覆盖 C# 中的通用约束

Question

Apologies if this would be a better fit for the software engineering stack exchange, but as this is fairly language-specific, I believe I'm asking in the right place.

I have a project which involves converting a sizable codebase from Java to C#. The original Java project has an interface roughly equivalent to the following C# snippet.

public interface IEntry<out T> where T : class, IEntry<T>{
  T SetName(string nameIn);

  string GetName();
}

It also contains a basic implementation of this interface.

public abstract class BaseEntry<T> : IEntry<T> where T : BaseEntry<T>{
  private string name;
  internal object extraData;

  public T SetName(string nameIn){
    name = nameIn;
    return (T)this;
  }

  public string GetName(){
    return name;
  }
}

The problem arises in the class which holds entries of types implementing the IEntry<T> interface.

public class ObjectHolder<T> where T : class, IEntry<T>{
  private readonly List<T> entries;
  private readonly bool canGetExtraData;

  public ObjectHolder(){
    //HasGenericBaseType simply checks if the first type implements the second type
    canGetExtraData = HasGenericBaseType(typeof(T), typeof(BaseEntry<>));
  }

  public void Add(T entry){
    entries.Add(entry);
  }

  public void DoSomething(int index){
    if(!canGetExtraData){
      T entry = entries[index];
      object extraData = ((BaseEntry<T>)entry).extraData; <-- This cast fails on compilation
      //Something gets done with the extraData object
    }
  }
}

The indicated line gives me the below error on compilation.

error CS0314: The type `T' cannot be used as type parameter `T' in the generic type or method `BaseEntry<T>'. There is no boxing or type parameter conversion from `T' to `BaseEntry<T>'

I understand why I get this error, but I'm struggling to find a way around it. How could the above class be redesigned to allow access to the internal object field on types extending BaseEntry<T> ?

Answer 1

So basically BaseEntry<T> is not equal to IEntry<T>

Analogy: You have a Mammal that implements ILegs , but not everything that has ILegs is a Mammal .

There are few fixes that you might want to look at, either pushing extraData back to IEntry<T> or as @ JeremyLakeman suggested, creating a separate generic or / non generic interface for IExtraData and constrain by that. The answer would depend on whether IEntry<T> always has extra data.