条件表达式返回类型

Question

我有两个类实现IMyInterface返回类型的异步方法Task<IMyInterface>.

为什么我的return语句收到编译器错误"没有隐式转换类型" return SomeBooleanDeterminedByTheMethod ? Class1 : new Class2();,解决此问题的最佳步骤是什么？

完整方法:

public static async Task<IMyInterface> MyMethodAsync(Subclass1 Class1Child)
{
    var listOfThings = new List<Tuple<int, Class1>>();
    await Task.Run(() =>
    {
        foreach (var item in SomeCollection)
        {
            var DummyClass1 = new Class1() {IntProperty = 0};
            var computationResult = new Tuple<int, Class1>( DummyClass1.IntProperty, DummyClass1);
            listOfThings.Add(computationResult);
        }
    }

    try 
    {
        var returnedClass1 = (from items in listOfThings
                          orderby items.Item1
                          select items.Item2).FirstOrDefault();
        return returnedClass1.BooleanProperty ? returnedClass1 : new Class2();
    }
    catch ... // Not relevant.
}

class Class1 : IMyInterface
{
    public int IntProperty { get; set;}
    public bool BooleanProperty => IntProperty % 2 == 1; // So, in my example, BooleanProperty will return false.
}

class Class2 : IMyInterface
{
    // This class serves as a separate class to indicate a different Type to be used for my program. 
}

interface IMyInterface { } 

Answer 1

正如运营商的文档所述?::

给出条件表达式: condition ? first_expression : second_expression;

first_expression和second_expression的类型必须相同,或者从一种类型到另一种类型必须存在隐式转换.

由于Class1或Class2都不是同一类型,两者之间也不存在隐式转换,因此可以将first_expression或者second_expression转换为接口类型,如下所示:

return SomeBooleanDeterminedByTheMethod ? (IMyInterface)Class1 : new Class2();

然后编译器将知道表达式应该求值为IMyInterface.