Generic foreach loop in C#

Question

The compiler, given the following code, tells me "Use of unassigned local variable 'x'." Any thoughts?

public delegate Y Function<X,Y>(X x);

public class Map<X,Y>
{
    private Function<X,Y> F;

    public Map(Function f)
    {
        F = f;
    }

    public Collection<Y> Over(Collection<X> xs){
        List<Y> ys = new List<Y>();
        foreach (X x in xs)
        {
            X x2 = x;//ys.Add(F(x));
        }
        return ys;
    }
}

After fixing the other errors in the code, I don't get that error. What does your code actually look like? — Guffa
– Guffa, Commented Mar 30, 2010 at 18:34
Shouldn't your constructor take an argument of type Function<X, Y> not Function? — Robert Davis
– Robert Davis, Commented Mar 30, 2010 at 18:35
Just wondering, but why would you ever need a class like this? Are you on an old version of .NET? — Juliet
– Juliet, Commented Mar 30, 2010 at 18:37

Lucero · Accepted Answer · 2010-03-30 18:35:20Z

8

After fixing the obvious errors it compiles fine for me.

public delegate Y Function<X,Y>(X x);

public class Map<X,Y>
{
    private Function<X,Y> F;

    public Map(Function<X,Y> f)
    {
        F = f;
    }

    public ICollection<Y> Over(ICollection<X> xs){
        List<Y> ys = new List<Y>();
        foreach (X x in xs)
        {
            X x2 = x;//ys.Add(F(x));
        }
        return ys;
    }
}

answered Mar 30, 2010 at 18:35

Lucero

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Comments

Mehrdad Afshari · Accepted Answer · 2010-03-30 18:34:08Z

2

The language specification defines foreach statement as the equivalent of a while loop, in which the loop variable is assigned to the Current property of the enumerator object. This definitely satisfies the definite assignment rules of any conforming C# compiler for that code snippet. Either you're using a non-conforming compiler or the error is from somewhere else.

answered Mar 30, 2010 at 18:34

Mehrdad Afshari

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Comments

JonH · Accepted Answer · 2010-03-30 18:40:36Z

2

This: public Map(Function f)

Should be:

public Map(Function<X,Y> f)

And this:

public Collection<Y> Over(Collection<X> xs)

Should be:

public ICollection<Y> Over(ICollection<X> xs)

Or:

public List<Y> Over(Collection<X> xs)

answered Mar 30, 2010 at 18:40

JonH

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1 Comment

Matthew Whited Over a year ago

or IEnumerable<Y> then you can toss the list and replace the .Add(...) with yield return ...

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