IEnumerable<T> interface is covariant, so IEnumerable<string> is IEnumerable<object>.
But why IEnumerable<int> is not IEnumerable<object> while int is object?
Variance conversions like this are only supported if there is an implicit reference or identity conversion between the type parameters. As the spec says:
A type T<Aᵢ, ..., Aᵥ> is variance-convertible to a type T<Bᵢ, ..., Bᵥ> if T is either an interface or a delegate type declared with the variant type parameters T<Xᵢ, ..., Xᵥ>, and for each variant type parameter Xᵢ one of the following holds:
Xᵢ is covariant and an implicit reference or identity conversion exists from Aᵢ to Bᵢ
Xᵢ is contravariant and an implicit reference or identity conversion exists from Bᵢ to Aᵢ
Xᵢ is invariant and an identity conversion exists from Aᵢ to Bᵢ
There is neither a reference not identity conversion between int and object, only a boxing conversion.
You might be wondering what's so special about implicit reference and identity conversions, that they allow variance conversions. Well, these conversions don't require the runtime to do anything at all!
To convert from a string to object, for example, is purely a compile-time matter. Nothing needs to be done to the bits representing the string, in order to turn it into an object. After all, every string is an object.
Because of this, converting from IEnumerable<string> to IEnumerable<object> is trivial too - the runtime does not need to do anything.
This is not true for the int-to-object conversion, however. Because int is a value type and object is a reference type. to maintain these semantics, the runtime needs to create a reference type wrapper around that int in order to do the conversion, "boxing" it.
This means that the runtime cannot just "do nothing" if it wants to convert a IEnumerable<int> to IEnumerable<object>. You would expect to get reference types out of that IEnumerable, not value types, so when and how does the "boxing" happen? When and how should this happen in general, not just in the case of IEnumerable<T>?
I'm sure this in theory can all be figured out, designed, and implemented, but it would certainly not be as trivial as the case with implicit reference conversions. In fact, I reckon it would be a lot more complicated and very drastic changes to both the language and runtime would be needed, which is probably why this is not implemented.
I'm working with a library that has languages represented by objects of a certain type Language whereas in my code I'm using an enum LanguageEnum from another library that I can't change. I'm trying to build an implicit conversion from the enum to the Lanuage class anyway.
My idea: Create a subclass of Lanuage -- let's say SubLanguage and add and implicit conversion from the language enum to this class.
Although I can do that, it doesn't solve my problem. When i use my LanugageEnum where a Language is expected, the compiler complains that there is no conversion although the compiler can convert to a SubLanguage which should actually be sufficient.
The code looks like this:
public void DoSomethingWith(Language lang) {
}
// somewhere else I call it like this
DoSomethingWith(LanguageEnum.German);
Is it possible to solve my problem without using explicit conversions?
If you want an implicit conversion, you need to specify it exactly in the target type, not its subtypes - otherwise the compiler simply won't consider it.
See the relevant section from the C# specification, 6.4.4 User-defined implicit conversions:
A user-defined implicit conversion from type S to type T is processed as follows:
Determine the types S0 and T0. If S or T are nullable types, S0 and T0 are their underlying types, otherwise S0 and T0 are equal to S and T respectively.
Find the set of types, D, from which user-defined conversion operators will be considered. This set consists of S0 (if S0 is a class or struct), the base classes of S0 (if S0 is a class), and T0 (if T0 is a class or struct).
Thus, if you are assigning to a variable or field or parameter of type Language, a conversion needs to be declared there, not in SubLanguage.
But, if I may say so, don't fixate on implicit conversions. In the long run, there's nothing wrong with making operations explicit and visible. Especially if they are hugely representation-changing operations.
I've seen both terms be used almost interchangeably in various online explanations, and most text books I've consulted are also not entirely clear about the distinction.
Is there perhaps a clear and simple way of explaining the difference that you guys know of?
Type conversion (also sometimes known as type cast)
To use a value of one type in a context that expects another.
Nonconverting type cast (sometimes known as type pun)
A change that does not alter the underlying bits.
Coercion
Process by which a compiler automatically converts a value of one type into a value of another type when that second type is required by the surrounding context.
Type Conversion:
The word conversion refers to either implicitly or explicitly changing a value from one data type to another, e.g. a 16-bit integer to a 32-bit integer.
The word coercion is used to denote an implicit conversion.
The word cast typically refers to an explicit type conversion (as opposed to an implicit conversion), regardless of whether this is a re-interpretation of a bit-pattern or a real conversion.
So, coercion is implicit, cast is explicit, and conversion is any of them.
