Given the following class
public class Component<TValue>
{
public TValue? MaxValue { get; set; }
public TValue? MinValue { get; set; }
public TValue Value { get; set; } = default!;
public override string ToString() =>
$"MinValue = {MinValue}, MaxValue = {MaxValue}, Value = {Value}";
}
Why is it that when I create an instance of Component<int> the MinValue and MaxValue properties are int instead of Nullable<int>?
static void Main(string[] args)
{
var instance = new Component<int>();
instance.Value = 42;
Console.WriteLine(instance.ToString());
}
I could get it to behave how I want to by adding where TValue: struct, but I am writing a Blazor component that needs to work with any numeric type (including nullables) so I can't add that constraint.
I'd like to know the logic behind why TValue? for an int should not be compiled as Nullable<Int32>.
You did not constrain your class's type parameter neither to struct nor to class.
When you create new instance of Component with generic type parameter which is not nullable (in your case 'int') all of the properties using this generic type, will become int.
If you want your TValue to be nullable, create instance providing 'int?' as a generic type parameter.
When the type parameter T in syntax T? is not constrained to a struct it takes on the nullable reference type meaning of T?. In the cases where T is instantiated with a reference or interface type then the result will continue to be a nullable reference type. For instance if it's instantiated with string the value is treated as string?. In the case T is instantiated with a value type the ? is effectively dropped as it has no meaning.
If the desire is to have T? mean a nullable value type then T must be constrained to struct
If the desire is to have T? mean Nullable<T> when instantiated with a value type and T? when instiated with a reference type, then unfortunately there is no such syntax for that. It would require significant runtime work to provide
Related
Rephrased question:
I have this equalitycomparer with Generic type constrained to a class
public class ReferenceEqualityComparer<T> : IEqualityComparer<T> where T : class
{
public static ReferenceEqualityComparer<T> Default => new();
public bool Equals(T? x, T? y) => ReferenceEquals(x, y);
public int GetHashCode(T? obj) => RuntimeHelpers.GetHashCode(obj);
}
when a class let's say
public class A
{
public string? P1{get; set;}
}
is consumed by a code generator
[Generator]
public class MyCodeGenerator : ISourceGenerator
{
}
the NetAnalyzer says that string? has a TypeKind of class
but when I do this,
#nullable enable
[TestMethod]
public void RefTest()
{
string? s1 = "adsad";
string? s3 = s1;
Assert.IsTrue(ReferenceEqualityComparer<string?>.Default.Equals(s1, s3));
}
#nullable restore
it says that string?does not match 'class' constraint. Even though the analyzer is telling me that its a class, am I missing something here? or have I misunderstood the concept?
Original question: According to the description of Nullable from Microsoft Documentation, they are classified as structs, but why is it that the CodeAnalyzer is telling me that the TypeKind of string? is a TypeKind.Class?
Here's some context, in a library I'm writing, classes are analyzed for Source Generation (C# 9 Source Generator) which is essentially using .NetAnalyzer. Each of the properties of the class will be checked whether their type is considered as a class. It turns out string? is considered as a Class.
There is a specific constraint for nullable classes in C#, so from constraint standpoint in compile time SomeRefType? does not match T:class, but it will match T:class?.
where T : class The type argument must be a reference type. This constraint applies also to any class, interface, delegate, or array type. In a nullable context in C# 8.0 or later, T must be a non-nullable reference type.
where T : class? The type argument must be a reference type, either nullable or non-nullable. This constraint applies also to any class, interface, delegate, or array type.
So possibly in nullable context you will want to use the second one:
public class ReferenceEqualityComparer<T> : IEqualityComparer<T> where T : class?
Which will not give any warnings neither for string? nor for string.
strings are classes and cannot be used as generic type argument for Nullable because of the generic constraint where T : struct. In your context string? is a nullable reference type, which can be used to incdicate to the compiler that a reference type should not have null as value (variables still can have null as value and should be checked for null values in public APIs, but the compiler will warn you when you use null in a non-nullable context)
In my current project, i have a Value<T> class.
