Null value for generic class [duplicate]

Question

I have a class like this:

using System; using System.Collections.Generic; using System.Linq; using System.Text; namespace WindowsFormsApplication1 { public class MyList<T> : List<T> { public int SelectedIndex { get; set; } public T CurrentItem { get { if (this.SelectedIndex > this.Count) return null; return this[this.SelectedIndex]; } } } }

I am creating a class that derive from list and create a property for getting the current item.

If SelectedIndex is a wrong value, I am returning null but it has an error

Cannot convert null to type parameter 'T' because it could be a non-nullable value type. Consider using 'default(T)' instead.

I want the return value to be null not default(T).

what should I do?

If you want it to be able to return null then you would either need to make it a nullable type, or place a constraint on T to make it a reference type by using T : class. — Evan Trimboli
– Evan Trimboli, Commented Nov 9, 2016 at 11:15
What do you think default(T) is? For reference-types this evaluates to null. For value-types (e.g. int) this is either zero or whatever the types default-type is. — MakePeaceGreatAgain
– MakePeaceGreatAgain, Commented Nov 9, 2016 at 11:21
Might-be-interesting-to-read: stackoverflow.com/questions/21692193/why-not-inherit-from-listt/… — Maarten
– Maarten, Commented Nov 9, 2016 at 11:22
As an alternative, you can consider throwing an IndexOutOfRangeException instead of returning null. Well, that would already occur if you just return this[this.SelectedIndex]. — Milack27
– Milack27, Commented Nov 9, 2016 at 11:25

Georg · Accepted Answer · 2016-11-09 11:18:32Z

null is an invalid value for value types such as int or double. Therefore, you have to restrict the generic type parameter to classes like so:

public class MyList<T> : List<T> where T : class

Then, the compiler error will disappear.

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