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Casting of generic type does not compile

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.

Casting T from and to Generic<T> (C#)

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.

Creating a class or interface for a complex generic type

I am trying to create an alias for a type of list of list of object. Specifically, I want to shorten all the typing I have to do for this type:
IReadOnlyList<IReadOnlyList<MyObject>>
My attempt is demonstrated here:
using System.Collections.Generic;
namespace MyApp
{
class Program
{
public class MyObject
{
public static IMyCollection GetCollection()
{
var a = new List<MyObject>();
a.Add(new MyObject());
var b = new List<IReadOnlyList<MyObject>>();
b.Add(a.AsReadOnly());
return b.AsReadOnly();
}
}
public interface IMyCollection : IReadOnlyList<IReadOnlyList<MyObject>>
{
}
static void Main(string[] args)
{
var collection = MyObject.GetCollection();
}
}
}
Unfortunately, this won't compile. There error is:
Cannot implicitly convert type
'System.Collections.ObjectModel.ReadOnlyCollection<System.Collections.Generic.IReadOnlyList<MyApp.Program.MyObject>>'
to 'MyApp.Program.IMyCollection'.
An explicit conversion exists (are you missing a cast?)
OK, so I'm close. Perhaps explicitly casting? So I change the return statement in GetCollection to
return (IMyCollection)b.AsReadOnly();
That compiles, albeit with a resharper warning: Suspicious cast: there is no type in the solution which is inherited from both 'System.Collections.ObjectModel.ReadOnlyCollection>' and 'MyApp.Program.IMyCollection'
And at runtime, I get an invalid cast exception: Unable to cast object of type 'System.Collections.ObjectModel.ReadOnlyCollection1[System.Collections.Generic.IReadOnlyList1[MyApp.Program+MyObject]]' to type 'IMyCollection'.
OK, I can accept all that. I'm the last person to ask about stuff like covariance and contravariance and stuff like that. But surely there's a way to define and create an object with a short name to stand in for a really long named datatype.
How can I create a type with a really long name and cast to a type with a really short name?
UPDATE:
A co-worker suggested using a using statement.
using IMyCollection= System.Collections.Generic.IReadOnlyList<System.Collections.Generic.IReadOnlyList<MyApp.Program.MyObject>>;
While that would work, it then becomes necessary to do that in every file that uses IMyCollection. Not exactly what I would consider a solution to my goal.

How badly do you want this?
You can manually implement your own wrapper class.
public interface IMyCollection : IReadOnlyList<IReadOnlyList<MyObject>>
{
}
public class MyCollectionImpl : IMyCollection
{
private readonly IReadOnlyList<IReadOnlyList<MyObject>> _wrappedCollection;
public MyCollectionImpl(IReadOnlyList<IReadOnlyList<MyObject>> wrappedCollection)
{
_wrappedCollection = wrappedCollection;
}
public int Count
{
get
{
return _wrappedCollection.Count;
}
}
public IReadOnlyList<MyObject> this[int index]
{
get
{
return _wrappedCollection[index];
}
}
public IEnumerator<IReadOnlyList<MyObject>> GetEnumerator()
{
return _wrappedCollection.GetEnumerator();
}
System.Collections.IEnumerator System.Collections.IEnumerable.GetEnumerator()
{
return _wrappedCollection.GetEnumerator();
}
}
Then you simply create an instance of this:
public class MyObject
{
public static IMyCollection GetCollection()
{
var a = new List<MyObject>();
a.Add(new MyObject());
var b = new List<IReadOnlyList<MyObject>>();
b.Add(a.AsReadOnly());
return new MyCollectionImpl(b.AsReadOnly());
}
}
This seems like a lot of extra work, but I would actually consider this a refactoring step.
I believe that the need to pass around types made up of complex set of generic parameters, is actually a bad smell in your code.
Ask yourself, what are you actually using IMyCollection for? Would you be able to add some specialized methods to this interface to make it easier to use?
Once you've created your own MyCollectionImpl class you can slowly add a number of methods to your IMyCollection interface to simplify it's usage. At some point, you might even get to the stage where you can stop exposing the <IReadonlyList<IReadonlyList<MyObject>> interface.

