What do DateTime? and using(var x = new y()) mean?

Question

I don't understand what the following lines mean, please explain them to me.

1.

DateTime? pInsertDate;

At this variable declaration, What does ? mean?

2.

using (TransactionScope scope = new TransactionScope())

At this object creation, What does using mean?

Take a look at this question regarding `using`: http://stackoverflow.com/questions/75401/uses-of-using-in-c — Hans Olsson, Dec 14 '10 at 08:15
You are asking two separate questions that both already have been answered for many times here at Stackoverflow. — Steven, Dec 14 '10 at 08:42

score 6 · Answer 1 · answered Dec 14 '10 at 08:12

6

The ? suffix is syntactic sugar for using Nullable<T>. So your declaration is equivalent to:
```
Nullable<DateTime> pInsertDate;
```
See the MSDN documentation for nullable value types for more information. Basically a nullable value type value can represent any of the values of the non-nullable underlying type (DateTime in this case) as well as the special "null" value. This isn't a null reference, but it's usually used with the same sort of connotations - "no value". For example, a Person class might have a DateTime DateOfBirth property, but a DateTime? DateOfDeath property, which would be null if the person was still alive.
A using statement is a way of automatically calling Dispose on the reference acquired in the first part of the statement at the end of the body. So your code is broadly equivalent to:
```
TransactionScope scope = new TransactionScope();
try
{
    // Body of the using statement
}
finally
{
    if (scope != null)
    {
        scope.Dispose();
    }
}
```
In this case of course we know thatscope won't be null because we're calling a constructor, but that's the general expansion. (You could have used a method call to obtain the transaction scope, for example - in which case it could have returned null, but the generated code won't throw a NullReferenceException.)

answered Dec 14 '10 at 08:12

Jon Skeet

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Jon, I need to ask you something. Is there any way to make an object nullable? I know that the objects are reference types and will be null if not referenced but my question is, is it possible to write myCustomObject? _temp = new myCustomObject() and assign the object to null on ctor if a condition is met? – Pabuc Dec 14 '10 at 08:18
1

@Pabuc: I think you're asking whether a constructor can return null - and the answer is no. Write a static method instead which either returns a reference to a new object or null. – Jon Skeet Dec 14 '10 at 08:21
So you can never write myCustomObject? _temp = new myCustomObject(); :/ – Pabuc Dec 14 '10 at 08:23
@Pabus: Not if `MyCustomObject` is a reference type. What would that even mean? It's already a nullable type, just not a nullable *reference* type... you can already test whether the current value is null, etc. – Jon Skeet Dec 14 '10 at 08:24
(constructor returning null) well, *it can* - both for structs (`T?`), and classes - but only in evil cases ;p – Marc Gravell Dec 14 '10 at 08:33

score 1 · Answer 2 · answered Dec 14 '10 at 08:13

1

this is syntax for Nullable Types

answered Dec 14 '10 at 08:13

Arseny

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score 0 · Answer 3 · answered Dec 14 '10 at 08:15

SomeType? is syntactic sugar for Nullable<SomeType>. It can only be applied to value types (not reference types) and indicates that the variable will also be able to store a value equivalent to what null is for reference types. (Value types cannot be null, so the Nullable<T> type was added to allow for such cases. It's extremely useful in database binding code, where nullable columns are common.)

using (SomeType x = y) { ... } is syntactic sugar for:

SomeType x = y;
try {
    ...
} finally {
    if (x != null)
        ((IDisposable)x).Dispose();
}

This pattern is common when using objects whose classes implement IDisposable, as a way to automatically clean up such objects when they are about to fall out of use.

What do DateTime? and using(var x = new y()) mean?

3 Answers3