Few examples (from the same source) :
Coercion (implicit):
double d;
int i;
if (d > i) d = i;
Cast (explicit):
double da = 3.3;
double db = 3.3;
double dc = 3.4;
int result = (int)da + (int)db + (int)dc; //result == 9
Usages vary, as you note.
My personal usages are:
A "cast" is the usage of a cast operator. A cast operator instructs the compiler that either (1) this expression is not known to be of the given type, but I promise you that the value will be of that type at runtime; the compiler is to treat the expression as being of the given type, and the runtime will produce an error if it is not, or (2) the expression is of a different type entirely, but there is a well-known way to associate instances of the expression's type with instances of the cast-to type. The compiler is instructed to generate code that performs the conversion. The attentive reader will note that these are opposites, which I think is a neat trick.
A "conversion" is an operation by which a value of one type is treated as a value of another type -- usually a different type, though an "identity conversion" is still a conversion, technically speaking. The conversion may be "representation changing", like int to double, or it might be "representation preserving" like string to object. Conversions may be "implicit", which do not require a cast, or "explicit", which do require a cast.
A "coercion" is a representation-changing implicit conversion.
Casting is the process by which you treat an object type as another type, Coercing is converting one object to another.
Note that in the former process there is no conversion involved, you have a type that you would like to treat as another, say for example, you have 3 different objects that inherit from a base type, and you have a method that will take that base type, at any point, if you know the specific child type, you can CAST it to what it is and use all the specific methods and properties of that object and that will not create a new instance of the object.
On the other hand, coercing implies the creation of a new object in memory of the new type and then the original type would be copied over to the new one, leaving both objects in memory (until the Garbage Collectors takes either away, or both).
As an example consider the following code:
class baseClass {}
class childClass : baseClass {}
class otherClass {}
public void doSomethingWithBase(baseClass item) {}
public void mainMethod()
{
var obj1 = new baseClass();
var obj2 = new childClass();
var obj3 = new otherClass();
doSomethingWithBase(obj1); //not a problem, obj1 is already of type baseClass
doSomethingWithBase(obj2); //not a problem, obj2 is implicitly casted to baseClass
doSomethingWithBase(obj3); //won't compile without additional code
}
obj1 is passed without any casting or coercing (conversion) because it's already of the same type baseClass
obj2 is implicitly casted to base, meaning there's no creation of a new object because obj2 can already be baseClass
obj3 needs to be converted somehow to base, you'll need to provide your own method to convert from otherClass to baseClass, which will involve creating a new object of type baseClass and filling it by copying the data from obj3.
A good example is the Convert C# class where it provides custom code to convert among different types.
According to Wikipedia,
In computer science, type conversion, type casting, type coercion, and type juggling are different ways of changing an expression from one data type to another.
The difference between type casting and type coercion is as follows:
TYPE CASTING | TYPE COERCION
|
1. Explicit i.e., done by user | 1. Implicit i.e., done by the compiler
|
2. Types: | 2. Type:
Static (done at compile time) | Widening (conversion to higher data
| type)
Dynamic (done at run time) | Narrowing (conversion to lower data
| type)
|
3. Casting never changes the | 3. Coercion can result in representation
the actual type of object | as well as type change.
nor representation. |
Note: Casting is not conversion. It is just the process by which we treat an object type as another type. Therefore, the actual type of object, as well as the representation, is not changed during casting.
I agree with #PedroC88's words:
On the other hand, coercing implies the creation of a new object in
memory of the new type and then the original type would be copied over
to the new one, leaving both objects in memory (until the Garbage
Collectors takes either away, or both).
Casting preserves the type of objects. Coercion does not.
Coercion is taking the value of a type that is NOT assignment compatible and converting to a type that is assignment compatible. Here I perform a coercion because Int32 does NOT inherit from Int64...so it's NOT assignment compatible. This is a widening coercion (no data lost). A widening coercion is a.k.a. an implicit conversion. A Coercion performs a conversion.
void Main()
{
System.Int32 a = 100;
System.Int64 b = a;
b.GetType();//The type is System.Int64.
}
Casting allows you to treat a type as if it were of a different type while also preserving the type.
void Main()
{
Derived d = new Derived();
Base bb = d;
//b.N();//INVALID. Calls to the type Derived are not possible because bb is of type Base
bb.GetType();//The type is Derived. bb is still of type Derived despite not being able to call members of Test
}
class Base
{
public void M() {}
}
class Derived: Base
{
public void N() {}
}
Source: The Common Language Infrastructure Annotated Standard by James S. Miller
Now what's odd is that Microsoft's documentation on Casting does not align with the ecma-335 specification definition of Casting.
