The reason I ask is that you can cast a nullable type to a regular type with the Value property. That makes me think that the regular type is just wrapped up in the nullable type.
Yes, it is a generic struct:
public struct Nullable<T> where T : struct, new()
This is probably more confusing if you've only seen the T? syntax - but that is just syntactic sugar, the compiler is changing it to Nullable<T>.
Source: http://msdn.microsoft.com/en-us/library/b3h38hb0.aspx , http://msdn.microsoft.com/en-us/library/1t3y8s4s.aspx
Yes - 'Nullable<T> Structure':
Represents an object whose underlying
type is a value type that can also be
assigned null like a reference type.
[BTW, if your curious you can use Reflector to 'look under the hood']
According to MSDN "Nullable types are instances of the System.Nullable<T> struct. A nullable type can represent the correct range of values for its underlying value type, plus an additional null value"
Related
This question already has answers here:
GetType on Nullable Boolean
(2 answers)
Closed 1 year ago.
I need a Nullable type but how do I initialize such a variable? Whenever I try to do this it automatically converts to a normal Int32.
Nullable<Int32> nullableInt32 = new Nullable<Int32>(3);
Console.WriteLine(nullableInt32.GetType()); // gives me System.Int32
Is this a bug? How can I actually initalize the Nullable?
From Microsoft's documentation:
If you want to determine whether an instance is of a nullable value type, don't use the Object.GetType method to get a Type instance to be tested with the preceding code. When you call the Object.GetType method on an instance of a nullable value type, the instance is boxed to Object. As boxing of a non-null instance of a nullable value type is equivalent to boxing of a value of the underlying type, GetType returns a Type instance that represents the underlying type of a nullable value type.
So your int? gets boxed to int, and GetType() is called on the boxed instance.
Unless you know the type at compile time and use typeof, there is no way to get a type of a nullable object:
var type = typeof(int?);
In practice this shouldn't matter because if you don't know the type at compile time, it means you're using some sort of type erasure (i.e. a cast to object), and that means boxing, and nullable value types don't exist there. You can't use polymorphism because that doesn't work with value types.
If you think you have a valid need for this, feel free to explain your use case in the comments.
This question already has answers here:
Why can't I write Nullable<Nullable<int>>?
(5 answers)
Closed 5 years ago.
Does anyone know why this code doesn't compile?
Nullable<Nullable<int>> n = null;
I realize Nullable has a constraint
where T : struct
But Nullable is struct. I also know this constraint has a restriction "The type argument must be a value type. Any value type except Nullable can be specified." (https://learn.microsoft.com/en-us/dotnet/csharp/programming-guide/generics/constraints-on-type-parameters).
So how does it work? Is this solved on compiler level?
The error message is:
The type int? must be a non-nullable value type in order to use it
as parameter 'T' in the generic type or method Nullable<T>
So it must not only be a value type but a non-nullable value type. But Nullable<int> is a nullable value type.
Here's the compiler error CS0453 which also shows this example:
This error occurs when you use a non-value type argument in
instantiating a generic type or method which has the value constraint
on it. It can also occur when you use a nullable value type argument.
Q: Is this solved on compiler level?
Yes, which means it' not very interesting to know how they achieved this constraint. It's an implementation detail of the compiler which doesn't need to use a C# language feature.
Why is not allowed?
Well, what would be the benefit of a Nullable<Nulable<int>>? Nullables were introduced to give value types the opportunity to be null(so undefined, without value). This is already achieved for a Nullable<int>, it can be null. So by nesting it in another nullable you would not get anything. It's not allowed for the same reason why you can't have a Nullable<string>, a string as every other reference type can already be null.
I'm new in C#. In c# I can't set value of a structure to null how can I create a structure with null value support?
Structs and value types can be made nullable by using the Generic Nullable<> class to wrap it. For instance:
Nullable<int> num1 = null;
C# provides a language feature for this by adding a question mark after the type:
int? num1 = null;
Same should work for any value type including structs.