This has nothing to do with covariance. IMyCollection inherits from IReadOnlyList<IReadOnlyList<MyObject>> so you can cast an instance of IMyCollection to IReadOnlyList<IReadOnlyList<MyObject>> but not the other way around.
If you wanna have some custom conversions then you can create a type instead with the short name you want and declare a conversion from IReadOnlyList<IReadOnlyList<MyObject>> to your type using operator overloading. This really seems unnecessary and unusual way to use operator overloading but it's the only way to do what you want to achieve.

C# casting to object in interface generic

i have a problem that I do not understand about casting in interfaces with generic (probably because covariance and contravariance are not completly clear to me at the moment).
I have an Interface where i define a getter and setter that should accept anything in a typed way (no object as type)
Eg :
public interface IDummy <T>
{
int SomeCommonMethod() ;
T Anything { get; set; }
}
Now i define some concretes implementation of interface defined before.
public class MyStrObj : IDummy <string>
{
private string _stirngVal = string.Empty ;
public int SomeCommonMethod()
{
return 0 ;
}
public string Anything
{
get { return _stirngVal ; }
set { _stirngVal = value ; }
}
}
public class MyFileObj : IDummy <File>
{
private File _file = null ;
public int SomeCommonMethod()
{
return 0 ;
}
public File Anything
{
get { return _file ; }
set { _file = value ; }
}
}
At time all works as expected, but now when try to use this two object their behaviour start becoming confusing for me.
I try to define an object that should be able to consume both previous classes (no matter which type they has in their generics, what matter is that they are IDummy).
public class Consumer
{
public static void Consume ( IDummy<object> obj )
{
//SOME CODE HERE.
}
}
Now if i try this code :
MyStrObj obj = new MyStrObj () ;
Consumer.Consume ( obj ) ;
then compiler notice to me that there are some invalid parameters over Consume method calling (obj sure), but there is not an implicit cast here?
If i try this way instead :
MyStrObj obj = new MyStrObj () ;
Consumer.Consume ( (IDummy<object>)obj ) ;
compiler seems to work as I suppose it should (at time I have not tested if the two calls are equivalent).
Thanks in advance for anyone that can help me to understand this behaviour, and sorry for my english (is not my language).

Your IDummy<T> is not covariant. That is why the implicit conversion of such does not work. If it were covariant, the conversion from more specific generic type to the more general one would work. However in your example you cannot make your interface covariant IDummy<out T>, since it has a property setter with your generic parameter.

The method in your Consumer class needs to be generic.
public class Consumer
{
public static void Consume<T> ( IDummy<T> obj )
{
//SOME CODE HERE.
}
}
Then you can do
var foo = new MyStrObj();
foo.Anything = "Hello";
Consumer.Consume(foo);

In order to achieve what you described using covariance you need to amend your interface to make it covariant.
Covariance allows you to assign more specific types to less specific. This is possible however only for the classes that only return your objects of the template type.
Hence in your interface you need to remove the setter ant mark the T as covariant using the keyword out. You can keep the setter on the classes that implement it though. So your interface would have to look like this:
public interface IDummy<out T>
{
int SomeCommonMethod();
T Anything { get; }
}
Your MyStrObj, MyFileObj and Consumer classes can stay as they are. After the change you can utilize the covariance when calling the Consume method.
MyStrObj obj = new MyStrObj();
obj.Anything = "My string";
Consumer.Consume(obj);

Generics in c# & accessing the static members of T

My question concerns c# and how to access Static members ... Well I don't really know how to explain it (which kind of is bad for a question isn't it?) I will just give you some sample code:
Class test<T>{
int method1(Obj Parameter1){
//in here I want to do something which I would explain as
T.TryParse(Parameter1);
//my problem is that it does not work ... I get an error.
//just to explain: if I declare test<int> (with type Integer)
//I want my sample code to call int.TryParse(). If it were String
//it should have been String.TryParse()
}
}
So thank you guys for your answers (By the way the question is: how would I solve this problem without getting an error). This probably quite an easy question for you!
Edit: Thank you all for your answers!
Though I think the try - catch phrase is the most elegant, I know from my experience with vb that it can really be a bummer. I used it once and it took about 30 minutes to run a program, which later on only took 2 minutes to compute just because I avoided try - catch.
This is why I chose the switch statement as the best answer. It makes the code more complicated but on the other hand I imagine it to be relatively fast and relatively easy to read. (Though I still think there should be a more elegant way ... maybe in the next language I learn)
Though if you have some other suggestion I am still waiting (and willing to participate)