Explicit conversions (casts): Explicit conversions require a cast
operator. 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.
...This sounds like Coercions not Casting.
For example,
object o = 1;
int i = (int)o;//Explicit conversions require a cast operator
i.GetType();//The type has been explicitly converted to System.Int32. Object type is not preserved. This meets the definition of Coercion not casting.
Who knows? Maybe Microsoft is checking if anybody reads this stuff.
From the CLI standard:
I.8.3.2 Coercion
Sometimes it is desirable to take a value of a type that is not assignable-to a location, and convert
the value to a type that is assignable-to the type of the location. This is accomplished through
coercion of the value. Coercion takes a value of a particular type and a desired type and attempts
to create a value of the desired type that has equivalent meaning to the original value. Coercion
can result in representation change as well as type change; hence coercion does not necessarily
preserve object identity.
There are two kinds of coercion: widening, which never loses information, and narrowing, in
which information might be lost. An example of a widening coercion would be coercing a value
that is a 32-bit signed integer to a value that is a 64-bit signed integer. An example of a
narrowing coercion is the reverse: coercing a 64-bit signed integer to a 32-bit signed integer.
Programming languages often implement widening coercions as implicit conversions, whereas
narrowing coercions usually require an explicit conversion.
Some coercion is built directly into the VES operations on the built-in types (see §I.12.1). All
other coercion shall be explicitly requested. For the built-in types, the CTS provides operations
to perform widening coercions with no runtime checks and narrowing coercions with runtime
checks or truncation, according to the operation semantics.
I.8.3.3 Casting
Since a value can be of more than one type, a use of the value needs to clearly identify which of
its types is being used. Since values are read from locations that are typed, the type of the value
which is used is the type of the location from which the value was read. If a different type is to
be used, the value is cast to one of its other types. Casting is usually a compile time operation,
but if the compiler cannot statically know that the value is of the target type, a runtime cast check
is done. Unlike coercion, a cast never changes the actual type of an object nor does it change the
representation. Casting preserves the identity of objects.
For example, a runtime check might be needed when casting a value read from a location that is
typed as holding a value of a particular interface. Since an interface is an incomplete description
of the value, casting that value to be of a different interface type will usually result in a runtime
cast check.
Below is a posting from the following article:
The difference between coercion and casting is often neglected. I can see why; many languages have the same (or similar) syntax and terminology for both operations. Some languages may even refer to any conversion as “casting,” but the following explanation refers to concepts in the CTS.
If you are trying to assign a value of some type to a location of a different type, you can generate a value of the new type that has a similar meaning to the original. This is coercion. Coercion lets you use the new type by creating a new value that in some way resembles the original. Some coercions may discard data (e.g. converting the int 0x12345678 to the short 0x5678), while others may not (e.g. converting the int 0x00000008 to the short 0x0008, or the long 0x0000000000000008).
Recall that values can have multiple types. If your situation is slightly different, and you only want to select a different one of the value’s types, casting is the tool for the job. Casting simply indicates that you wish to operate on a particular type that a value includes.
The difference at the code level varies from C# to IL. In C#, both casting and coercion look fairly similar:
static void ChangeTypes(int number, System.IO.Stream stream)
{
long longNumber = number;
short shortNumber = (short)number;
IDisposable disposableStream = stream;
System.IO.FileStream fileStream = (System.IO.FileStream)stream;
}
At the IL level they are quite different:
ldarg.0
conv.i8
stloc.0
ldarg.0
conv.i2
stloc.1
ldarg.1
stloc.2
ldarg.1
castclass [mscorlib]System.IO.FileStream
stloc.3
As for the logical level, there are some important differences. What’s most important to remember is that coercion creates a new value, while casting does not. The identity of the original value and the value after casting are the same, while the identity of a coerced value differs from the original value; coersion creates a new, distinct instance, while casting does not. A corollary is that the result of casting and the original will always be equivalent (both in identity and equality), but a coerced value may or may not be equal to the original, and never shares the original identity.
It’s easy to see the implications of coercion in the examples above, as the numeric types are always copied by value. Things get a bit trickier when you’re working with reference types.
class Name : Tuple<string, string>
{
public Name(string first, string last)
: base(first, last)
{
}
public static implicit operator string[](Name name)
{
return new string[] { name.Item1, name.Item2 };
}
}
In the example below, one conversion is a cast, while the other is a coercion.
