C#•7d ago

Why `where T: class`

I am reading a book on c# 12. In this example the author includes where T: class but why is that needed? It seems to work fine without

5 Replies

Jason_Bjorn•7d ago

class Program
{
    public static T GetLast<T>(T[] items) => items[^1];

    public static T[] GetNonNullItems<T>(T[] items)
        => Array.FindAll(items, value => value != null);

    public static void PrintArray<T>(T[] array)
    {
        Console.Write("[ ");
        foreach (var e in array)
        {
            Console.Write($"{e} ");
        }

        Console.WriteLine("]");
    }


    public static void Main()
    {
        int[] values = [1, 2, 3];
        int?[] values2 = [1, null, 3];

        string[] words = ["Hello", "World"];
        string?[] words2 = ["Howdy", null, "Partner"];

        PrintArray(GetNonNullItems(values));
        PrintArray(GetNonNullItems(values2));
        PrintArray(GetNonNullItems(words));
        PrintArray(GetNonNullItems(words2));
    }
}

class Program
{
    public static T GetLast<T>(T[] items) => items[^1];

    public static T[] GetNonNullItems<T>(T[] items)
        => Array.FindAll(items, value => value != null);

    public static void PrintArray<T>(T[] array)
    {
        Console.Write("[ ");
        foreach (var e in array)
        {
            Console.Write($"{e} ");
        }

        Console.WriteLine("]");
    }


    public static void Main()
    {
        int[] values = [1, 2, 3];
        int?[] values2 = [1, null, 3];

        string[] words = ["Hello", "World"];
        string?[] words2 = ["Howdy", null, "Partner"];

        PrintArray(GetNonNullItems(values));
        PrintArray(GetNonNullItems(values2));
        PrintArray(GetNonNullItems(words));
        PrintArray(GetNonNullItems(words2));
    }
}

Angius•7d ago

To not accept structs

Pobiega•7d ago

generics are a bit finnicky with how they deal with null. Im sure you are aware that a struct can't be null, so a bool? can't actually be null, its instead handled by a wrapper called Nullable<T> you can look up exactly how it works, but due to structs not being able to be null, there are generic constraints like class, struct, notnull and class? https://learn.microsoft.com/en-us/dotnet/csharp/programming-guide/generics/constraints-on-type-parameters for more info

Jason_Bjorn•7d ago

thank you @ZZZZZZZZZZZZZZZZZZZZZZZZZ @Pobiega

arion•7d ago

Because they already rejected structnt as a constraint name

Gaming

Programming

Why `where T: class`