How to using List RemoveAll method with using interfaces?

Question

I can create list and use RemoveAll to remove some elements, but I need to do this using interfaces. How to do it? List elements is String.

Welcome to StackOverflow. Please read this carefully: http://stackoverflow.com/help/how-to-ask — Jens, Mar 14 '15 at 07:35
Please be more specific. Why do you "need to do this using interfaces"? If `RemoveAll()` works for you, why don't you just use that? What code have you tried? What did that code do? How was that different from what you wanted? Please see http://stackoverflow.com/help/mcve and http://stackoverflow.com/help/how-to-ask for advice on how to present your question in a clear, useful way. — Peter Duniho, Mar 14 '15 at 07:35
This is my homework on programming in university. I need to remove from the list of all the rows with large letters using interfaces. — user3750141, Mar 14 '15 at 07:41
See [Why doesn't IList support AddRange](http://stackoverflow.com/questions/11538259/why-doesnt-ilist-support-addrange), the concept is the same. `IList<>` doesn't support many methods. — xanatos, Mar 14 '15 at 07:41
I'm voting to close this question as off-topic because because it exists other place for homework. — JPBlanc, Mar 14 '15 at 08:20
When you say "interfaces", do you mean `IList` instead of `List`? In that case use [this extension method](http://stackoverflow.com/questions/4086772/list-not-lose-the-reference/4086877#4086877). Or do you need to represent the predicate as an interface? (a dumb idea, but fitting for homework) — CodesInChaos, Mar 14 '15 at 08:22
@JPBlanc no, homework is welcome here, but it needs to be an answerable question that is written in a way people can understand. Don't close for being homework, close for needing editing to be an answerable question. — Kate Gregory, Apr 06 '15 at 21:33
@Kate Gregory homework is a personal work not a community one. Once anyone put a minimum personal reflexion on the top of his homewok exercise, I will no longer detect it as an homework. — JPBlanc, Apr 07 '15 at 03:57

xanatos · Answer 1 · 2015-03-14T08:20:19.350

Here it is what you asked:

// An interface
public interface IMySelector
{
    bool IDontLike(string str);
}

// A class implementing the interface
public class MySelector : IMySelector
{
    public bool IDontLike(string str)
    {
        if (str.StartsWith("foo"))
        {
            return true;
        }

        return false;
    }
}

List<string> list = new List<string> { "foo1", "foo2", "bar1", "bar2" };

// Using the interface
IMySelector selector = new MySelector();

// Begin from last, it will be faster to remove
for (int i = list.Count - 1; i >= 0; i--)
{
    // Your condition
    if (selector.IDontLike(list[i]))
    {
        list.RemoveAt(i);
    }
}

There is an interface, a class implementing the interface and the code that uses the interface to select which elements to remove. Note how I remove the elements from the bottom to the top. It's faster and it requires one less row of code :-) (if you had a for [0... list.Count) you would have if (selector...) { list.RemoveAt(1); i--; })

As a small note, in C# often you use delegates instead of single-method interfaces.

With IEquatable<T>

public class MySelector : IEquatable<string>
{
    public bool Equals(string str)
    {
        // Strange concept of equality... All the 
        // words that start with foo are equal :-)
        if (str.StartsWith("foo"))
        {
            return true;
        }

        return false;
    }
}

List<string> list = new List<string> { "foo1", "foo2", "bar1", "bar2" };

// Using the interface
IEquatable<string> selector = new MySelector();

// Begin from last, it will be faster to remove
for (int i = list.Count - 1; i >= 0; i--)
{
    // Your condition
    if (selector.Equals(list[i]))
    {
        list.RemoveAt(i);
    }
}

In my task you need to inherit from the interface and override the appropriate method. — user3750141, Mar 14 '15 at 07:47
@user3750141 Then you should show us your interface and something of your code. "I can create list and use RemoveAll to remove some elements, but I need to do this using interfaces" doesn't mean anything. — xanatos, Mar 14 '15 at 07:49
I have List ls = new List(); ls.add("dsfsd"); ls.add("DSFSDsd"); I need to remove all elements in list who have uppercase letters. For example ls.Remove(new Compare()), where new Compare() - it's a class, who inherit some interface. — user3750141, Mar 14 '15 at 07:55
For example https://msdn.microsoft.com/en-us/library/ms131190(v=vs.110).aspx — user3750141, Mar 14 '15 at 08:16
@user3750141 There is no standard *interface* for this, because in C# you'd use standard *delegates* for this, either `Predicate` or `Func`. — CodesInChaos, Mar 14 '15 at 08:24

score 0 · Answer 2 · answered Mar 14 '15 at 08:16

Write an extension method. Call it RemoveAll<TItem>. You want to extend the interface IList<TItem> so choose the this-marked parameter accordingly. Also make a Func<TItem, bool> parameter. In the body of the method, foreach, use the delegate instance, and call the IList<>.Remove instance method.

Since this is homework, I think I provided enough details.

How to using List RemoveAll method with using interfaces?

2 Answers2