Currently T can be anything, but we must support Null values, so currently we use it in two forms:
Value<string> or Value<int?>
Can I in any way create a Value class that would allow me to specify Value<string> or Value<int>, but where the effect is that Value holds T for classes, but holds T? for structs?
The goal is to avoid the case where a developer specifies Value<int> and we later have problems, because we dont handle Null values properly.
Ie, id like Compiler suport for avoiding errors.
Can I in any way create a Value class that would allow me to specify Value<string> or Value<int>, but where the effect is that Value holds T for classes, but holds T? for structs?
No, because you cannot have "or" logic in generic type constraints.
To achieve "or" logic in generic type constraints, you would have to create two separate generic classes, each with their own generic type constraint. Note that these two classes could inherit from a shared generic base class which has no type constraint at all.
If that base type is abstract, then you can be sure that consumers must have passed one of the derived classes' type constraint check (assuming no one added some other derived classes)
It would be more appropriate to change your expectation instead of trying to make it the way you currently want to.
but we must support null values
If instead of null, you use default(T), then the problem is resolved.
For class types, default(MyClass) effectively resolves to null
For structs, default(MyStruct) returns a struct whose properties all contain their default values
default(MyStruct) is the common way to use the concept of "nullability" without having to use a literal null value.
A usage example:
public class Container<T>
{
private readonly Random _random = new Random();
public T DoSomething(T input)
{
//always returns "nothing"
return default(T);
}
}
public class MyClass
{
public int Value { get; set; }
}
public struct MyStruct
{
public int Value { get; set; }
}
public class Test
{
public bool ContainerReturnsNothing<T>(T input)
{
var container = new Container<T>();
var output = container.DoSomething(input);
return output.Equals(default(T));
}
public void TestAClassAndAStruct()
{
ContainerReturnsNothing(new MyClass() {Value = 1}); // true
ContainerReturnsNothing(new MyStruct() {Value = 1}); // true
}
}
In the TestAClassAndAStruct method you can see that this generic call stack works exactly the same regardless of whether you use a class or a struct.
If you change the container to return input; then ContainerReturnsNothing will return false in both cases.
One thing you may need to be aware of is that a "nothing" struct cannot be differentiated from a struct that was created but whose properties all happen to have default values. If a struct whose properties all happen to have default values, is a meaningful (i.e. not-nothing) struct value in your case, then this is an issue.
A simple example here is int, whose default(int) resolves to 0. If 0 is a meaningful value, then it is therefore not nothing, which means that you cannot use 0 to represent nothingness.
But you can work around that issue by adding a property which is forced to contain a non-default value when a constructor is executed:
public struct MyStruct
{
public bool IsConstructed { get; } // default false
public int Value { get; }
public MyStruct(int myValue)
{
this.IsConstructed = true;
this.Value = myValue;
}
}
default(MyStruct) will always have IsConstructed set to false.
Every instantiated struct will always have IsConstructed set to true.
This avoid the issue since they will never be equal to one another. In other words:
var myStruct = new MyStruct(0);
var isNull = myStruct.Equals(default(MyStruct));
isNull is false because myStruct and default(MyStruct) contain a different value for IsConstructed (true and false, respectively).
If you run this same check using the MyStruct class from the original example, isNull would be true because all the properties match, i.e. myStruct and default(MyStruct) both have a Value property set to 0.
I am trying to make a model class in C# in which i require object/List properties as optional property:
public class Customer
{
[JsonProperty("Custid")]
public string CustId { get; set; }
[JsonProperty("CustName")]
public string CustName { get; set; }
}
public class Store
{
[JsonProperty("id")]
public string Id { get; set; }
[JsonProperty("Name")]
public string? Name { get; set; }
[JsonProperty("Customer")]
public List<Customer>? Customers{ get; set; } *//Error 1*
[JsonProperty("OtherProperty")]
public object? OtherProperty{ get; set; } *//Error 2*
}
The above code is giving error as :-
Error 1: The type 'object' must be a non-nullable value type in order to use it as parameter 'T' in the generic type or method 'Nullable'
Error 2: The type 'List' must be a non-nullable value type in order to use it as parameter 'T' in the generic type or method 'Nullable'
Please Explain me the above scenario and provide me with the alternate solution.
string, List and object are all reference types. Those are nullable by default. The Nullable type (e.g. int? is a shorthand for Nullable<int>) is only used for value types.