Tuple<string, string> tuple = name;
string[] strings = name;
After these conversions, tuple and name are equal, but strings is not equal to either of them. You could make the situation slightly better (or slightly more confusing) by implementing Equals() and operator ==() on the Name class to compare a Name and a string[]. These operators would “fix” the comparison issue, but you would still have two separate instances; any modification to strings would not be reflected in name or tuple, while changes to either one of name or tuple would be reflected in name and tuple, but not in strings.
Although the example above was meant to illustrate some differences between casting and coercion, it also serves as a great example of why you should be extremely cautious about using conversion operators with reference types in C#.
What does "From any class-type S to any interface-type T, provided S is not sealed and provided S does not implement T." actually mean?
I came across this in the C# Language Specifications here:
6.2.4 Explicit reference conversions
The explicit reference conversions
are:
...
From any class-type S to any interface-type T, provided S is not
sealed and provided S does not
implement T.
I can understand what "provided S is not sealed" means, but I'm not sure if I understand what "provided S does not implement T" really mean.
For example:
class S {}//not sealed, nor does it implement T
interface T {}
...
T t = (T)new S();//will throw InvalidCastException.
Could it be that it is in the specs only to enumerate all syntactically correct ways of expressing an explicit reference conversion, regardless of whether it will throw an exception or not? Or does it mean some other thing which I do not know (as of now)?
Thanks in advance.
The pun is in the "not sealed" part:
class S {} //not sealed, nor does it implement T
interface T {}
class S2 : S, T { }
S s = new S2(); // S reference to an S2 instance, implicit conversion
T t = (T)s; // OK, s refers to an S2 instance that does implement T
Could it be that it is in the specs only to enumerate all syntactically correct ways of expressing an explicit reference conversion, ...
Yes, the compiler must allow it unless it knows the conversion to be impossible.
If you look at T t = (T)s;, if S was sealed then the compiler could know with certainty that the conversion was impossible. But with an unsealed S, it would have to eliminate the possibility that s is referencing an S2 type, and that is not practical/possible (S2 could be in another assembly).
Are you sure it's not an error in the text?
Provided S does implement T.
Edit: I've found the reference: http://msdn.microsoft.com/en-us/library/aa691291(v=VS.71).aspx
That is explicit reference conversion, it means that is not implicit!
So the sentence "From any class-typeS to any interface-typeT, provided S is not sealed and provided S does not implement T" is correct, it can't be an implicit conversion, so it must be explicit (and is not guaranteed it would succeed).
The spec goes to say:
The explicit reference conversions are those conversions between reference-types that require run-time checks to ensure they are correct.
In other words, the types of conversion listed here are those explicit conversion, where the compiler cannot make the check if such conversion is possible and valid at compile time, due to a lack of explicit relationship between the two types. Such conversion attempt will be resolved during the execution of the program and might result in an exception, if the conversion is impossible.
The canonical example of a run-time explicit conversion that might fail, but also might succeed, would be conversion from interface-type S to interface-type T, where S and T are not related. The success depends on whether the underlying object implements both interfaces or not.
Particular curious example of the case you asked about - from class-type S to interface-type T, where S does not implement T (and is not sealed), would be when S is a COM object, and T is a COM interface not explicitly listed in the type library as implemented by the coclass. In this case, the conversion might fail, but it also might succeed, depending on what the particular IUnknown::QueryInterface implementation of the coclass is. (Note that technically, the coclass in this case is implementing the interface, but is not advertising it to the compiler)
And of course, #Henk Holterman example of S2 : S, T is also a good example where that conversion can succeed.
It seems it's all about user-defined conversions.
You definitely can't cast class S to interface T if S does not implement T.
This question already has answers here:
Why isn't it possible to define implicit cast operator from interface to class?
(2 answers)
Closed 9 years ago.
I'm just wondering if anyone knows the reason why you are not allowed to use interfaces with the implicit or explicit operators?
E.g. this raises compile time error:
public static explicit operator MyPlayer(IPlayer player)
{
...
}
"user-defined conversions to or from an interface are not allowed"
Thanks,
Section 10.9.3 of the C# spec spells this out. The short version is that it's disallowed so that the user can be certain that conversions between reference types and interfaces succeed if and only if the reference type actually implements that interface, and that when that conversion takes place that the same object is actually being referenced.
Defining an implicit or explicit conversion between reference types gives the user the expectation that there will be a change in reference; after all, the same reference cannot be both types. On the other hand, the user does not have the same expectation for conversions between reference types and interface types.
User-defined conversions are not allowed to convert from or to interface-types. In particular, this restriction ensures that no user-defined transformations occur when converting to an interface-type, and that a conversion to an interface-type succeeds only if the object being converted actually implements the specified interface-type.