enum Season { spring, summer, fall, winter }
int a = (int)Season.spring;
Is this a unboxing or just a normal casting?
If this is a unboxing, could you explain why?
I thought this is just a normal casting because 'enum' and 'int' is both a value-type data.
As documented in ECMA-334 11.3.3, C# defines a conversion to and from an enum type and its underlying type:
However, this does not specify whether or not it is an unboxing conversion. That could be deduced, though, from the fact that both the enum and the integer are value-types, therefore no unboxing is involved.
ECMA-335, which defines the CLR, makes it more clear that an enum is not just convertible to an integer, it actually is the integer, there is no conversion done at all:
14.3 Enums
For binding purposes (e.g., for locating a method definition from the method reference used to call it) enums shall be distinct from their underlying type. For all other purposes, including verification and execution of code, an unboxed enum freely interconverts with its underlying type. Enums can be boxed (§13) to a corresponding boxed instance type, but this type is not the same as the boxed type of the underlying type, so
boxing does not lose the original type of the enum.
Related
//Main method
Belle jhbh = new();
IOpera kujb = (IOpera)jhbh;
interface IOpera {
}
interface IFire:IOpera {
}
class Belle :fff,IFire {
}
Can you please tell me, how does the casting work?
To me the following two variants would do the same thing:
1. IOpera kujb = (IOpera)jhbh;
2. IOpera kujb = jhbh;
I mean, at the end of the day It is kujb that decides what member is accessible and what is not, right.
and since, In both cases, kujb points to the same object, what is the point of casting then?
I think I am getting something horribly wrong.
class Belle and interface IOpera are:
two different types
the point of the interface is that it guarantees that what ever implements it will provide the methods declared within it.
what is the point of casting then?
As these are two different types.
IOpera kujb = (IOpera)jhbh;
As msdn says about this:
These kinds of operations are called type conversions. In C#, you can
perform the following kinds of conversions:
Implicit conversions: No special syntax is required because the
conversion always succeeds and no data will be lost. Examples include
conversions from smaller to larger integral types, and conversions
from derived classes to base classes.
Explicit conversions (casts): Explicit conversions require a cast
expression. Casting is required when information might be lost in the
conversion, or when the conversion might not succeed for other
reasons. Typical examples include numeric conversion to a type that
has less precision or a smaller range, and conversion of a base-class
instance to a derived class.
User-defined conversions: User-defined conversions are performed by
special methods that you can define to enable explicit and implicit
conversions between custom types that do not have a base class–derived
class relationship. For more information, see User-defined conversion
operators.
Conversions with helper classes: To convert between non-compatible
types, such as integers and System.DateTime objects, or hexadecimal
strings and byte arrays, you can use the System.BitConverter class,
the System.Convert class, and the Parse methods of the built-in
numeric types, such as Int32.Parse. For more information, see How to
convert a byte array to an int, How to convert a string to a number, and How to convert between hexadecimal strings and numeric > types.
UPDATE:
Upcasting can be done without explicit cast. So this is an example of implicit cast:
Engineer engineer = new Engineer();
IEmployee employee = engineer;
i am stuck in programming terminology here, which is getting me confused and i cannot gather my thoughts on how to actually and correctly express(write) these few MSDN theory sentences from the Common Type System page.
Would anyone help me on this one, i want to understand this!
And if someone would be so kind to write some code and comment on this issue,
it would be awesome and praiseworthy of you!
//This is the text(it is taken from the "Structures" paragraph):
https://msdn.microsoft.com/en-us/library/zcx1eb1e(v=vs.110).aspx#
"For each value type, the common language runtime supplies a corresponding boxed type, which is a class that has the same state and behavior as the value type.
An instance of a value type is boxed when it is passed to a method that accepts a parameter of type System.Object.
It is unboxed (that is, converted from an instance of a class back to an instance of a value type) when control returns from a method call that accepts a value type as a by-reference parameter.
Some languages require that you use special syntax when the boxed type is required; others automatically use the boxed type when it is needed.
When you define a value type, you are defining both the boxed and the unboxed type."
Thank You in advance, best regards!
