Compilation error regarding declaring generic nested class

Question

I get a compilation errors regarding this piece of code:

Error 1 Invalid token '(' in class, struct, or interface member declaration
Error 2 Cannot use more than one type in a for, using, fixed, or declaration

Any idea why? In addition, is it possible to declare dictionary as follows?

public class S
{
        private class ObInfo<T>
        {
            private string _type;
            private T _value;

            public ObInfo<T>(string i_Type, T Value)
            {
                this._type = i_Type;
                this._value = Value;
            }

            public ObInfo() 
               {}
       }

       private static Dictionary<int,ObInfo> sObj= new Dictionary<int,ObInfo>();
}

score 7 · Accepted Answer · edited May 23 '17 at 12:12

7

public ObInfo<T>(...) {

Constructors cannot take generic parameters.
Remove the <T> and everything will work.

All methods (and types) inside a class inherit that class's generic parameters; you should only create generic methods inside generic classes if the methods need a separate type parameter. (this should be avoided; it's very confusing)

Also, open generic types are not actually types; you cannot have a Dictionary<int,ObInfo> without specifying the type parameter for ObjInfo.
Instead, you can either use a non-generic interface for the dictionary, or move the type parameter to the outer class and have a separate dictionary per type parameter.

edited May 23 '17 at 12:12

Community

1
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answered Dec 17 '12 at 15:24

SLaks

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Well that was fast, but if I need to pass generic parameter to the c'tor? – JavaSa Dec 17 '12 at 15:26
@JavaSa: See my second paragraph. – SLaks Dec 17 '12 at 15:26
@SLaks: Just to make something clear, the example you referred me to is a little bit hard for me to get the main idea, did you mean to do something like Artur was trying to? (Below answer) – JavaSa Dec 17 '12 at 15:47
@SLaks: Great, now if in `Class S` there is already another method that I can't touch, which has type parameter `` also like: `public static void someMethod() {..}`. Referring what you said, S don't have type parameter `` aside and it's legal, how can it be? Second, The `` of the method I've mentioned has nothing to do with the `` of the nested class, so should I choose another Type parameter name? – JavaSa Dec 17 '12 at 16:00

score 7 · Answer 2 · answered Dec 17 '12 at 17:20

SLaks' answer is fine, but to clarify one point: you asked why the error was happening, not how to fix it. The error is being reported because the compiler is reasoning that you intended to say:

   private class ObInfo<T>
   {
        public ObInfo<T> SomeMethodNameHere(string i_Type, T Value)

That is, it thinks you are trying to make a method -- or possibly a field or event -- and you have typed the return type, ObInfo<T> but forgotten the method name. Whether this is a method, field or event, the ( is unexpected, and so that's the error.

Obviously this is not the best error message possible, as it confused you. It might be better to add another heuristic to speciifcally detect the situation that you're in.

I'd be interested to know why you made this error. Did you think:

The ctor is a method whose name is the same as the class name, and the T is part of the class name.
The ctor is a generic method, and a generic method has to have a type parameter declared for it.
Something else.

?

If you thought the first thing: the T is not part of the class name. If you thought the second thing: if that were true then you'd be declaring a second type parameter in scope called T, which is a little confusing, no?

Another possible explanation is that one sees a constructor as a nameless method with return type `ObInfo` | Personally I think it was a bad idea to use the class name to declare constructors instead of a using a keyword. Makes the code harder to read, and hinders copy&paste of constructor signatures across classes. — CodesInChaos, Dec 23 '12 at 19:12

score 1 · Answer 3 · answered Dec 17 '12 at 15:32

You can simply put static field inside the class. So you will have different static dictionaries for each implementation of generic class

public class S
{
        private class ObInfo<T>
        {
            private string _type;
            private T _value;

            public ObInfo(string i_Type, T Value)
            {
                this._type = i_Type;
                this._value = Value;
            }

            public ObInfo() 
            {}

           private static Dictionary<int,ObInfo<T>> sObj= new Dictionary<int,ObInfo<T>>();
       }
}

Compilation error regarding declaring generic nested class

3 Answers3