如何将List.ForEach（）的结果打包到新列表中？

Question

I'm learning C#.

I have two lists of same type.

struct Foo
{
    string fKey;
    Bar bVal;
}
List<Foo> f1; List<Foo> f2;

Now f1 and f2 contains same number of elements, with each fKey in f1 present in f2 also, but in a different position.

What I want to do is to join these two lists on the fKey, do some operation on the Bar values (some kind of merging), and store the results to a new List.

Foo Merge(Foo f1, Foo f2)
{
// Merge
    return new Foo(f1);
}

For this I have reached until here.

f1.Join(f2, oldFoo => oldFoo.fKey, newFoo => newFoo.fKey, (oldFoo, newFoo) => new { myOldFoo = oldFoo, myNewFoo = newFoo } //  Join on the fKey attribute
                        ).ToList().         //  Dump the results to a list
                            ForEach(aPair => Merge(aPair.myNewFoo, aPair.myOldFoo));

In the imperative programming, I would push the return value of Merge() to a new List. How I can do that in this consruct?

Answer 1

Perform your Merge operation within a Select projection, then use ToList to materialize the results into a list.

var l = f1.Join(f2, oldFoo => oldFoo.fKey, newFoo => newFoo.fKey, 
                    (oldFoo, newFoo) => new { myOldFoo = oldFoo, myNewFoo = newFoo })
          .Select(aPair => Merge(aPair.myNewFoo, aPair.myOldFoo))
          .ToList();

Edit : I assume you could combine the Merge into the Join for conciseness:

var l = f1.Join(f2, oldFoo => oldFoo.fKey, newFoo => newFoo.fKey, 
                    (oldFoo, newFoo) => Merge(newFoo, oldFoo))
          .ToList();

Answer 2

这正是ConvertAll()函数的作用。