So I have a generic class, with Type as its generic parameter. In this class is a method, which has object parameter called value. Kinda like this:
public class Foo<Type> where Type : IComparable
{
public void Bar(object value)
{
DoSomething((Type)value);
}
}
As you can notice, I need to "DoSomething" with value (stored in object) that I first need to cast to Type. I even have my own overridden cast, which works on its own.
In this specific case, Type is generic, lets call it GenericType, and has this user-defined cast:
public static implicit operator GenericType<T>(T value)
{
return new GenericType<T>(value);
}
and value is an enum, lets say
public enum Number: short
{
Zero = 0,
One = 1,
Two = 2
}
The 'DoSomething((Type)value)' in this case is where Type is GenericType and value is Number.Zero. For some reason, this causes the cast to throw InvalidCastException: Specified cast is not valid. When I try it directly, i mean like..
GenericType<Number> z = (GenericType<Number>)Number.Zero;
..it works (I know, there is not explicit cast even needed). But for some reason, it does not work in the complex example I stated above. Can anyone help me understand and potentially fix that?
Why not just let your class use the generic type?
public class Foo<T> where T : IComparable
{
public void Bar(T value)
{
DoSomething(value);
}
}
No casting needed... and please don't use reserved words to name stuff.
Related
Please have a look at the following code. Why do I get a compile-error?
I don't get it!
Casting is a way of telling the compiler that I know more about the objects than it does. And in this case, I know for fact, that "x" does actually contain an instance of "SomeClass". But the compiler seems to be unwilling to accept that information.
https://dotnetfiddle.net/0DlmXf
public class StrangeConversion
{
public class SomeClass { }
public interface ISomeInterface { }
public class Implementation : SomeClass, ISomeInterface { }
public void Foo<T>() where T : class
{
T x = (T)Factory();
//Compile-error: Cannot convert type 'T' to 'SomeClass'
SomeClass a = (SomeClass)x;
//This is perfectly fine:
SomeClass b = (SomeClass)(object)x;
if (x is SomeClass c)
{
//This works as well and 'c' contains the reference.
}
}
private object Factory()
{
return new Implementation();
}
}
Edit:
#Charles Mager has the correct answer in the comment: There does not seem to be a valid reason. The language designers just didn't want to allow this cast.
I fixed using the as casting e.g.
SomeClass a = x as SomeClass;
This Answer explains is very well https://stackoverflow.com/a/132467/16690008
Essentially it's because it would throw an exception if T is not type of that class
It's hard to make sense of exactly what you're trying to achieve, but it seems like a generic constraint is what you're after
public void Foo<T>()
where T : SomeClass // constrain T to be inheriting from SomeClass
{
T x = Factory<T>(); // x is guaranted to be SomeClass
}
private T Factory<T>()
where T : SomeClass // same here
{
return new Implementation();
}
You constrain the generic to only reference types by specifying where T : class, but the compiler needs to know with certainty if the cast is possible. This means you are asking the compiler to trust that SomeClass can be cast from any reference type you pass to it, which is something it won't do. The microsoft docs state that for the class generic type constraint:
The type argument must be a reference type. This constraint applies also to any class, interface, delegate, or array type.
Its important to note that SomeClass b = (SomeClass)(object)x; works because of the cast to object which is the root of the object hierarchy. But as you can see from the list of supported reference types, SomeClass a = (SomeClass)x; has to account for things such as delegates, array types, etc., at which point the compiler will throw you the error
Don't do SomeClass b = (SomeClass)(object)x;, it is much cleaner to make proper use of type constraints along with the as & is operators which were designed for this exact purpose of type checking and safe casting
Short answer:
This behaviour is correct according to the spec. The spec is just bad here since this might convert a compile-error into a runtime-error.
Long answer:
I did some more research on the matter. This is an oversight in the language's spec.
C# uses the same syntax for two totally different things:
int i = (int)1.9
This converts the double 1.9 to an integer. The value is actually changed.
object o = "abc";
string s = (string) o;
This looks the same, but does not change the object referenced by "o" at all. It does only convert the type of the reference.
When it comes to generics, this kind of ambiguity is an issue:
function f(T x) {
var x = (string) x;
}
What should the language do if T is "int"?
That's why the spec forces the developer to cast to object first:
function f(T x) {
var x = (string)(object)x;
}
Now, the behaviour is clear: X might still be a value-type. But if it is, it will be converted to a reference-type first.
This ambiguity does not exist in my example, since T is guaranteed to be a reference type:
public void Foo<T>() where T : class
Thus the cast to object is not necessary. It could even be harmful if the "where" specifies an actual type. In that case, the forced cast to object might convert a compile-time-error (impossible cast) to a runtime-error.
