Generic interfaces

Question

Here is my code

public interface ITranslator<E, R>
{       
    E ToEntity<T>(R record);
}

class Gens : ITranslator<string, int>
{
    #region ITranslator<string,int> Members

    public string ToEntity<MyOtherClass>(int record)
    {
        return record.ToString();
    }

    #endregion
}

When I compile this, I get an error Type parameter declaration must be an identifier not a type

Why is that I cannot have ToEntity<MyOtherClass> but can only have ToEntity<T> ??

Edit: what is MyOtherClass doing ? I am converting between entities(POCOs equivalent of Entity framework) and record(Object returned by the framework) for multiple tables/classes. So I would want to use this to do my class specific conversion

Answer 1

Your interface has a generic ToEntity<T> method that you've made non-generic in your implementation class Gens as ToEntity<MyOtherClass> . (A generic method could take any type parameter, possibly given certain constraints on T . Your Gens class is trying to provide a definition for ToEntity only for the type parameter MyOtherClass , which defeats the purpose of generics.)

In your code example, it's unclear how your Gens class is trying to use the MyOtherClass type; it's certainly not involved in the logic of ToEntity . We'd need more information to be able to guide you further.

To illustrate, here's what your current definition of the ITranslator<E, R> interface offers, in plain English:

"I provide a mechanism to translate any record of type R into an entity of type E , this mechanism being dependent upon any user-specified type T ."

Your Gens class, on the other hand, the way it's currently designed, "implements" the above interface like so:

"I can translate integers to strings. I provide the illusion of allowing the user to specify a type to control how this translation is performed, but in fact there is no choice of type. The MyOtherClass class is involved somehow; that's all I can say."

From these two descriptions, it's clear that the Gens class is not really doing what the ITranslator<E, R> interface guarantees . Namely, it is not willing to accept a user-specified type for its ToEntity method. That's why this code won't compile for you.

Answer 2

您必须在泛型类型上声明约束。

public string ToEntity<T>(int record) where T : MyOtherClass

Answer 3

That compiles OK for me in LINQpad. Maybe you have a type named E, R, or T somewhere?

Ahh I see what you're trying to do... you have MyOtherClass defined as a class somewhere yet you're trying to use it as a type argument in ToEntity. How exactly do you want MyOtherClass involved in ToEntity?