C#•9mo ago

List extension

I created this extesion

public static class ListUtilExtension
{
    public static void AddOptional<T>(this List<T> list, T? item)
    {
        if (item is null)
        {
            return;
        }
        list.Add(item);
    }
}

public static class ListUtilExtension
{
    public static void AddOptional<T>(this List<T> list, T? item)
    {
        if (item is null)
        {
            return;
        }
        list.Add(item);
    }
}

In my test, to string worked

    [Fact]
    [Description("Should not add null value in list")]
    public void ShouldNotAddNullValue()
    {
        // Setup
        var list = new List<string>();

        // Exercise
        list.AddOptional(null);

        // Verify
        Assert.Empty(list);
    }

    [Fact]
    [Description("Should not add null value in list")]
    public void ShouldNotAddNullValue()
    {
        // Setup
        var list = new List<string>();

        // Exercise
        list.AddOptional(null);

        // Verify
        Assert.Empty(list);
    }

But when I try with int, not worked

    [Fact]
    [Description("Should add value after tried to add null value in list")]
    public void ShouldAddValueAfterTryAddNullValue()
    {
        // Setup
        const int expectedCountInList = 2;
        var list = new List<int>();
        var firstIntToAdd = Random.Shared.Next(1, int.MaxValue);
        var secondIntToAdd = Random.Shared.Next(1, int.MaxValue);
        list.AddOptional(firstIntToAdd);
        list.AddOptional(null);

        // Exercise
        list.AddOptional(secondIntToAdd);

        // Verify
        Assert.Equal(expectedCountInList, list.Count);
    }

    [Fact]
    [Description("Should add value after tried to add null value in list")]
    public void ShouldAddValueAfterTryAddNullValue()
    {
        // Setup
        const int expectedCountInList = 2;
        var list = new List<int>();
        var firstIntToAdd = Random.Shared.Next(1, int.MaxValue);
        var secondIntToAdd = Random.Shared.Next(1, int.MaxValue);
        list.AddOptional(firstIntToAdd);
        list.AddOptional(null);

        // Exercise
        list.AddOptional(secondIntToAdd);

        // Verify
        Assert.Equal(expectedCountInList, list.Count);
    }

Show to me: Argument type 'null' is not assignable to parameter type 'int' What I did wrong?

2 Replies

reflectronic•9mo ago

T? on a generic method, by default, does not let you pass null for structs you need to write two generic methods

public static void Add<T>(this List<T>, T? item) where T : class { … }

public static void Add<T>(this List<T>, T? item) where T : struct { … }

public static void Add<T>(this List<T>, T? item) where T : class { … }

public static void Add<T>(this List<T>, T? item) where T : struct { … }

the code can be identical for both in this case the where T : struct one will use the special Nullable<T> type that allows you to pass null

rcnespoliOP•9mo ago

oh nice! Is there some doc to I see more details about generics? Thank you

Gaming

Programming

List extension