The problem is that TryParse isn't defined on an interface or base class anywhere, so you can't make an assumption that the type passed into your class will have that function. Unless you can contrain T in some way, you'll run into this a lot.
Constraints on Type Parameters

Short answer, you can't.
Long answer, you can cheat:
public class Example
{
internal static class Support
{
private delegate bool GenericParser<T>(string s, out T o);
private static Dictionary<Type, object> parsers =
MakeStandardParsers();
private static Dictionary<Type, object> MakeStandardParsers()
{
Dictionary<Type, object> d = new Dictionary<Type, object>();
// You need to add an entry for every type you want to cope with.
d[typeof(int)] = new GenericParser<int>(int.TryParse);
d[typeof(long)] = new GenericParser<long>(long.TryParse);
d[typeof(float)] = new GenericParser<float>(float.TryParse);
return d;
}
public static bool TryParse<T>(string s, out T result)
{
return ((GenericParser<T>)parsers[typeof(T)])(s, out result);
}
}
public class Test<T>
{
public static T method1(string s)
{
T value;
bool success = Support.TryParse(s, out value);
return value;
}
}
public static void Main()
{
Console.WriteLine(Test<int>.method1("23"));
Console.WriteLine(Test<float>.method1("23.4"));
Console.WriteLine(Test<long>.method1("99999999999999"));
Console.ReadLine();
}
}
I made a static dictionary holding a delegate for the TryParse method of every type I might want to use. I then wrote a generic method to look up the dictionary and pass on the call to the appropriate delegate. Since every delegate has a different type, I just store them as object references and cast them back to the appropriate generic type when I retrieve them. Note that for the sake of a simple example I have omitted error checking, such as to check whether we have an entry in the dictionary for the given type.

To access a member of a specific class or interface you need to use the Where keyword and specify the interface or base class that has the method.
In the above instance TryParse does not come from an interface or base class, so what you are trying to do above is not possible. Best just use Convert.ChangeType and a try/catch statement.
class test<T>
{
T Method(object P)
{
try {
return (T)Convert.ChangeType(P, typeof(T));
} catch(Exception e) {
return null;
}
}
}

One more way to do it, this time some reflection in the mix:
static class Parser
{
public static bool TryParse<TType>( string str, out TType x )
{
// Get the type on that TryParse shall be called
Type objType = typeof( TType );
// Enumerate the methods of TType
foreach( MethodInfo mi in objType.GetMethods() )
{
if( mi.Name == "TryParse" )
{
// We found a TryParse method, check for the 2-parameter-signature
ParameterInfo[] pi = mi.GetParameters();
if( pi.Length == 2 ) // Find TryParse( String, TType )
{
// Build a parameter list for the call
object[] paramList = new object[2] { str, default( TType ) };
// Invoke the static method
object ret = objType.InvokeMember( "TryParse", BindingFlags.InvokeMethod, null, null, paramList );
// Get the output value from the parameter list
x = (TType)paramList[1];
return (bool)ret;
}
}
}
// Maybe we should throw an exception here, because we were unable to find the TryParse
// method; this is not just a unable-to-parse error.
x = default( TType );
return false;
}
}
The next step would be trying to implement
public static TRet CallStaticMethod<TRet>( object obj, string methodName, params object[] args );
With full parameter type matching etc.

