4

I have the need to make a class that can hold the value for the following types: int?, decimal?, date, bool? string.

I want to be able to do soemthing like:

var x = new MyClass<int?>()  
x.Value = null;
x.value = 99;         

// or

var y = new MyClass<bool?>();
y.Value = null;
y.Value = true

// and so on

I have been trying to create a class of type T along the lines of:

public class TestClass<T>
{             
    public T? Value{ get; set; }
}

I want to use Value to take any of the allowed types but I can't make T nullable. The error is:

Only non nullable value type could be underlying of Sysytem.Nullable

is there anyway of doing what I'm trying to achieve?

davy
  • 4,474
  • 10
  • 48
  • 71
  • Please see the answer in the post "https://stackoverflow.com/questions/187406/c-sharp-nullable-string-error/54878479#54878479". In upcoming version of c# 8, reference types are non nullable by default. – ANewGuyInTown Feb 26 '19 at 04:34

2 Answers2

13

Try this:

public class TestClass<T> where T : struct
{             
    public T? Value{ get; set; }
}

Classes cannot be nullable as class is a reference type. You have to add constraint to your generic class to allow only structs (which can be nullable).

string is a reference type and your class won't work with string it this case as it can't be nullable.

gzaxx
  • 17,312
  • 2
  • 36
  • 54
  • thanks. so is it possible to use the same class with value and reference types like: var y = new TestClass or new TestClass? – davy Nov 26 '13 at 10:06
  • It is possible as long as you are not trying to force them to be nullable by writing `public T? Value{ get; set; }` – gzaxx Nov 26 '13 at 10:07
5

The problem is that you are passing a nullable into your generic, and then trying to make it nullable.

Try this:

public class TestClass<T>
{             
    public T Value{ get; set; }
}

Now when you do this:

var x = new MyClass<int?>();

Value will be defined as int? so you can use it in the way you want. If you define it as

var x = new MyClass<int>();

Value will be defined as int - and won't accept nulls.

Obsidian Phoenix
  • 4,083
  • 1
  • 22
  • 60