I have some C# code with the following array declaration. Note the single ? after Color.
private Color?[,] scratch;
In my investigating I have found that if you you have code such as:
int? a;
The variable a is not initialized. When and where would you use this?
? is just syntactic sugar, it means that the field is Nullable. It is actually short for Nullable<T>.
In C# and Visual Basic, you mark a value type as nullable by using the ? notation after the value type. For example, int? in C# or Integer? in Visual Basic declares an integer value type that can be assigned null.
You can't assign null to value types, int being a value type can't hold null value, int? on the other hand can store null value.
Same is the case with Color, since it is a structure (thus value type) and it can't hold null values, with Color?[,] you are making an array of nullable Color.
For your question:
The variable 'a' is not initialized. When and where would you use this?
By using ? with variable doesn't make it initialized with null or any other value, it is still has to be initialized.
int? a = null;
int b = null;//error since b is not nullable
It means that the type defined is nullable. Check out this link for more info.
With nullables you can do:
int? i = null;
And of course do null checks:
if (i == null)
{
// do stuff here ...
}
Color? refers to a Nullable Color and is equivalent to Nullable<Color>.
Color c = null;//ERROR Color is a struct, cannot be null!
Color? c = null;//OK
The syntax T? is shorthand for System.Nullable, where T is a value type. The two forms are interchangeable.
http://msdn.microsoft.com/en-us/library/1t3y8s4s(v=vs.80).aspx
Its a Nullable Structure
http://msdn.microsoft.com/en-us/library/b3h38hb0.aspx
Also a duplicate https://stackoverflow.com/questions/5407552/what-does-question-mark-after-a-type-name-mean-in-c
This means that the object can have a null-value in Addition to their normal range.
From MSDN:
Nullable types are instances of the Nullable struct. A nullable type can represent the correct range of values for its underlying value type, plus an additional null value. For example, a Nullable, pronounced "Nullable of Int32," can be assigned any value from -2147483648 to 2147483647, or it can be assigned the null value.
...
Example
class NullableExample
{
static void Main()
{
int? num = null;
// Is the HasValue property true?
if (num.HasValue)
{
System.Console.WriteLine("num = " + num.Value);
}
else
{
System.Console.WriteLine("num = Null");
}
// y is set to zero
int y = num.GetValueOrDefault();
// num.Value throws an InvalidOperationException if num.HasValue is false
try
{
y = num.Value;
}
catch (System.InvalidOperationException e)
{
System.Console.WriteLine(e.Message);
}
}
}
The ? operator signifies that the value type immediately preceding the operator may have the value null.
One way to think of it, and by no means a formal definition, is its an "exception" to how the value would "normally" be assigned a value.
A classic use case of the operator is in a simple controller in a web app. Suppose we have a typical DB that stores a list of movies. We can access the details of these movies by adding into the URL the following:
.../Movies/Details?MovieName=TopGun
, or,
.../Movies/Details?MovieName=DieHard
But what if we wanted to see the details of a movie where the value of MovieName is not defined? What will you show on your HTML page? Well, this controller will notify us of a bad request:
Check this out:
// GET: Movies/Details/5
public ActionResult Details(int? id)
{
if (id == null)
{
return new HttpStatusCodeResult(HttpStatusCode.BadRequest);
}
Movie movie = db.Movies.Find(id);
if (movie == null)
{
return HttpNotFound();
}
return View(movie);
}
nullabletag beside you don't know what is?mean?