Unfortunately, the people who created the spec, did not see this issue and did not include it.
This generates an error saying I cannot convert type ClassType
to T. Is there any workaround for this?
Is there any way to specify that the type of this can in fact be converted to T?
public void WorkWith<T>(Action<T> method)
{
method.Invoke((T)this);
}
Two possible solutions:
Not type-safe:
public void WorkWith<T>(Action<T> method)
{
method.Invoke((T)(object)this);
}
This isn't typesafe because you can pass it any method that has a single parameter and no return value, like:
WorkWith((string x) => Console.WriteLine(x));
The typesafe "version" (using generic constraints):
public class MyClass
{
public void WorkWith<T>(Action<T> method) where T : MyClass
{
method.Invoke((T)this);
}
}
The point here is that to be able to cast this to T, the compiler wants to be sure that this is always castable to T (so the need for the constraint). As shown in the not-type-safe example, the "classical" (unsafe) solution used with generics is passing through a cast to object.
public void WorkWith<T>(Action<T> method) where T: ClassType {
method.Invoke((T)this);
}
public interface ILovable<T> where T : IEquatable<T>
{
T Care(T t);
}
public class Me : ILovable<int>
{
public int Care(int i)
{
return i;
}
}
Say I have the above. Now below function fails:
private static void Colour<T>(ILovable<T> me) where T : IEquatable<T>
{
var z = me.Care(1); //cannot convert from 'int' to 'T'
}
What's failing the above piece of code? ILovable<T> has a Care function which intakes a T which is IEquatable<T>. In the above function I'm calling the same Care function and passing T which is int type. int is after all IEquatable<int>.
What am I doing wrong? Is there any work around to get it fixed?
Your method signature does not specify a ILovable<int>, it specifies an ILovable<T>. This, for example, would work:
private static void Colour(ILovable<int> me)
{
var z = me.Care(1); //cannot convert from 'int' to 'T'
}
The problem is the compiler doesn't know that T is an 'int' in your example; it could be any type that meets the constraint. Here is another way that would work:
private static void Colour<T>(ILovable<T> me, T valueToCareAbout) where T : IEquatable<T>
{
var z = me.Care(valueToCareAbout);
}
//use like this
Colour(me, 1);
The error I get is:
Argument type 'int' is not assignable to parameter type 'T'
I'm pretty sure this is because you are defining me as an ILovable<T>. Therefore, it doesn't automatically resolve to the Me type where int is defined as T.
This will fix the error because Me defines T as an int:
private static void Colour<T>(Me me) where T : IEquatable<T>
{
var z = me.Care(1);
}
Well that's because method Colour says that there will be parameter of type ILovable< T > whereas T would be resolved later, so at compile time either I tell method that T is int type.
So either you pass ILovable as parameter and grantee that T is int
void Colour<T>(ILovable<int> me)
or pass type Me directly
void Colour<T>(Me me)
Because otherwise me.Care is expecting type T not int as specific
Change following
private static void Colour<T>(ILovable<T> me) where T : IEquatable<T>
To
private static void Colour<Int32>(ILovable<int> me)
and above will work.
Now the mystry portion
You are getting error in following
private static void Colour<T>(ILovable<T> me) where T : IEquatable<T>
because Care is expecting T, and you are providing int.
It is same as
Care((T)1).
or
T t = (T)1; //This is the cause of error as int cannot be changed to T. Remember Int32 is sealed so T cannot derive from int
Care(t); // This is fine
To make above work, T has to int. To make it so, Colur method syntax should be like
private static void Colour<Int32>(ILovable<int> me)
If you want to pass string to Care, T should be string.
private static void Colour<string>(ILovable<string> me)
{
me.Care("Hello");
}
Now if we have to fix T then question arises why T is required at all in Colour definition.
Answer -> For non taking care of inheritance type in non sealed class.
The short answer is overriding variable of type T inside a generic method (or class) with a more derived type is not possible since compiler doesn't explicitly know T is that more derived type (in our case T is int), because T can be any other more derived type at run time.
Long answer: me variable is of type ILovable<T>. Now me.Care function is expecting parameter of type that is specified on ILovable<T> which is T. Outside the Care function T can be anything that is IEquatable<T>, so int is ok. But inside the function, T has to be just T and not another derived type of IEquatable<T>. Otherwise there will be runtime error for scenarios like this:
private static void Colour<T>(ILovable<T> me) where T : IEquatable<T>
{
var z = me.Care(1);
}
...