In C# 8.0, a new feature was introduced that allows for non-nullable reference types - i.e. reference types that explicitly disallow null assignment. This is an opt-in feature - you can enable it to allow you to more clearly show intent about the references. If you use this, the syntax used to define nullable reference types is the same as for nullable value types:
string nonNullableString = null; // Error
string? nullableString = null; // Ok
Keep in mind that enabling non-nullable reference types means that all of the reference types that aren't followed by ? will be non-nullable; this might require you to make lots of changes in your application.
So there's your two choices. Either enable non-nullable reference types, and then you need to explicitly mark types that you want to have nullable, or stick with nullable reference types, and just use string instead of string? for the same result. I would encourage the use of non-nullable types by default, since it holds some promise for avoiding an entire class of very common programming mistakes.
If you aren't using C# 8:
object? doesn't exists. object is already nullable.
List<Customer>? doesn't exists. List<Customer> is already nullable.
If you want to use nullable reference types you must update your compiler version!
The Nullable<T> type requires that T is a non-nullable value type, for example int or DateTime. Reference types like string or List can already be null. There would be no point in allowing things like Nullable<List<T>> so it is disallowed.
In the following code, I pass a struct into a constructor that is expecting a class. Why does this compile and run without error (and produce the desired output)?
class Program
{
static void Main()
{
var entity = new Foo { Id = 3 };
var t = new Test<IEntity>(entity); // why doesn't this fail?
Console.WriteLine(t.Entity.Id.ToString());
Console.ReadKey();
}
}
public class Test<TEntity> where TEntity : class
{
public TEntity Entity { get; set; }
public Test(TEntity entity)
{
Entity = entity;
}
public void ClearEntity()
{
Entity = null;
}
}
public struct Foo : IEntity
{
public int Id { get; set; }
}
public interface IEntity
{
int Id { get; set; }
}
If I change my Main() method so that it includes a call to ClearEntity(), as shown below, it still generates no error. Why?
static void Main()
{
var entity = new Foo { Id = 3 };
var t = new Test<IEntity>(entity);
Console.WriteLine(t.Entity.Id.ToString());
t.ClearEntity(); // why doesn't this fail?
Console.ReadKey();
}
where TEntity : class forces TEntity to be a reference type, but an interface such as IEntity is a reference type.
See here:
http://msdn.microsoft.com/en-us/library/d5x73970(v=vs.80).aspx
where T : class | The type argument must be a reference type, including any class, interface, delegate, or array type
Regarding your second question, you might think t.ClearEntity() would fail because it's assigning null to a variable whose type is a value type, but that's not the case. The compile-time type of Entity is the reference type IEntity, and the runtime type (after assignment) is the null type. So you never have a variable of type Foo but value null.
from the C# documentation:
where T : class
The type argument must be a reference type, including any class, interface, delegate, or array type. (See note below.)
Because you're passing the struct via an interface, it's still considered a reference type.
Within the .net runtime, every non-nullable value type has an associated reference type (often referred to as a "boxed value type") which derives from System.ValueType. Saying Object Foo = 5; won't actually store an Int32 into Foo; instead it will create a new instance of the reference type associated with Int32 and store a reference to that instance. A class constraint on a generic type specifies that the type in question must be some sort of a reference type, but does not by itself exclude the possibility that the type may be used to pass a reference to a boxed value-type instance. In most contexts outside generic type constraints, interface types are regarded as class types.