An object and a value type are stored differently. An object is a pointer to memory in the heap which contains the binary representation of that object. The stack is memory allocated to store pointers and value types. So a function doesn't have a pointer to an integer or bool. It is passed a copy of the actual value.
But if you have a method like this:
string GetString(object o)
{
return o.ToString();
}
That method expects an object, a pointer to a location in memory, even if you pass a value type to it. So in order to do that, the framework has to create an object stored on the heap containing that int so that it can pass a reference (pointer) to that value to the function. That's boxing.
Boxing is implicit. You don't have to call some conversion function to convert an int to an object.
Unboxing occurs when you take that object and cast it as a value type. For example,
object x = 5; //Boxes the value to create an object with a pointer
var y = (int)x; //Unboxes the value, creating an int on the stack.
When you unbox the object stored in the heap and referenced by x is inspected and its value retrieved. Unboxing is explicit. When you convert anything from object to a value type you must specify the type to which you are converting it.
.NET is language agnostic which allows programmers to write code in different languages (which can be compiled to IL), and that code can interact with other code written in different languages.
This feature is provided by the CTS (Common Type System), a standard that specifies how type definitions are represented in the memory and how types are declared, used, and managed in the CLR (Common Language Runtime).
Example
C# has an int data type and VB.NET has a Integer data type. After the compilation, both instances of int and Integer will use the same structure Int32 from CTS.
Can we consider value type conversions like int to float conversion as upcasting and float to int as downcasting? I believe when we talk about upcasting and downcasting, we specifically mean reference conversions.
No, conversions between value types cannot be seen as upcasting or downcasting, as these do not share a hierarchical link (they do not inherit from each other in any way or form).
Upcasting means specifically that you are changing a subtype reference to a super type reference.
Downcasting is the other way around.
No, a float is not a special case (I.E. subclass of) an int. They are different types.
No, up- and downcasting is only valid in the context of inheritance. When you are converting between value types that can hold more or less data you are using plain type conversions. Of that, there are two different types:
1. Implicit conversions. These are conversions that can be made as the conversion is "safe", that is there will be no data lost. For example, you can safely convert an int value to a long value.
2. Explicit conversions. These are conversions that can result in the losing of data and therefore you have to do an explicit cast to do the conversion. For example, converting a long value to an int is unsafe.
For more importation, see: http://msdn.microsoft.com/en-us/library/ms173105.aspx
These are not examples of upcasting and downcasting, as others have described. However, we can talk about widening and narrowing primitive type conversions (for instance, going from int to long is a widening conversion and hence can be done implicitly.) In this particular case (float/int), however, the conversion is a bit different.
There is an implicit conversion from int to float, but the conversion can actually still lose precision, due to the inability of float to represent integral numbers exactly. This is worth knowing, as it bends the general rule that implicit conversions should never lose information. This is really only noticeable if you convert integers to floats, do a bunch of accumulating math with them and then try converting back to an int. In these cases, the approximation error can accumulate, producing incorrect results if you assumed the floats were perfect representations of decimal numbers.
In Nullable micro-optimizations, part one, Eric mentions that Nullable<T> has a strange boxing behaviour that could not be achieved by a similar user-defined type.
What are the special features that the C# language grants to the predefined Nullable<T> type? Especially the ones that could not be made to work on a MyNullable type.
Of course, Nullable<T> has special syntactic sugar T?, but my question is more about semantics.
What I was getting at is: there is no such thing as a boxed nullable. When you box an int, you get a reference to a boxed int. When you box an int?, you get either a null reference or a reference to a boxed int. You never get a boxed int?.
You can easily make your own Optional<T> struct, but you can't implement a struct that has that boxing behaviour. Nullable<T>'s special behaviour is baked into the runtime.
This fact leads to a number of oddities. For example:
C# 4: Dynamic and Nullable<>
C# Reflection: How to get the type of a Nullable<int>?
Cannot change type to nullable in generic method
And FYI there are other ways in which the Nullable<T> type is "magical". For instance, though it is a struct type, it does not satisfy the struct constraint. There's no way for you to make your own struct that has that property.
