Question: Greetings everybody. I am sorry, if this was already asked before (searched in vain) or is really very simple, but i just can't get it. The [MSDN definition](http://msdn.microsoft.com/en-us/library/b3h38hb0.aspx) of a Nullable type, states, that it is defined in a following manner:

```
[SerializableAttribute]
public struct Nullable<T>
where T : struct, new()

```

So the question is quite straightforward: How is this definition possible? Or this is just a typo? Every value type already has a default constructor. Indeed, when i try to compile something like this, the compiler reasonably says, that it is illegal to apply both constraints at the same time, because the second one is implicitly included in a first one.

Thanks in advance.

Answer: 
I think it's just a mistake in the documentation. If you look at the `Nullable<T>` type in Reflector or using the "Go To Definition" command in VS, it shows only the `struct` constraint.

---

**EDIT**

I was thinking again about that, and I did a little test :

```
var attributes = typeof(Nullable<>).GetGenericArguments()[0].GenericParameterAttributes;
Console.WriteLine(attributes);

```

This code produces the following output :

> 
> NotNullableValueTypeConstraint, DefaultConstructorConstraint
> 
> 
> 

So according to reflection, the `T` in `Nullable<T>` *does* have the `new()` constraint... Which means that even though it's invalid in C#, it must be valid for the CLR.

So the documentation is both right and wrong : it's true that the `T` in `Nullable<T>` has the "default constructor" constraint, but the C# declaration it shows is wrong...

This is actually not very surprising, since the documentation is generated from assembly metadata (and XML comments of course). There must be a bug in the documentation generator...
If you look at the IL disassembly, you can see that it does have a constructor constraint:

```
.class public sequential ansi serializable sealed beforefieldinit Nullable<valuetype (System.ValueType) .ctor T>

```

If the documentation is generated directly from the assembly metadata, maybe this is the cause?