Cast to generic type in C

Question

I have a Dictionary to map a certain type to a certain generic object for that type  For example   typeof LoginMessage  maps to MessageProcessor lt LoginMessage gt    Now the problem is to retrieve this generic object at runtime from the Dictionary  Or to be more specific  To cast the retrieved object to the specific generic type   I need it to work something like this   Type key   message GetType    MessageProcessor lt key gt  processor   messageProcessors key  as MessageProcessor lt key gt     Hope there is a easy solution to this   Edit  I do not want to use Ifs and switches  Due to performance issues I cannot use reflection of some sort either

User · Answer

You can write a method that takes the type as a generic parameter:

void GenericProcessMessage<T>(T message)
{
    MessageProcessor<T> processor = messageProcessors[typeof(T)]
        as MessageProcessor<T>;

    //  Call method processor or whatever you need to do
}

Then you need a way to call the method with the correct generic argument. You can do this with reflection:

public void ProcessMessage(object message)
{
    Type messageType = message.GetType();
    MethodInfo method = this.GetType().GetMethod("GenericProcessMessage");
    MethodInfo closedMethod = method.MakeGenericMethod(messageType);
    closedMethod.Invoke(this, new object[] {message});
}

User · Answer

Please see if following solution works for you  The trick is to define a base processor interface which takes a base type of message    interface IMessage      class LoginMessage   IMessage      class LogoutMessage   IMessage      class UnknownMessage   IMessage      interface IMessageProcessor       void PrcessMessageBase IMessage msg      abstract class MessageProcessor lt T gt    IMessageProcessor where T   IMessage       public void PrcessMessageBase IMessage msg                ProcessMessage  T msg              public abstract void ProcessMessage T msg       class LoginMessageProcessor   MessageProcessor lt LoginMessage gt        public override void ProcessMessage LoginMessage msg                System Console WriteLine  Handled by LoginMsgProcessor             class LogoutMessageProcessor   MessageProcessor lt LogoutMessage gt        public override void ProcessMessage LogoutMessage msg                System Console WriteLine  Handled by LogoutMsgProcessor             class MessageProcessorTest            lt summary gt          IMessage Type and the IMessageProcessor which would process that type          It can be further optimized by keeping IMessage type hashcode          lt  summary gt      private Dictionary lt Type  IMessageProcessor gt  msgProcessors                                    new Dictionary lt Type  IMessageProcessor gt         bool processorsLoaded   false       public void EnsureProcessorsLoaded                 if  processorsLoaded                        var processors                   from processorType in Assembly GetExecutingAssembly   GetTypes                   where processorType IsClass  amp  amp   processorType IsAbstract  amp  amp                        processorType GetInterface typeof IMessageProcessor  Name     null                 select Activator CreateInstance processorType                foreach  IMessageProcessor msgProcessor in processors                                MethodInfo processMethod   msgProcessor GetType   GetMethod  ProcessMessage                    msgProcessors Add processMethod GetParameters   0  ParameterType  msgProcessor                              processorsLoaded   true                       public void ProcessMessages                 List lt IMessage gt  msgList   new List lt IMessage gt             msgList Add new LoginMessage             msgList Add new LogoutMessage             msgList Add new UnknownMessage              foreach  IMessage msg in msgList                        ProcessMessage msg                        public void ProcessMessage IMessage msg                EnsureProcessorsLoaded            IMessageProcessor msgProcessor   null          if msgProcessors TryGetValue msg GetType    out msgProcessor                         msgProcessor PrcessMessageBase msg                     else                       System Console WriteLine  Processor not found                         public static void Test                 new MessageProcessorTest   ProcessMessages

User · Answer

Does this work for you       interface IMessage       void Process object source      class LoginMessage   IMessage       public void Process object source                 abstract class MessageProcessor       public abstract void ProcessMessage object source  object type      class MessageProcessor lt T gt    MessageProcessor where T  IMessage       public override void ProcessMessage object source  object o                 if    o is T                 throw new NotImplementedException                      ProcessMessage source   T o              public void ProcessMessage object source  T type                type Process source             class Program       static void Main string   args                Dictionary lt Type  MessageProcessor gt  messageProcessors   new Dictionary lt Type  MessageProcessor gt             messageProcessors Add typeof string   new MessageProcessor lt LoginMessage gt              LoginMessage message   new LoginMessage            Type key   message GetType            MessageProcessor processor   messageProcessors key           object source   null          processor ProcessMessage source  message             This gives you the correct object  The only thing I am not sure about is whether it is enough in your case to have it as an abstract MessageProcessor   Edit  I added an IMessage interface  The actual processing code should now become part of the different message classes that should all implement this interface

User · Answer

The following seems to work as well  and it s a little bit shorter than the other answers   T result    T Convert ChangeType otherTypeObject  typeof T

