Question

Just one curious question from me, is there any transformation function in linq. I mean if i have List<int> or List<Foo>, i would like to change the elements at say index x or whoever satisfy the condition in Where.

Answer 1

If you’re looking to conditionally project a Foo to another Foo (leaving others untouched), you can do something like:

IEnumerable<Foo> foos = ...

var transformed = foos.Select(foo => myCondition(foo) ? transform(foo) : foo);

On the other hand, if you only want to project Foos that match the condition:

var transformed = foos.Where(foo => myCondition(foo))
                      .Select(foo => transform(foo));

Do note that both of these return new sequences – LINQ isn’t normally used to modify existing collections. You could of course materialize the results into a collection, overwriting an existing variable if necessary.

// assuming the transform is from Foo -> Foo
foos = foos.Select(foo => transform(foo)).ToList();

Since you specifically mention lists, there is another non-LINQ immediate-execution alternative to the first query – the List<T>.ConvertAll method:

List<Foo> foos = ...

// implicitly List<Foo> assuming the transform is from Foo -> Foo
var transformed = foos.ConvertAll
                  (foo => myCondition(foo) ? transform(foo) : foo);

---

EDIT: Sounds like you’re looking for a “ReplaceWhere” method – as far as I know, there is no direct framework method that replaces the elements of a list based on a predicate. It’s easy to write one yourself though:

/// <summary>
/// Replaces items in a list that match the specified predicate,
/// based on the specified selector. 
/// </summary>
public static void ReplaceWhere<T>(this IList<T> list,
                                   Func<T, bool> predicate,
                                   Func<T, T> selector)
{
    // null-checks here.

    for (int i = 0; i < list.Count; i++)
    {
        T item = list[i];

        if (predicate(item))
            list[i] = selector(item);
    }
}

Usage:

List<int> myList = ...
myList.ReplaceWhere(i => i > 0, i => i * i);