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[C#] Nullable types
Posted by Matthijs on Jan 22 2006 @ 20:34 :: 3781 unique visits
C# v2.0 has the ability to "null" value types. This can be useful if you want to know if a value has been initialized (database usage etc).The syntax for a nullable type is T?, that shorthand for System.Nullable<T>.
An instance of the nullable type T? will have two read-only property's:
- bool HasValue;
- T Value;
Obviously the underlying type of the nullable type can't be another nullable type (for example int??) or a reference type (string?).
This type will also beside the two property's include an additional constructor that takes an instance of type T? (T x; for example) to set the initial value:
new T?(x);
When using them, watch out with boxing a nullable type that is not initialized and then trying to unbox it to the underlying type.
CODE: CSHARP
int? a = null;
int? b = 10;
object oa = (object)a;
object ob = (object)b;
int c = (int)oa; // Will cause a System.NullReferenceException
int d = (int)ob; // This will work
//So if unsure cast to the nullable type:
int? e = (int?)oa;
//Obviously casting an int to a nullable type will work:
int? f = (int?)d;
int? b = 10;
object oa = (object)a;
object ob = (object)b;
int c = (int)oa; // Will cause a System.NullReferenceException
int d = (int)ob; // This will work
//So if unsure cast to the nullable type:
int? e = (int?)oa;
//Obviously casting an int to a nullable type will work:
int? f = (int?)d;
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