User · Answer

I struggled to solve a similar problem around data table classes instead of messages  The root issue mentioned above of casting a non-generic version of the class to a derived generic version was the same    In order to allow injection into a portable class library which did not support database libraries  I introduced a set of interface classes  with the intent that I could pass a type and get a matching generic  It ended up needing to implement a generic method      Interface for injection public interface IDatabase          Original  non-functional signature      IDatatable lt object gt  GetDataTable Type dataType           Functional method using a generic method      IDatatable lt T gt  GetDataTable lt T gt         And this the whole implementation using the generic method above   The generic class that will be cast from a dictionary      Non-generic base class allows listing tables together abstract class Datatable       Datatable Type storedClass              StoredClass   storedClass             Type StoredClass   get  private set          Generic inheriting class abstract class Datatable lt T gt   Datatable  IDatatable lt T gt        protected Datatable            base typeof T                   This is the class that stores the generic class and casts it to satisfy the generic method in the interface  class Database          Dictionary storing the classes using the non-generic base class     private Dictionary lt Type  Datatable gt   tableDictionary       protected Database List lt Datatable gt  tables                 tableDictionary   new Dictionary lt Type  Datatable gt             foreach  var table in tables                         tableDictionary Add table StoredClass  table                           Interface implementation  casts the generic     public IDatatable lt T gt  GetDataTable lt T gt                  Datatable table   null            tableDictionary TryGetValue typeof T   out table            return table as IDatatable lt T gt             And finally the calling of the interface method   IDatatable lt CustomerAccount gt  table    database GetDataTable lt CustomerAccount gt

User · Answer

I had a similar problem  I have a class   Action lt T gt    which has a property of type T   How do I get the property when I don t know T  I can t cast to Action lt   unless I know T   SOLUTION   Implement a non-generic interface   public interface IGetGenericTypeInstance       object GenericTypeInstance        Now I can cast the object to IGetGenericTypeInstance and GenericTypeInstance will return the property as type object

User · Answer

As mentioned  you cannot cast it directly  One possible solution is to have those generic types inherit from a non-generic interface  in which case you can still invoke methods on it without reflection  Using reflection  you can pass the mapped object to any method expecting it  then the cast will be performed for you  So if you have a method called Accept expecting a  MessageProcessor as a parameter  then you can find it and invoke it dynamically

User · Answer

public delegate void MessageProcessor lt T gt  T msg  where T   IExternalizable        virtual public void OnRecivedMessage IExternalizable msg                Type type   msg GetType            ArrayList list   processors Get type           if  list    null                        object   args   new object   msg               for  int i   list Count - 1  i  gt   0  --i                                Delegate e    Delegate list i                   e Method Invoke e Target  args

User · Answer

This is simply not allowed   Type key   message GetType    MessageProcessor lt key gt  processor   messageProcessors key  as MessageProcessor lt key gt     You cannot get a generic type as a variable value   You d have to do a switch or something   Type key   message GetType    if  key    typeof Foo         MessageProcessor lt Foo gt  processor    MessageProcessor lt Foo gt  messageProcessors key          Do stuff with processor   else if  key    typeof Bar         MessageProcessor lt bar gt  processor    MessageProcessor lt Bar gt  messageProcessors key          Do stuff with processor

User · Answer

The answer of  DanielPlaisted before generally works  but the generic method must be public or one must use BindingFlags NonPublic   BindingFlags Instance  Couldn t post it as a comment for lack of reputation

User · Answer

You can t do that  You could try telling your problem from a more high level point of view  i e  what exactly do you want to accomplish with the casted variable  for a different solution   You could go with something like this    public abstract class Message                    public class Message lt T gt    Message        public abstract class MessageProcessor        public abstract void ProcessMessage Message msg       public class SayMessageProcessor   MessageProcessor        public override void ProcessMessage Message msg             ProcessMessage  Message lt Say gt  msg               public void ProcessMessage Message lt Say gt  msg                do the actual processing                Dispatcher logic   Dictionary lt Type  MessageProcessor gt  messageProcessors           typeof Say   new SayMessageProcessor            typeof string   new StringMessageProcessor            properly initialized   messageProcessors msg GetType   GetGenericArguments   0   ProcessMessage msg

User · Answer

To convert any type object to a generic type T  the trick is to first assign to an object of any higher type then cast that to the generic type  object temp   otherTypeObject  T result    T temp

User · Answer

Type type   typeof MessageProcessor lt  gt   MakeGenericType key     That s the best you can do  however without actually knowing what type it is  there s really not much more you can do with it   EDIT  I should clarify  I changed from var type to Type type  My point is  now you can do something like this   object obj   Activator CreateInstance type     obj will now be the correct type  but since you don t know what type  key  is at compile time  there s no way to cast it and do anything useful with it

[c#] Cast to generic type in C#

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