I found these two in the C# specifications:
The is operator works on T? as it would have on T, and the as operator can convert to nullable types.
Predefined and user-defined operators that operate on non-nullable value types are lifted to the nullable forms of those types.
Now, here are the features that I think are not limited to Nullable<T>:
The value in a switch can be of a nullable type. I don't think this counts, because switch also accepts user-defined implicit conversions that could be defined on a MyNullable type.
Nullable IDisposable types are supported, with a null check being inserted before the generated calls to Dispose(). I don't think this counts, because I could define MyNullable as a class and then it would be the same.
Here is what I am not sure about:
The specs mentions boxing/unboxing and implicit/explicit conversions, but I do not understand whether the same results can be achieved with a MyNullable.
C# lifts operators on nullable types. For example:
int? SumNullableInts(int? a, int? b)
{
return a + b;
}
You would have to do a lot of reflection work in MyNullable<T> to support that, and then the following would compile, where it shouldn't:
MyNullable<List<string>.Enumerator> SumNullableEnumerators(MyNullable<List<string>.Enumerator> a, MyNullable<List<string>.Enumerator> b)
{
return a + b;
}
Do interface variables have value-type or reference-type semantics?
Interfaces are implemented by types, and those types are either value types or reference types. Obviously, both int and string implement IComparable, and int is a value type, and string is a reference type. But what about this:
IComparable x = 42;
IComparable y = "Hello, World!";
(The question I was trying to answer was presumably deleted because it asked whether interfaces are stored on the stack or the heap, and, as we should all be aware, it's more constructive to think of differences between value and reference types in terms of their semantics rather than their implementation. For a discussion, see Eric Lippert's The stack is an implementation detail.)
Usually, as per the existing answers, it is a reference-type and requires boxing; there is an exception though (isn't there always?). In a generic method with a where constraint, it can be both:
void Foo<T>(T obj) where T : ISomeInterface {
obj.SomeMethod();
}
This is a constrained operation, and is not boxed even if it is a value-type. This is achieved via constrained. Instead, the JIT performs the operation as virtual-call for reference-types, and static-call for value-types. No boxing.
This is about understanding boxing and unboxing of types. In your example, the int is boxed upon assignment and a reference to that "box" or object is what is assigned to x. The value type int is defined as a struct which implements IComparable. However, once you use that interface reference to refer to the value type int, it will be boxed and placed on the heap. That's how it works in practice. The fact that using an interface reference causes boxing to occur by definition makes this reference type semantics.
MSDN: Boxing and Unboxing
A variable or field whose type is IComparable is a reference-type variable or field, regardless of the type of the value assigned to that field. This means that x in the sample code is boxed.
A simple test will demonstrate that. The test is based on the fact that you can only unbox a value type to its original type (and the nullable version of that type):
[TestMethod, ExpectedException(typeof(InvalidCastException))]
public void BoxingTest()
{
IComparable i = 42;
byte b = (byte)i; //exception: not allowed to unbox an int to any type other than int
Assert.AreEqual(42, b);
Assert.Fail();
}
EDIT
On top of that, the C# specification specifically defines reference-type as comprising class types, interface types, array types and delegate types.
EDIT 2
As Marc Gravell points out in his answer, a generic type with an interface constraint is a different case. This doesn't cause boxing.
Variables of interface type will have always have either immutable semantics, mutable reference semantics, or "oddball" semantics (something other than normal reference or value semantics). If variable1 and variable2 are both declared as the same interface type, one performs variable2 = variable1, and one never again writes to either variable, the instance referred to by variable1 will always be indistinguishable from the one referred to be variable2 (since it will be the same instance).
Generic types with interface constraints may have immutable semantics, mutable reference semantics, or "quirky" semantics, but may also have mutable value semantics. This can be dangerous if the interface is not documented as having mutable value semantics. Unfortunately, there is no way to constrain an interface to have either immutable semantics or mutable value semantics (meaning that following variable2 = variable1, it should not be possible to change variable1 by writing variable2, nor vice versa). One could add a "struct" constraint along with the interface constraint, but that would exclude classes which have immutable semantics while not excluding structs that have reference semantics.