It's important to note that not only are boxed value types stored like reference types; they behave like reference types. For example, List<string>.Enumerator is a value type which implements IEnumerator<string>. If one has two variables of type List<string>.Enumerator, copying one to the other will copy the state of the enumeration, such that there will be two separate and independent enumerators which point to the same list. Copying one of those variables to a variable of type IEnumerator<string> will create a new instance of the boxed value type associated with List<string.Enumerator and store in the latter variable a reference to that new object (which will be a third independent enumerator). Copying that variable to another of type IEnumerator<string>, however, will simply store a reference to the existing object (since IEnumerator<string> is a reference type).
The C# language tries to pretend that value types derive from Object, but within the guts of the .net Runtime they really don't. Instead, they're convertible to types which derive from System.ValueType (which in turn derives from Object). The latter types will satisfy a type constraint, even though the former ones will not. Incidentally, despite its name, System.ValueType is actually a class type.
I, likewise, assumed that constraint keyword class meant the same class as the type declaration keyword class, but it doesn't.
As explained in the other answers, the term class here is over-loaded, which seems to me to be a horrible decision for the C# language design. Something like referencetype would have been more helpful.
I am designing a custom attribute class.
public class MyAttr: Attribute
{
public ValueRange ValRange { get; set; }
}
Then I am attempting to assign this attribute to a property in an adjoining class:
public class Foo
{
[MyAttr(ValRange= new ValueRange())]
public string Prop { get; set; }
}
However, the compiler is complaining the following:
'ValRange' is not a valid named attribute argument because it is not a valid attribute parameter type
I also tried converting the ValueRange class to a struct in hopes that become a value type might solve the problem. Is there any way around this?
Is there any way around this?
No.
For more details I refer you to section 17.1.3 of the C# 4 specification, which I reproduce here for your convenience:
The types of positional and named parameters for an attribute class are limited to the attribute parameter types, which are:
One of the following types: bool, byte, char, double, float, int, long, sbyte, short, string, uint, ulong, ushort.
The type object.
The type System.Type.
An enum type, provided it has public accessibility and the types in which it is nested (if any) also have public accessibility.
Single-dimensional arrays of the above types.
A constructor argument or public field which does not have one of these types, cannot be used as a positional or named parameter in an attribute specification.
Remember, the point of an attribute is to at compile time add information to the metadata associated with the entity upon which you've placed the attribute. That means that all the information associated with that attribute must have a well-defined, unambiguous way to serialize it into and out of metadata. By restricting the set of legal types to a small subset of all possible types we ensure that the compiler can always emit legal metadata that the consumer can understand.
Attribute parameter values need to be resolvable at compile time (i.e constants).
See Attribute Parameter Types on MSDN:
Values passed to attributes must be known to the compiler at compile time.
If you can create a ValueRange that is a constant, you can use it.
Is there any way around this?
Yes.
You can have your attribute use a Type property and then use types that implement a defined interface, for which the code that processes that attribute would have to assume, and as such also create an implicit, but hopefully documented, requirement to its clients:
public interface IValueRange {
int Start { get; }
int End { get; }
}
public class MyAttr : Attribute {
// The used type must implement IValueRange
public Type ValueRangeType { get; set; }
}
// ....
public class Foo {
class FooValueRange : IValueRange {
public int Start { get { return 10; } }
public int End { get { return 20; } }
}
[MyAttr(ValueRangeType = typeof(FooValueRange))]
public string Prop { get; set; }
}
This is not unlike many classes in the System.ComponentModel namespace, like DesignerAttribute.
Attribute parameters must be values of the following types (quoting the article):
Simple types (bool, byte, char, short, int, long, float, and double)
string
System.Type
enums
object (The argument to an attribute parameter of type object must be a constant value of one of the above types.)
One-dimensional arrays of any of the above types
Edit: Changed "compile-time constant" to "value", since types and arrays are not constants (thanks to the commenter who pointed this out (and subsequently deleted his comment for some reason...))
Attributes can only receive compile-time-constants as parameters (e.g. 3, "hello", typeof(MyClass), "path to a resource defining whatever non constant data you need").
The last example (passing a type) I gave may help you design a workaround (pass a type implementing an interface with the method you need).