Question: I have an `IList<T>` that I need to sort, and I would rather not copy the list if possible. I've noticed that `ArrayList` has an `Adapter` static method that wraps the passed list without copying it, but this takes an `IList` and I have an `IList<T>`. Is it safe to cast from a `System.Collections.Generic.IList<T>` to a `System.Collections.IList` and just use the `Adapter` method?

Note that this is .Net 2.0, so LINQ is not an option.

Answer: 
You cannot cast IList(T) to IList.

After some sniffing with Reflector, it seems like ArrayList.Adapter(IList).Sort() will first copy the list to an object array, sort the array and then copy the array back to a list:

```
object[] array = new object[count];
this.CopyTo(index, array, 0, count);
Array.Sort(array, 0, count, comparer);
for (int i = 0; i < count; i++)
{
    this._list[i + index] = array[i];
}

```

You might get boxing overhead if T in your List(T) a value type. 

If you need to alter the sequence of the objects in the list that you have, you can do it similarly:

```
IList<object> unsorted = ...
List<object> sorted = new List<object>(unsorted);
sorted.Sort(); 
for (int i = 0; i < unsorted.Countt; i++)
{
    unsorted[i] = sorted[i];
}

```

If the list is so huge (as in hundreds of million items) that you cannot make an extra copy in memory, I suggest using a List(T) in the first place or implement your favorite in-place sorting algorithm.
Since the Sort method isn't on the IList interface you might consider creating your own:

```
interface ISortableList<T> : IList<T>
{
    void Sort();
    void Sort(IComparer<T> comparer);
}

class SortableList<T> : List<T>, ISortableList<T> { }

/* usage */
void Example(ISortedList<T> list)
{
    list.Sort();
    list.Sort(new MyCustomerComparer());
}

```

In general the parameter type you specify in your method should be the lowest common denominator of members you actually need to call. If you really need to call the Sort() method then your parameter should have that member defined. Otherwise you should probably load it into another object that can do what you want such as:

```
void Example(IList<T> list)
{
    list = new List<T>(list).Sort();
}

```

This should actually be pretty fast, almost certainly faster still than writing your own custom inline sort algorithm.