UpdateLastAccessedTimestamp(Entry entry)
{
    entry.LastAccessedOn = SystemTime.Now();
    this._unitOfWork.Entries.Update(entry);
    this._unitOfWork.Save();
}

单元测试2

[Fact]
public void do_something_different
{
    SystemTime.Now = () => DateTime.Now;
}

所以让我们假设单元测试 2(它是完整的)在单元测试 1 的行之间触发:

SystemTime.Now = () => expectedTimestamp;

// Unit test 2 starts execution here

// Act
service.UpdateLastAccessedTimestamp(this._testEntry); 

如果发生这种情况，那么您的 UpdateLastAccessedTimestamp 将不会(必然)具有您在 SystemTime.Now = () 中设置的预期 DateTime 值=> expectedTimestamp;，因为另一个测试已经覆盖了单元测试 1 中提供的功能。

这就是为什么我认为您最好将 DateTime 作为参数传递，或者像这样使用可注入(inject)的日期时间:

/// <summary>
/// Injectable DateTime interface, should be used to ensure date specific logic is more testable
/// </summary>
public interface IDateTime
{
    /// <summary>
    /// Current Data time
    /// </summary>
    DateTime Now { get; }
}

/// <summary>
/// DateTime.Now - use as concrete implementation
/// </summary>
public class SystemDateTime : IDateTime
{
    /// <summary>
    /// DateTime.Now
    /// </summary>
    public DateTime Now { get { return DateTime.Now; } }
}

/// <summary>
/// DateTime - used to unit testing functionality around DateTime.Now (externalizes dependency on DateTime.Now
/// </summary>
public class MockSystemDateTime : IDateTime
{
    private readonly DateTime MockedDateTime;

    /// <summary>
    /// Take in mocked DateTime for use in testing
    /// </summary>
    /// <param name="mockedDateTime"></param>
    public MockSystemDateTime(DateTime mockedDateTime)
    {
        this.MockedDateTime = mockedDateTime;
    }

    /// <summary>
    /// DateTime passed from constructor
    /// </summary>
    public DateTime Now { get { return MockedDateTime; } }
}

使用这种情况，您的服务类可能会从(类似于)这样改变:

public class Service
{
    public Service() { }

    public void UpdateLastAccessedTimestamp(Entry entry)
    {
        entry.LastAccessedOn = SystemTime.Now();
        this._unitOfWork.Entries.Update(entry);
        this._unitOfWork.Save();
    }
}

对此:

    public class Service
    {

        private readonly IDateTime _iDateTime;

        public Service(IDateTime iDateTime)
        {
            if (iDateTime == null)
                throw new ArgumentNullException(nameof(iDateTime));
            // or you could new up the concrete implementation of SystemDateTime if not provided

            _iDateTime = iDateTime;
        }

        public void UpdateLastAccessedTimestamp(Entry entry)
        {
            entry.LastAccessedOn = _iDateTime.Now;
            this._unitOfWork.Entries.Update(entry);
            this._unitOfWork.Save();
        }           
    }

对于您的 Service 的实际实现，您可以像这样新建(或使用 IOC 容器):

Service service = new Service(new SystemDateTime());

对于测试，您可以使用模拟框架，或者您的 Mock 类:

Service service = new Service(new MockDateTime(new DateTime(2000, 1, 1)));

你的单元测试可以变成:

[Fact]
public void it_should_update_the_last_accessed_timestamp_on_an_entry()
{
    // Arrange
    MockDateTime mockDateTime = new MockDateTime(new DateTime 2000, 1, 1);
    var service = this.CreateClassUnderTest(mockDateTime);

    // Act
    service.UpdateLastAccessedTimestamp(this._testEntry); 

    // Assert
    Assert.Equal(mockDateTime.Now, this._testEntry.LastAccessedOn);
}