MSDN Explanation: Nullable Types (c#)
You can use Nullable<T> which has an alias in C#. Keep in mind that the struct itself is not really null (The compiler treats the null differently behind the scenes). It is more of an Option type.
Struct? value = null;
As #CodeInChaos mentions Nullable<T> is only boxed when it is in a non-null state.
Nullable Types
Boxing Nullable Types
you can use Nullable<T> for structs, or the shorthand form (?) of the same:
Represents an object whose underlying
type is a value type that can also be
assigned null like a reference type.
struct Foo
{
}
Nullable<Foo> foo2 = null;
Foo? foo = null; //equivalent shorthand form
Since the "Struct" is not a reference type, you cannot assign "null" as usual manner. so you need to use following form to make it "nullable"
[Struct Name]? [Variable Name] = null
eg.
Color? color = null;
then you can assign null for the object and also check the nullability using conditional statements.
Can someone give me a list, or point me to where I can find a list of C# data types that can be a nullable type?
For example:
I know that Nullable<int> is ok
I know that Nullable<byte[]> is not.
I'd like to know which types are nullable and which are not. BTW, I know I can test for this at runtime. However, this is for a code generator we're writing, so I don't have an actual type. I just know that a column is string or int32 (etc).
All value types (except Nullable<T> itself) can be used in nullable types – i.e. all types that derive from System.ValueType (that also includes enums!).
The reason for this is that Nullable is declared something like this:
struct Nullable<T> where T : struct, new() { … }
A type is said to be nullable if it can be assigned a value or can be assigned null, which means the type has no value whatsoever. Consequently, a nullable type can express a value, or that no value exists. For example, a reference type such as String is nullable, whereas a value type such as Int32 is not. A value type cannot be nullable because it has enough capacity to express only the values appropriate for that type; it does not have the additional capacity required to express a value of null.
The Nullable structure supports using only a value type as a nullable type because reference types are nullable by design.
The Nullable class provides complementary support for the Nullable structure. The Nullable class supports obtaining the underlying type of a nullable type, and comparison and equality operations on pairs of nullable types whose underlying value type does not support generic comparison and equality operations.
From Help Docs
http://msdn.microsoft.com/en-us/library/b3h38hb0.aspx
It can be any value type including struct, it cannot be a reference type, as those are inherently nullable already.
Yes:
Int32
double
DateTime
CustomStruct
etc.
No:
string
Array
CustomClass
etc.
For more information, see MSDN: http://msdn.microsoft.com/en-us/library/2cf62fcy(v=VS.80).aspx
Any data type can be nullable.
Nullable works for you because normal int is not nullable, so the conversion to nullable int is necessary.
Nullable<int[]> not works because int[] is already a nullable type.
Nullable does not support types that are already nullable.
Note, that you need to convert a type to be nullable only if he is not already a nullable type.
Value types are not nullable, value types are int, float, double, long etc, as well as structs you create.
While reference types, for example, are already nullable (classes for example).
You can use the ? operator to make the type to be a nullable type.
Note, that even tho it's not a good practice if you would use the question mark operator on a type that is already nullable it will not raise an error.
int? myNullableVar = null;
int[]? MyNullableArr = null; //not necessary but does not raise an error
If I declare a nullable (either via Nullable or the ? symbol) variable from a value type, does it still respect the rules of value types (i.e. pass by value by default, deallocated when it falls out of scope, not when the garbage collector runs) or does it turn into a reference type since it's actually of type Nullable?
The documentation is here:
http://msdn.microsoft.com/en-us/library/b3h38hb0.aspx
As you can see, the documentation describes this as the "nullable structure", indicating that it is a value type. Also, the documentation gives the first lines of the declaration of the type:
public struct Nullable<T> where T : struct, new()
again showing that the type is a value type.
Yes, System.Nullable is a generic Struct, ie value type.
Nullable value types are essentially just a struct wrapper around a value type which allows them to be flagged as null.
something like this:
struct Nullable<T>
{
public T Value;
public bool HasValue;
}
I believe it's actually more complex than that under the hood, and the compiler does lots of nice stuff for you so you can write for example if(myInt == null). But yes, they are still value types.