有哪些方法可以模拟结构以在单元测试中获取类？

Question

我有一门课，我正试图通过单元测试。该类将结构公开为公共属性。该结构还具有一些公共方法（比结构中的方法做得更多）。我无法对结构进行更改（我不拥有该代码，目前风险太大）。

Mock 不适用于值类型。有没有办法有效地“模拟结构”？

public class SystemUnderTest
{
    public FragileStructCantChange myStruct {get; set;}

    public string MethodUnderTest()
    {
        if(myStruct.LongRunningMethod() == "successful")
            return "Ok";
        else
            return "Fail";        
    }
}

public struct FragileStructCantChange
{
    public string LongRunningMethod()
    {
        var resultString = DoStuff(); //calls other subsystems            
        return resultString;
    }
}

Answer 1

通过一个接口引用一个结构体，通过一个称为"boxing" 的过程有效地将其转换为一个引用类型。对结构进行装箱可能会导致一些细微的行为变化，您应该在做出决定之前read about。

另一种选择是在结构和被测代码之间添加一些间接性。一种方法是添加一个来指定测试值。

public class SystemUnderTest
{
    public FragileStructCantChange myStruct { get; set; }

    // This value can be overridden for testing purposes
    public string LongRunningMethodOverride { get; set; }

    public string MethodUnderTest()
    {
        // Call the indirect Func instead of referencing the struct directly
        if (LongRunningMethod() == "successful")
            return "Ok";
        else
            return "Fail";
    }

    private string LongRunningMethod()
        => LongRunningMethodOverride ?? myStruct.LongRunningMethod();
}

public class Tests
{

    [Fact]
    public void TestingSideDoor()
    {
        var sut = new SystemUnderTest();

        // Override the func to supply any test data you want
        sut.LongRunningMethodOverride = "successful";

        Assert.Equal("Ok", sut.MethodUnderTest());
    }
}

另一种方法是公开一个可覆盖的函数指针...

public class SystemUnderTest
{
    public FragileStructCantChange myStruct { get; set; }

    // This Func can be overridden for testing purposes
    public Func<string> LongRunningMethod;

    public SystemUnderTest() {
        LongRunningMethod = () => myStruct.LongRunningMethod();
    }

    public string MethodUnderTest()
    {
        // Call the indirect Func instead of referencing the struct directly
        if (LongRunningMethod() == "successful")
            return "Ok";
        else
            return "Fail";
    }
}

public class Tests
{

    [Fact]
    public void TestingSideDoor()
    {
        var sut = new SystemUnderTest();

        // Override the func to supply any test data you want
        sut.LongRunningMethod = () => "successful";

        Assert.Equal("Ok", sut.MethodUnderTest());
    }
}

另一种选择是使用虚拟方法，它可以由子类或模拟框架（在本例中为Moq）伪造...

public class SystemUnderTest
{
    public FragileStructCantChange myStruct { get; set; }

    // This method can be overridden for testing purposes
    public virtual string LongRunningMethod()
        => myStruct.LongRunningMethod();

    public string MethodUnderTest()
    {
        // Call the indirect method instead of referencing the struct directly
        if (LongRunningMethod() == "successful")
            return "Ok";
        else
            return "Fail";
    }
}

public class Tests
{

    [Fact]
    public void TestingSideDoor()
    {
        var sut = new Mock<SystemUnderTest>();

        // Override the method to supply any test data you want
        sut.Setup(m => m.LongRunningMethod())
            .Returns("successful");

        Assert.Equal("Ok", sut.Object.MethodUnderTest());
    }
}

我不会声称这些选项中的任何一个都很漂亮，并且在将这种后门放入共享库之前，我会认真考虑安全性。如果可行，接口通常更可取。

不过，这种方法是可行的，而且我认为当您面对不希望进行侵入式重构的测试不友好代码时，这可能是一个公平的折衷方案。