Extend C# List.Last

Question

The .Last() method for Lists only returns a value. I would like to be able to do something like this.

  List<int> a = new List<int> { 1, 2, 3 };
  a.Last() = 4;

This is my attempt at writing an extension method (it does not compile)

public unsafe static T* mylast<T>(this List<T> a)
{
   return &a[a.Count - 1];
}

Is what I want to do possible?

edit:

This is an example of where I would want to use it.

 shapes.last.links.last.points.last = cursor;   //what I want the code to look like
 //how I had to write it.
 shapes[shapes.Count - 1].links[shapes[shapes.Count - 1].links.Count - 1].points[shapes[shapes.Count - 1].links[shapes[shapes.Count - 1].links.Count - 1].points.Count-1] = cursor;

This is why doing

shapes[shapes.Count-1]

isn't really a solution.

Are you saying you want a method to set the last item in a list to a value specified on the right hand side? — George Duckett, Jul 26 '12 at 15:42
Is a.Add(4) not enough? or you want to change the last element? — Tallmaris, Jul 26 '12 at 15:42
If so just make a new version of `Last` that takes a parameter of type `T` and sets the value within the method its self. — George Duckett, Jul 26 '12 at 15:43

score 7 · Answer 1 · answered Jul 26 '12 at 15:43

7

Just use

a[a.Count-1] = 4;

Or write an extension method

a.SetLast(4);

Even if you could create a faux-extension property, it's not a good idea. This goes double if the solution involves unsafe code.

answered Jul 26 '12 at 15:43

Thom Smith

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Sorry I didn't explain fully the first time, see my edit. I'll look into faux-extension properties. – blooop Jul 26 '12 at 15:56

score 5 · Answer 2 · edited May 23 '17 at 11:55

There are no extension properties in C#. But here is an extension method you can use:

public static class ListEx
{
    public static void SetLast<T>(this IList<T> list, T value)
    {
        if (list == null)
            throw new ArgumentNullException("list");
        if(list.Count == 0)
            throw new ArgumentException(
                "Cannot set last item because the list is empty");

        int lastIdx = list.Count - 1;
        list[lastIdx] = value;
    }

    //and by symmetry
    public static T GetLast<T>(this IList<T> list)
    {
        if (list == null)
            throw new ArgumentNullException("list");
        if (list.Count == 0)
            throw new ArgumentException(
                "Cannot get last item because the list is empty");

        int lastIdx = list.Count - 1;
        return list[lastIdx];
    }
}

Here is how to use it

class Program
{
    static void Main(string[] args)
    {
        List<int> a = new List<int> { 1, 2, 3 };
        a.SetLast(4);
        int last = a.GetLast(); //int last is now 4
        Console.WriteLine(a[2]); //prints 4
    }
}

If you want you can adjust validation behaviour.

Mark Byers · Answer 3 · 2012-07-26T15:49:44.693

You can create an extension method that sets the last element.

For simplicity, this is how it would look without error checking:

public static class ListExtensions
{
    public static void SetLast<T>(this IList<T> source, T value)
    {
        source[source.Count - 1] = value;
    }
}

Of course if you want to do it properly, you would also want error checking:

public static void SetLast<T>(this IList<T> source, T value)
{
    if (source == null)
    {
        throw new ArgumentNullException("source");
    }

    if (source.Count == 0)
    {
        throw new ArgumentException("cannot be empty", "source");
    }

    source[source.Count - 1] = value;
}

score 1 · Accepted Answer · answered Jul 26 '12 at 15:45

I'd recommend Thom Smith's solution, but if you'd really like to have property-like access, why not just use a property?

public class MyList<T> : List<T>
{
    public T Last
    {
        get
        {
            return this[this.Count - 1];
        }
        set
        {
            this[this.Count - 1] = value;
        }
    }
}

Used like:

var m = new MyList<int> { 1, 2, 3 };
m.Last = 4;
Console.WriteLine(m.Last);

No unsafe code, so it's better. Also, it doesn't preclude the use of LINQ's Last method (the compiler can tell the two apart due to how they're used).

Thanks. Your method does what I want to do syntactically. I asked the question to begin with as I was hoping I wouldn't have to do this, but it looks like this is the easiest way after all. — blooop, Jul 26 '12 at 16:18

HatSoft · Answer 5 · 2012-07-26T16:40:05.327

0

public class AdvancedList : List<int>
{
    public int Last
    {
        get { return this[Count - 1]; }
        set
        {
            if(Count >= 1 )
                this[Count - 1] = value;
            else
                Add(value);
        }
    }
}

AdvancedList advancedList = new AdvancedList();
advancedList.Add(100);
advancedList.Add(200);

advancedList.Last = 10;

advancedList.Last = 11;

edited Jul 26 '12 at 16:40

answered Jul 26 '12 at 15:45

HatSoft

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The get is ok but the set will add another value at the bottom of the list instead of swapping it. – Tallmaris Jul 26 '12 at 15:50
@Tallmaris i just tested it does not, please can you tell me a bit more – HatSoft Jul 26 '12 at 15:55
try doing var foo = new AdvancedList(); foo.Last = 3; this will add 3. Now do foo.Last = 4 and you have two elements in foo, 3 and 4. What OP wants is to swap the last element, so keep the list the same length but just change the last int. – Tallmaris Jul 26 '12 at 16:05

Embedd_0913 · Answer 6 · 2012-07-26T16:02:57.830

-2

You may say: If you wana set the last value: even more cool would be to pass a new value:

 public static void SetLast(this List<int> ints, int newVal)
    {
         int lastIndex = ints.Count-1;
         ints[lastIndex] = newVal;
    }

edited Jul 26 '12 at 16:02

answered Jul 26 '12 at 15:43

Embedd_0913

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OP is looking to set the last value. – NominSim Jul 26 '12 at 15:43

Extend C# List.Last

6 Answers6