This isn't really a solution, but in certain scenarios it could be a good alternative: We can pass an additional delegate to the generic method.
To clarify what I mean, let's use an example. Let's say we have some generic factory method, that should create an instance of T, and we want it to then call another method, for notification or additional initialization.
Consider the following simple class:
public class Example
{
// ...
public static void PostInitCallback(Example example)
{
// Do something with the object...
}
}
And the following static method:
public static T CreateAndInit<T>() where T : new()
{
var t = new T();
// Some initialization code...
return t;
}
So right now we would have to do:
var example = CreateAndInit<Example>();
Example.PostInitCallback(example);
However, we could change our method to take an additional delegate:
public delegate void PostInitCallback<T>(T t);
public static T CreateAndInit<T>(PostInitCallback<T> callback) where T : new()
{
var t = new T();
// Some initialization code...
callback(t);
return t;
}
And now we can change the call to:
var example = CreateAndInit<Example>(Example.PostInitCallback);
Obviously this is only useful in very specific scenarios. But this is the cleanest solution in the sense that we get compile time safety, there is no "hacking" involved, and the code is dead simple.

Do you mean to do something like this:
Class test<T>
{
T method1(object Parameter1){
if( Parameter1 is T )
{
T value = (T) Parameter1;
//do something with value
return value;
}
else
{
//Parameter1 is not a T
return default(T); //or throw exception
}
}
}
Unfortunately you can't check for the TryParse pattern as it is static - which unfortunately means that it isn't particularly well suited to generics.

The only way to do exactly what you're looking for would be to use reflection to check if the method exists for T.
Another option is to ensure that the object you send in is a convertible object by restraining the type to IConvertible (all primitive types implement IConvertible). This would allow you to convert your parameter to the given type very flexibly.
Class test<T>
{
int method1(IConvertible Parameter1){
IFormatProvider provider = System.Globalization.CultureInfo.CurrentCulture.GetFormat(typeof(T));
T temp = Parameter1.ToType(typeof(T), provider);
}
}
You could also do a variation on this by using an 'object' type instead like you had originally.
Class test<T>
{
int method1(object Parameter1){
if(Parameter1 is IConvertible) {
IFormatProvider provider = System.Globalization.CultureInfo.CurrentCulture.GetFormat(typeof(T));
T temp = Parameter1.ToType(typeof(T), provider);
} else {
// Do something else
}
}
}

Ok guys: Thanks for all the fish. Now with your answers and my research (especially the article on limiting generic types to primitives) I will present you my solution.
Class a<T>{
private void checkWetherTypeIsOK()
{
if (T is int || T is float //|| ... any other types you want to be allowed){
return true;
}
else {
throw new exception();
}
}
public static a(){
ccheckWetherTypeIsOK();
}
}

You probably cant do it.
First of all if it should be possible you would need a tighter bound on T so the typechecker could be sure that all possible substitutions for T actually had a static method called TryParse.

You may want to read my previous post on limiting generic types to primitives. This may give you some pointers in limiting the type that can be passed to the generic (since TypeParse is obviously only available to a set number of primitives ( string.TryParse obviously being the exception, which doesn't make sense).
Once you have more of a handle on the type, you can then work on trying to parse it. You may need a bit of an ugly switch in there (to call the correct TryParse ) but I think you can achieve the desired functionality.
If you need me to explain any of the above further, then please ask :)

Best code: restrict T to ValueType this way:
class test1<T> where T: struct
A "struct" here means a value type.
String is a class, not a value type.
int, float, Enums are all value types.
btw the compiler does not accept to call static methods or access static members on 'type parameters' like in the following example which will not compile :(
class MyStatic { public static int MyValue=0; }
class Test<T> where T: MyStatic
{
public void TheTest() { T.MyValue++; }
}
=> Error 1 'T' is a 'type parameter', which is not valid in the given context
SL.

That is not how statics work. You have to think of statics as sort of in a Global class even if they are are spread across a whole bunch of types. My recommendation is to make it a property inside the T instance that can access the necessary static method.
Also T is an actual instance of something, and just like any other instance you are not able to access the statics for that type, through the instantiated value. Here is an example of what to do:
class a {
static StaticMethod1 ()
virtual Method1 ()
}
class b : a {
override Method1 () return StaticMethod1()
}
class c : a {
override Method1 () return "XYZ"
}
class generic<T>
where T : a {
void DoSomething () T.Method1()
}