C＃…并非所有代码路径都返回值

Question

I am trying to create a Collection with properties and their respective accessors.

Here is my code:

class SongCollection : List<Song>
{
    private string playedCount;
    private int totalLength;

    public string PlayedCount
    {
        get
        {
            foreach (Song s in this)
            {
                if (s.TimesPlayed > 0)
                {
                    return s.ToString();
                }
            }
        }
    }


    public int TotalLength
    {
        get
        {
            foreach (Song s in this)
            {
                int total = 0;
                total += s.LengthInSeconds;
            }
            return total;
        }
    }
}

I'm receiving the error at the "get" point. It tells me that not all code paths return a value... What exactly does this mean, and what am I missing?

Answer 1

Firstly, the reason you're getting that message is that if this is empty, then the code within the foreach block (which is where the required return statement is) would never be executed.

However, your TotalLength() function would always return the length of the first Song , as you're declaring your variable, setting its value, then returning it within the foreach block. Instead, you'd need to do something like this:

int totalLength = 0;

foreach(Song s in this)
{
    total += s.LengthInSeconds;
}

return totalLength;

Your PlayedCount function suffers from similar issues (if the collection is empty or contains no elements whose TimesPlayed property is greater than 0, then there would be no way for it to return a value), so judging by your comment you could write it this way:

public int PlayedCount()
{
    int total = 0;

    foreach(Song s in this)
    {
        if (s.TimesPlayed > 0)
        {
            total++;
        }
    }

    return total;
}

Answer 2

It means just as it says, not all code paths return a value.

In this case, if your list is empty, then it cannot call return. In a foreach, there must be at least one item for the code to execute. Now, maybe you know that the list will always contain a value, but the compiler can't know that

Answer 3

What would your method return if this did not evaluate?

 if (s.TimesPlayed > 0)
                {
                    return s.ToString();
                }

try using an else to return an empty string or something

Answer 4

The fact that 'this' could have no songs- in that case the loops will not execute at all and there is no implicit return value in C#.

Furthermore, your getters don't really make sense unless you only ever had one song in the collection. You need something more like this:

public int TotalLength()
{
    get
    {
        int total = 0;
        foreach (Song s in this)
        {
            total += s.LengthInSeconds;
        }
        return total;    
    }
}

Finally, without knowing how you keep track of TimesPlayed for each individual song, I wouldn't know how to implement that getter, but I am sure you can figure it out with this much.