Does List<T>.ToList() enumerate the collection

Question

Through the travels in StackOverflow there are many usages of the ToList() extension method for an ICollection (or classes that derive from ICollection<T>).

I have pulled out the good ol' decompiler to have a look at how the ToList() extension method is executed and does it enumerate the collection at anytime. Going through the source I came across:

[__DynamicallyInvokable]
public static List<TSource> ToList<TSource>(this IEnumerable<TSource> source)
{
    if (source == null)
    {
        throw Error.ArgumentNull("source");
    }
    return new List<TSource>(source);
}

Now this is pretty straight forward, so the ToList() extension method just creates a new List<T> with the source object. Digging deeper into the List constructor shows this line.

ICollection<T> ts = collection as ICollection<T>;
//.. ommited code
ts.CopyTo(this._items, 0);

As you would, you would dig into the CopyTo method. This is where I got stuck. After all the digging through method calls we get to the final extern method call

[ReliabilityContract(Consistency.MayCorruptInstance, Cer.MayFail)]
[SecurityCritical]
internal static extern void Copy(Array sourceArray, int sourceIndex, Array destinationArray, int destinationIndex, int length, bool reliable);

My question is (although I cant seem to find it) does the ToList() extension method (ICollection<T>.CopyTo()) iterate through all objects of the Collection (Array) when executed?

Subquestion: Where can I find the source for the extern void Copy() above?

Edit: I have edited the question directly as I realized after that I am trying to get a specification on an Interface and not implementation. Sorry

Answer 1

My question is (although I cant seem to find it) does the ToList() extension method (ICollection<T>.CopyTo()) iterate through all objects of the Collection (Array) when executed?

No, it does not. It just copies the underlying array storage using Array.Copy.

---

Update

Just to make sure my answer is interpreted correctly. ToList() does not enumerate the source collection when it's being called directly on another List<T> or T[] array (or other collection which implements ICollection.CopyTo without enumerator).

When you call ToList<> e.g. on Enumerable.Range(0, 1000).ToList() there will be an enumeration. Enumeration will be also performed when you use any LINQ method, e.g. myList.Select(x => x.PropertyName).ToList() will cause enumeration.

End of update

---

void ICollection.CopyTo(Array array, int arrayIndex)
{
    if (array != null && array.Rank != 1)
    {
        ThrowHelper.ThrowArgumentException(ExceptionResource.Arg_RankMultiDimNotSupported);
    }
    try
    {
        Array.Copy(this._items, 0, array, arrayIndex, this._size);
    }
    catch (ArrayTypeMismatchException)
    {
        ThrowHelper.ThrowArgumentException(ExceptionResource.Argument_InvalidArrayType);
    }
}

which uses unmanaged c++ code to do that in efficient way:

This method is equivalent to the standard C/C++ function memmove, not memcpy.

from Array.Copy Method (Array, Array, Int32)

Answer 2

does the ToList() extension method (ICollection.CopyTo()) iterate through all objects of the Collection

It depends on how ICollection<T> is implemented in particular class.

For example, List<T> implementation just copies array (without iteration), because internally List<T> uses array as a storage. But nothing prevents you (or someone else) to write implementation, which will iterate though the source collection.

Where can I find the source for the extern void Copy() above

methods, marked by MethodImpl(MethodImplOptions.InternalCall), are implemented in unmanaged C++ code. I'm not sure, that there are any opened sources for them. But I suppose, that, ultimately, there's something like memcpy(...).

Answer 3

extern it means that function will be implemented in outside of C# code. It could be implemented in C/C++. More detail :

extern keyword