public static int Count<TSource>(this IEnumerable<TSource> source)
{
    if (source == null)
    {
        ThrowHelper.ThrowArgumentNullException(ExceptionArgument.source);
    }
    ICollection<TSource> tSources = source as ICollection<TSource>;
    if (tSources != null)
    {
        return tSources.Count;
    }
    IIListProvider<TSource> tSources1 = source as IIListProvider<TSource>;
    if (tSources1 != null)
    {
        return tSources1.GetCount(false);
    }
    ICollection collections = source as ICollection;
    if (collections != null)
    {
        return collections.Count;
    }
    int num = 0;
    using (IEnumerator<TSource> enumerator = source.GetEnumerator())
    {
        while (enumerator.MoveNext())
        {
            num = checked(num + 1);
        }
    }
    return num;
}

internal class DontTryToCountMe : IReadOnlyCollection<int>
{
    public int Count => throw new NotImplementedException();

    public IEnumerator<int> GetEnumerator()
    {
        var i = 0;
        while (true)
        {
            yield return i++;
        }
    }

    IEnumerator IEnumerable.GetEnumerator() => GetEnumerator();
}

ICollection<int> collection = new ReadOnlyCollection<int>(new int[0]);
collection.Add(42);

public record SomeDto(string SomeName, IEnumerable<SomeOtherType> SomeItems);