Colour("");
Right now, T is string when calling Colour(""). So me is ILovable<string>. So me.Care function expects a string as parameter but provided is an int and that is disaster.
TL;DR: (the title gives it away) Can I "Implicitly drop the generic argument from List<T>" in one single conversion?
A friend asked me today if it would be possible to implicitly convert a List<T> to a non-generic wrapper.
var list = new List<_some_type_>();
ObjectResult result = list;
The method would look something like this:
public static implicit operator ObjectResult(List<T> list) { ... }
Clearly, the T here is not defined, and the implicit operator's method name is the type name, ergo you can't include a generic parameter to the method name unless the type were actually generic, i.e.:
class ObjectResult<T> { ... }
Instead of
class ObjectResult { ... }
The constraints we have with user-defined conversions are (am I missing any?):
Cannot convert to or from a Base type
Cannot convert to or from an interface.
Must make conversion enclosed in one of the two types.
What makes List<T> so hard:
List<T>'s only derivation comes from Object; thus we'd have to convert directly from List<T>
List<T> has only interfaces, again, must be directly from List<T>
List<T>, obviously, is compiled up in the framework, so, it must be from our wrapper
I thought of a 2-step solution where there is a middle man we can convert from (whereas the only middle man with List<T> is Object, and due to rule #1, that's not an option).
public class ObjectResult
{
public static implicit operator ObjectResult(WrapperBase arg) { ... }
}
public class WrapperBase { }
public class ObjectResultWrapper<T> : WrapperBase
{
public static implicit operator ObjectResultWrapper<T>(List<T> arg) { ... }
}
Then, the invoking code would look like this:
var list = new List<int>();
ObjectResultWrapper<int> wrap = list;
ObjectResult result = wrap;
This doesn't solve the problem really, it's only a work around to drop T implicitly (but in two steps, not one). At this point, it'd be easier to have a helper method, and not use user-defined conversions.
There may be arguments against the goal of implicitly dropping the generic argument - I don't have anything else of why he feels this was important. Consider this simply an academic question.
The answer: No, You can't do that with an implicit cast.
Alternatives:
I think the best thing would be a static ObjectWrapper.FromList<T>(List<T>) method.
The wrapper cast is an option as well, though not nearly as elegant.
How about declaring a static "Convert" function instead of trying to declare a conversion operator? Then you could use the compiler's type inference do something like this (calling the conversion method From):
List<int> list = new List<int>();
ObjectResult result = ObjectResult.From(list);
The From method might look like this:
public class ObjectResult
{
//...
public static ObjectResult From<T>(T arg) { return new ObjectResult<T>(arg); }
//...
}
public class ObjectResult<T> : ObjectResult
{
//...
public ObjectResult(T obj) { /* ... some implementation ... */ }
//...
}
I have the following code:
public interface IDrilldown
{
void AddCriteria<T>(T Criterion);
}
public class MyClass<W> : IDrilldown // where W : class
{
void IDrilldown.AddCriteria<T>(T Criterion)
{
W value = Criterion as W;
...
}
}
Unfortunately, the cast I have above will not work unless W has the constaint in the code. I would like to have this using value types. Is it at all possible?
I cannot make W and T the same type. My interface does not have a type associated with it globally, only the internal data types.
This is so that I can have a List all having different T's
I was able to find a way to do it, it's a little hacky but allows it to work:
class MyClass<W> : IDrilldown {
void IDrilldown.AddCriteria<T>(T Criterion) {
if (Criterion is W) {
W value = (W)Convert.ChangeType(Criterion, typeof(W));
// value is W, have fun
// or - as Snowbear pointed out in the comments
W value = (W)(object)Criterion;
// works just as well....
} else {
// value is NOT W and could not be converted.
}
}
}
The only drawback with this is, Convert.ChangeType will use converters to change between internal objects, so string value = (string)Convert.ChangeType(1, typeof(string)) will work and return "1" instead of throwing an exception.
To clarify on how this works, the documentation states:
For the conversion to succeed, value must implement the IConvertible interface, because the method simply wraps a call to an appropriate IConvertible method. The method requires that conversion of value to conversionType be supported.
so for this method to work with custom types you will need to implement the IConvertible interface to convert from one custom type to any other type. In the code sample above, if both T and W are the same type, the Convert.ChangeType will succeed, even if the custom object does not implement IConvertiable.
Would the dynamic keyword help you out?
Something like this:
public interface IDrilldown
{
void AddCriteria<T>(T Criterion);
}
public class MyClass : IDrilldown
{
void IDrilldown.AddCriteria<T>(T criterion)
{
dynamic value = criterion;
// can use typeof() to figure out type if needed...
...
}
}