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delegate void f<in T>(T p) where T : Google.Protobuf.IMessage;

// class ProtoMessageA is protobuf generated class which derived from Google.Protobuf.IMessage
void handler(ProtoMessageA m){}

f<ProtoMessageA> h = handler;

Then put it into dict, due to Dictionary<int, f<T>> is illegal, so change value type to object:

Dictionary<int, object> d = new Dictionary<int, object>();
d[1] = h;

Now get delegate from dict and invoke it:

ProtoMessageA m;
var h = d[1];
((f<ProtoMessageA>)h).Invoke(m);

Works fine so far.

However, if message obejct is created at runtime, the specific type cannot be determined at compile time, it might be ProtoMessageA, ProtoMessageB... so use their base class IMessage to hold:

Google.Protobuf.IMessage m = descriptor.Parser.ParseFrom(bytes array);
// simplfied here, the int key can make sure get 
// correct delegate instance(parameter type same as m)
var h = d[1];
((f<???>)h).Invoke(m);

If cast h to f<Google.Protobuf.IMessage>, it will report Specified cast is not valid

Also I can get specific type of message by

var t = m.GetType();

But t is a variable, it can not as template argument as:

f<t>

The paradox is if not cast to the right type, Invoke cannot be call, however, the template type cannot know at compile time, it is runtime info.

Does it possible to invoke it?

jean
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    You can close a generic through reflection. – silkfire Feb 24 '21 at 08:31
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    Your question lacks crucial information. It seems that your delegate instances have already been created, and it's just that the invoking code has no way to specify the generic type for the purpose of casting. If so, there's no need. Call `DynamicInvoke()` on the `Delegate` instance without applying a type. See duplicate. If you don't even have the delegate instantiated, you can create generic delegates through reflection. There duplicates on the site for that scenario too. Just use the search feature or your favorite web search page to find them. – Peter Duniho Feb 24 '21 at 08:39

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