Count is required when enumerable is not a collection

Extension method WithProgress will throw exception is collection has no items

else if (count < 0) Count is required when enumerable is not a collection .

You receive and IEnumerable and return an IEnumerable implementation named progress enumerable.

Instead of throwing, could you just return the enumerable parameter ?

Suggested implementation :

public static IEnumerable WithProgress(this IEnumerable enumerable, IProgressBar progressBar, int count = -1)
        {
            if (enumerable is ICollection)
            {
                count = ((ICollection)enumerable).Count;
            }
            else if (count < 0)
            {
               return enumerable;
            }

            return new ProgressEnumerable(enumerable, progressBar, count);
        }

[ronaldbarendse]

It’s probably better to use more specific WithProgress extension methods, e.g.:

public static IEnumerable<T> WithProgress<T>(this ICollection<T> collection, IProgressBar progressBar)
{
    return collection.WithProgress<T>(progressBar, collection.Count);
}

public static IEnumerable<T> WithProgress<T>(this IEnumerable<T> enumerable, IProgressBar progressBar, int count)
{ }

So when using ICollection, it automatically uses the count, otherwise you just have to pass it manually (e.g. in case of an IReadOnlyCollection, adding this as additional method would make the signature ambiguous). And if count is less than 0, it should throw an ArgumentOutOfRangeException…

[pieceofsummer]

I was considering to write

    }
    else if (count < 0)
    {
        count = enumerable.Count();
    }

but I’m not sure that is always a better option than materialization (for example, for complex DB queries), and even if any enumerator is guaranteed to be allowed enumerating twice, so it is better to specify that explicitly.