Get the Type (T) of a Nullable at runtime?

Question

I don't really know what to add to the title, I'd like to get the Type (T) of a Nullable. For example, I got a an object whose Type is Nullable and I'd like to get something like "System.Int32" (the type of course, not the string)

I believe this is a dupe but can't find it, but you're looking for [Nullable.GetUnderlyingType](http://msdn.microsoft.com/en-us/library/system.nullable.getunderlyingtype.aspx). Could be http://stackoverflow.com/questions/5174423/getting-basic-datatype-rather-than-weird-nullable-one-via-reflection-in-c-sha or http://stackoverflow.com/questions/5181494/determine-if-a-generic-param-is-a-nullable-type — user7116, Nov 29 '11 at 15:23
http://blog.dotnetclr.com/archive/2008/03/11/get-underlying-type-from-a-nullable-type.aspx — Oded, Nov 29 '11 at 15:23
The question is ambiguous. Do you have a Type object representing a nullable value type, and you want to know the underlying value type? Or do you have an *instance* of an object of nullable value type and you want to know the underlying value type? The answers to those two questions are completely different. In the first case, you can use GetUnderlyingType. In the second case, either the object instance is boxed or it is not. If it is boxed then *it is not of nullable type*. There is no such thing as a boxed nullable value type. If it is not boxed then *you already know its type*. — Eric Lippert, Nov 29 '11 at 15:40
I had a type representing the object. For an instance, I would have to use Activator.CreateInstance which could be possible but kinda useless imo, since I can get what i want from the Type ! — Guillaume Slashy, Nov 30 '11 at 09:48

Steven Muhr · Accepted Answer · 2011-11-29T15:52:17.290

12

Try this : Nullable.GetUnderylingType(myType)

FYI : myType must be a Type

edited Nov 29 '11 at 15:52

answered Nov 29 '11 at 15:24

Steven Muhr

3,339
28
46

Wow, didn't know this existed, very nice! – Kolky Nov 29 '11 at 15:24
got the following error : Error 1 'System.Nullable' does not contain a definition for 'GetUnderylingType' – Guillaume Slashy Nov 29 '11 at 15:46
My test shows that Nullable.GetUnderlyingType(new Nullable(2)) return null. But using GetType() will work, see my answer below. – Casperah Nov 29 '11 at 15:48
Are you referecing using System; ? – Steven Muhr Nov 29 '11 at 15:50
yep, System.Nullable.GetUnderylingType(...). Anyway, the other answer worked like a charm :) – Guillaume Slashy Nov 29 '11 at 15:52
2

Yes. Nullable.GetUnderlyingType has been added with .Net 2.0. I recommand you use this Method to get your underlyingType. Its cleaner – Steven Muhr Nov 29 '11 at 15:57

score 2 · Answer 2 · answered Nov 29 '11 at 15:27

2

This method should do the trick:

Type GetValueType(Type t)
{
   return t.GetGenericArguments().First();
}

answered Nov 29 '11 at 15:27

Bas

26,772
8
53
86

score 1 · Answer 3 · answered Nov 29 '11 at 15:45

Use GetType() it works as (un)expected.

if (typeof(int?) == typeof(int))
    MessageBox.Show("It works with definitions");
if (new Nullable<int>(2).GetType() == typeof(int))
    MessageBox.Show("It works with instances!");

This sample code will show the last message box. GetType() will return the underlying type.

score 0 · Answer 4 · answered Nov 29 '11 at 15:24

0

A Nullable is a Generic construction, so you should get it from the GenericArguments from the type, like this: typeof(T).GetGenericArguments()[0]

answered Nov 29 '11 at 15:24

Kolky

2,917
1
21
42

Dylan Meador · Answer 5 · 2011-11-29T16:08:53.303

0

System.Nullable.GetUnderlyingType(typeof(Nullable<int>));

edited Nov 29 '11 at 16:08

answered Nov 29 '11 at 15:24

Dylan Meador

2,381
1
19
32

This will throw NullReferenceException if myInt is currently null. – David Ruttka Nov 29 '11 at 15:32

score 0 · Answer 6 · answered Nov 29 '11 at 15:25

Warning: horrible code, no checks, just an example etc:

using System;

static class Program {

    public static void Main(params string[] args) {
        Type t = typeof(int?);
        Type genericArgument = t.GetGenericArguments()[0];
        Console.WriteLine(genericArgument.FullName);
    }
}

Get the Type (T) of a Nullable at runtime?

6 Answers6