C＃方法（收益）未检测到返回路径

Question

Please consider the very small method below -:

IEnumerable<ObjectTest> CreateObjectList()
{
    bool aTest = false;
    ObjectTest anObject = null;
    if (aTest == true)
    {
        foreach (var item in aTestList)
        {
            yield return anObject;
        }
    }
}

I feel that this code should not compile. There are paths that do not return as the boolean test fails. The compiler should report-:

Not all code paths return a value.

But this isn't the case - can someone please explain?

(.NET 4.0)

Richard

Answer 1

As yield return wil return an iterator in any case it is guaranteed to never return null . Thus a method that yields has allways a return-value, namely the iterator. Wheather the iterator returns any elements or not is not relevant here.

Maybe this and this may also be interesting.

Answer 2

Just because you're using yield return doesn't mean that it covers all paths through your code that are outside of the scope of the yield return .

Expanding on what HimBromBeere said in the comments, please consider this:

IEnumerable<ObjectTest> CreateObjectList()
{
    bool aTest = false;
    ObjectTest anObject = null;
    List<ObjectTest> objects = new List<ObjectTest>();
    if (aTest == true)
    {
        foreach (var item in aTestList)
        {
            objects.Add(anObject);
        }

        return objects;
    }
}

That is basically what you're doing, only allowing for processing "from within" the foreach part of you code. That above code will throw the same error.

As was said in the comments, the part that you're missing is that when your if (aTest == true) is not valid, the rest of your code doesn't return anything. You can maintain the code that you have above, but just include the following:

if (aTest == true)
{
    ...
}

yield break;

Answer 3

Think about what would happen if aTestList would be empty in the following case:

IEnumerable<ObjectTest> CreateObjectList()
{
    foreach (var item in aTestList)
    {
        yield return item;
    }
}

Do you understand what will happen? The enumerator won't yield anything. That's exactly what happens in the code in your question.

Answer 4

Remember that what you write in C# is not necessarily what is exactly compiled. The yield keyword is one of those "syntactic sugar" things that do not result in exactly what you have written.

Try to open the compiled code with a .Net decompiler (JustCode or dotPeek for instance), and check out the IL (Intermediary Language) result. You will see that what is compiled is not what you might expect.

In short, yield return is just a shortcut for "create a variable, fill it, then return it".

You can see a bit more explanation here : http://www.dotnetperls.com/yield