Convert a for loop nested in a ForEach loop to LINQ

Question

I'm getting a compilation error "not all code paths return a value" on the following code, How come?!

 public class SomeEntity
    {
        public int m_i;
        public SomeEntity(int i)
        {
            m_i = i;
        }

        public override string ToString()
        {
            return m_i.ToString();
        }


        public static int someFunction(int i) { return i + 100; }

        public static IEnumerable GetEntities()
        {
            int [] arr = {1,2,3};
            foreach (int i in arr)
            {

                //        for (int i = 0; i < someArray.Count();i++)
                //            yield return new SomeEntity(someFunction(i));

                // *** Equivalent linq function ***    
                return Enumerable.Range(0, 7).Select(a => new SomeEntity(someFunction(a)));
            }
        }
    }

I can't seem to figure this out..... I tried converting the outer foreach loop to a linq expression

public static IEnumerable GetEntities()
        {
            int [] arr = {1,2,3};

            return arr.Select(Xenv =>
                 Enumerable.Range(0, 7).Select(a => new SomeEntity(someFunction(a)))
                );
        }

but then I just got an error :/

Besides your issue, why use a loop if you will always exit in the first iteration? I hope that's something temporal and you will actually use the yield keyword, right? — Andrew
– Andrew, Commented Sep 1, 2016 at 9:19
if you want to replace the for loop with a Linq expression then you should also remove the loop. You don't use int i anyway — Mong Zhu
– Mong Zhu, Commented Sep 1, 2016 at 9:20

Phillip Ngan · Accepted Answer · 2016-09-01 09:18:32Z

6

Because it is possible that arr is empty and you'll not return inside the foreach loop. Put a return after the foreach loop.

public static IEnumerable GetEntities()
{
    int[] arr = { 1, 2, 3 };
    foreach (int i in arr)
    {

        //        for (int i = 0; i < someArray.Count();i++)
        //            yield return new SomeEntity(someFunction(i));

        // *** Equivalent linq function ***    
        return Enumerable.Range(0, 7).Select(a => new SomeEntity(someFunction(a)));
    }
    return Enumerable.Empty<int>(); // <<<< this is what you need
}

answered Sep 1, 2016 at 9:18

Phillip Ngan

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1 Comment

Hamawi Over a year ago

there is major difference between the yield code he tries to replace and code you suggests. instead of returning IEnumrable<SomeEntity> it returns IEnumarable<IEnumrable<SomeEntity>

Hamawi · Accepted Answer · 2016-09-01 09:34:13Z

1

The yield code you are replacing returned IEnumrable<SomeEntity> while the new code returns IEnumarable<IEnumrable<SomeEntity>>.

you can use SelectMany

public static IEnumerable GetEntities()
        {
            int [] arr = {1,2,3};

            return arr.SelectMany(Xenv =>
                 Enumerable.Range(0, 7).Select(a => new SomeEntity(someFunction(a)))
                );
        }

on side note, you use the old non-generic IEnumerable which prevent .Net compiler from doing type consistency checks. always use the generic one IEnumerable<> with the specific type.

edited Sep 1, 2016 at 9:34

answered Sep 1, 2016 at 9:18

Hamawi

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Comments

jthomperoo · Accepted Answer · 2016-09-01 09:23:02Z

The problem is with the 'foreach' loop: the program cannot assume that the loop will always iterate it through it at least once.

If the array being iterated through was a length 0 the code within the loop would not be called; and therefore the return statement would not be triggered; resulting in the error that not all code paths return a value. In order to fix this you need to put a return statement outside of the loop:

public static IEnumerable GetEntities()
    {
        int [] arr = {1,2,3};
        foreach (int i in arr)
        {

            //        for (int i = 0; i < someArray.Count();i++)
            //            yield return new SomeEntity(someFunction(i));

            // *** Equivalent linq function ***    
            return Enumerable.Range(0, 7).Select(a => new SomeEntity(someFunction(a)));
        }
        //INSERT RETURN STATEMENT HERE
    }

Shakir Ahamed · Accepted Answer · 2016-09-01 09:32:41Z

0

If you are using any return type on your method you have to return anything with type of method in before the final braces of method like below.you no need to return anything if your using return type as void

public static IEnumerable GetEntities()
{
     int [] arr = {1,2,3};
     foreach (int i in arr)
     {

            //        for (int i = 0; i < someArray.Count();i++)
            //            yield return new SomeEntity(someFunction(i));

            // *** Equivalent linq function ***    
            var numcol = Enumerable.Range(0, 7).Select(a => new SomeEntity(someFunction(a)));
     }

    return numcol;
}

answered Sep 1, 2016 at 9:32

Shakir Ahamed

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