Question

I am sure I am making a foolish mistake here but it beats me. Here is a piece of code:

void Main()
{
    //Key is a parent while the list contains its children
    Dictionary<string,List<string>> d = new Dictionary<string,List<string>>();
    d.Add("1",new List<string>(){"2","3"});//valid.should return false 
    d.Add("2",new List<string>(){"4"});//valid.should return false
    d.Add("3",new List<string>(){"5"});//valid.should return false
    d.Add("4",new List<string>(){"1"});//invalid.should return true 

   IsChildAlreadyAParent("4","2",d);
 }

private bool IsChildAlreadyAParent( string child, string parent, Dictionary<string, List<string>> d )
{           
    if( !d.ContainsKey( child ) || ( d.ContainsKey( child ) && d[child].Count == 0 ))
    {
        return false;
    }

    foreach( string childOfChild in d[child] )
     {
        if( childOfChild == parent )
            return true;

        if( IsChildAlreadyAParent( childOfChild, parent, d ) ) return true;
    }
}

Compiling this gives me this error:

IsChildAlreadyAParent(string, string, System.Collections.Generic.Dictionary<string,System.Collections.Generic.List<string>>)': not all code paths return a value

I have read the code a few times and I cant see how a return condition would be missed. I know I can rectify it by adding a method return statement before the method end but it doesn’t help me understand the issue at hand. Where is the gap??

Answer 1

You might think this executes the loop body exactly once, always returning from the function:

foreach( string childOfChild in d[child] )
{
    if( childOfChild == parent )
        return true;

    return IsChildAlreadyAParent( childOfChild, parent, d );
}

But what if d[child] has no elements at all?

Also, testing only the first child probably isn’t the right solution either.

Better:

foreach( string childOfChild in d[child] )
{
    if( childOfChild == parent ) return true;
    if (IsChildAlreadyAParent( childOfChild, parent, d )) return true;
}
return false;