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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.
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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.
This question already has answers here:
C# nullable string error
(5 answers)
Closed 8 years ago.
Look into following code block:
//Declaring nullable variables.
//Valid for int, char, long...
Nullable<int> _intVar;
Nullable<char> _charVar;
//trying to declare nullable string/object variables
//gives compile time error.
Nullable<string> _stringVar;
Nullable<object> _objVar;
While compiling code compiler gives following error message:
The type 'string'/'object' must be a non-nullable value type in order to use it as parameter 'T' in the generic type or method 'System.Nullable'
I read it several times but still unable to understand. Can anyone clarify this? Why object or string dont support nullable reference type?
object and string are reference types, so they're already nullable. For example, this is already valid:
string x = null;
The Nullable<T> generic type is only for cases where T is a non-nullable value type.
In the declaration for Nullable<T> there is a constraint on T:
public struct Nullable<T> where T : struct
That where T : struct is precisely the part that constrains T to be a non-nullable value type.
Nullable<T> is defined as:
public struct Nullable<T> where T : struct
meaning: it only works on value-type T (excluding Nullable<TSomethingElse> itself).
You cannot use Nullable<T> on reference-types (or on Nullable<T>), but you don't need to since all reference-types (including object and string) are already "nullable", in that you can assign null to them.
string and object are reference types, and therefore are "nullable" already. The Nullable<T> type exists as a wrapper around value types that don't support null out of the box.
string myString = null //fine
int myInt = null //compiler error
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Is there a technical reason why there is no implicit conversion from DBNull to the various nullable and/or sql types? I understand why the conversions don't currently happen, but don't understand why an implicit conversion wasn't created at the time or added in subsequent versions of the framework.
Just to be clear, I'm looking for technical reasons, not "because that's they way they did it" or "I like it that way".
The most straightforward explanation comes from Microsoft's documentation:
CLR nullable types are not intended for storage of database nulls because an ANSI SQL null does not behave the same way as a null reference (or Nothing in Visual Basic).
Well, I don't know about the SqlTypes case, but there definitely are some technical reasons why adding an implicit conversion between DBNull.Value and values of Nullable<T> with HasValue = false wouldn't work.
Remember, DBNull is a reference type, and despite the fact that Nullable<T> acts like a reference type -- by pretending to be able to take on the null value -- it's actually a value type, with value semantics.
In particular, there's a weird edge case when values of type Nullable<T> are boxed. The behavior is special-cased in the runtime to box values of type Nullable<T> to a boxed version of T, not a boxed version of Nullable<T>.
As the MSDN documentation explains it:
When a nullable type is boxed, the common language runtime automatically boxes the underlying value of the Nullable(Of T) object, not the Nullable(Of T) object itself. That is, if the HasValue property is true, the contents of the Value property is boxed. When the underlying value of a nullable type is unboxed, the common language runtime creates a new Nullable(Of T) structure initialized to the underlying value.
If the HasValue property of a nullable type is false, the result of a boxing operation is Nothing. Consequently, if a boxed nullable type is passed to a method that expects an object argument, that method must be prepared to handle the case where the argument is Nothing. When Nothing is unboxed into a nullable type, the common language runtime creates a new Nullable(Of T) structure and initializes its HasValue property to false.
Now we get into a tricky problem: the C# language spec (§4.3.2) says we can't use an unboxing conversion to convert DBNull.Value into Nullable<T>:
For an unboxing conversion to a given nullable-type to succeed at run-time, the value of the source operand must be either null or a reference to a boxed value of the underlying non-nullable-value-type of the nullable-type. If the source operand is a reference to an incompatible object, a System.InvalidCastException is thrown.
And we can't use a user-defined conversion to convert from object to Nullable<T>, either, according to §10.10.3:
It is not possible to directly redefine a pre-defined conversion. Thus, conversion operators are not allowed to convert from or to object because implicit and explicit conversions already exist between object and all other types.
OK, you or I couldn't do it, but Microsoft could just amend the spec, and make it legal, right? I don't think so.
Why? Well, imagine the intended use case: you've got some method that is specified to return object. In practice, it either returns DBNull.Value or int. But how could the compiler know that? All it knows is that the method is specified to return object. And the conversion operator to be applied must be selected at compile time.
OK, so assume that there is some magical operator that can convert from object to Nullable<T>, and the compiler has some way of knowing when it is applicable. (We don't want it to used for every method that is specified to return object -- what should it do if the method actually returns a string?) But we still have an issue: the conversion could be ambiguous! If the method returns either long, or DBNull.Value, and we do int? v = Method();, what should we do when the method returns a boxed long?
Basically, to make this work as intended, you'd have to use the equivalent of dynamic to determine the type at runtime and convert based on the runtime type. But then we've broken another rule: since the actual conversion would only be selected at runtime, there's no guarantee that it would actually succeed. But implicit conversions are not supposed to throw exceptions.
So at this point, it's not only a change to the specified behavior of the language, a potentially significant performance hit, and on top of that it could throw an unexpected exception! That seems like a pretty good reason not to implement it. But if you need one more reason, remember that every feature starts out minus 100 points.
In short: what you really want here can't be done with an implicit conversion anyway. If you want the behavior of dynamic, just use dynamic! This does what you want, and is already implemented in C# 4.0:
object x = 23;
object y = null;
dynamic dx = x;
dynamic dy = y;
int? nx = (int?) dx;
int? ny = (int?) dy;
Console.WriteLine("nx.HasValue = {0}; nx.Value = {1}; ny.HasValue = {2};",
nx.HasValue, nx.Value, ny.HasValue);
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"
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