CommonUtils.TryGetCollectionCount.cs

using System;
using System.Collections;
using System.Collections.Generic;

public static class CommonUtils
{
    /// <exception cref="ArgumentNullException">
    /// Thrown when <paramref name="enumerable"/> is <see langword="null"/>.
    /// </exception>
    public static bool TryGetCollectionCount<T>(IEnumerable<T> enumerable, out int count)
    {
        switch (enumerable)
        {
            case null:
                throw new ArgumentNullException(nameof(enumerable));

            case IReadOnlyCollection<T> collection:
                count = collection.Count;
                return true;

            case ICollection<T> collection:
                count = collection.Count;
                return true;

            case ICollection collection:
                count = collection.Count;
                return true;

            default:
                count = default;
                return false;
        }
    }
}

TryGetCollectionCountTests.cs

using System;
using System.Collections;
using System.Collections.Generic;
using NSubstitute;
using NUnit.Framework;
using Shouldly;

public static class TryGetCollectionCountTests
{
    [Test]
    public static void Detects_size_of_IReadOnlyCollection()
    {
        var collection = Substitute.For<IReadOnlyCollection<int>>();
        collection.Count.Returns(42);

        CommonUtils.TryGetCollectionCount(collection, out var count).ShouldBeTrue();
        count.ShouldBe(42);

        collection.ReceivedCalls().ShouldHaveSingleItem();
    }

    [Test]
    public static void Detects_size_of_generic_ICollection()
    {
        var collection = Substitute.For<ICollection<int>>();
        collection.Count.Returns(42);

        CommonUtils.TryGetCollectionCount(collection, out var count).ShouldBeTrue();
        count.ShouldBe(42);

        collection.ReceivedCalls().ShouldHaveSingleItem();
    }

    [Test]
    public static void Detects_size_of_nongeneric_ICollection()
    {
        var collection = Substitute.For<ICollection, IEnumerable<int>>();
        collection.Count.Returns(42);

        CommonUtils.TryGetCollectionCount((IEnumerable<int>)collection, out var count).ShouldBeTrue();
        count.ShouldBe(42);

        collection.ReceivedCalls().ShouldHaveSingleItem();
    }

    [Test]
    public static void Does_not_detect_size_of_non_collection_enumerable()
    {
        var collection = Substitute.For<IEnumerable<int>>();

        CommonUtils.TryGetCollectionCount(collection, out _).ShouldBeFalse();

        collection.ReceivedCalls().ShouldBeEmpty();
    }

    [Test]
    public static void Null_is_not_allowed()
    {
        var ex = Should.Throw<ArgumentNullException>(() =>
            CommonUtils.TryGetCollectionCount<int>(enumerable: null, count: out _));

        ex.ParamName.ShouldBe("enumerable");
    }
}