为什么我不能声明通用列表 <T> 作为接口方法的返回类型？

Question

Why can't I create an interface method with return type List<T> ? I'm encountering this compile error "The type or namespace name 'T' could not be found"

interface ILoader 
{        
    List<T> LoadValues();
}

Appreciate any of your input.

Answer 1

What is T supposed to be in your interface? For the compiler, T is just an unknown identifier at that point.

You need to specify something for T , some type that is used as the type argument for List<T> .

---

Depending on what you want to achieve, there are four options:

• Declare T as a generic parameter on the interface/class level:

   interface ILoader<T> { List<T> LoadValues(); } 

• Declare T as a generic parameter on the method level:

   interface ILoader { List<T> LoadValues<T>(); } 

• Assign a fixed type as T ; eg int (wlog):

   interface ILoader { List<int> LoadValues(); } 

• Declare T as a generic parameter on a higher nesting level (thanks to Jeppe Stig Nielsen for pointing out this possibility):

   class Outer<T> { interface ILoader { List<T> LoadValues(); } } 

Answer 2

The T generic parameter must also be declared in the interface. Something like this:

interface ILoader<T>
{        
    List<T> LoadValues();
}

Answer 3

You should do something like this:

interface ILoader<T>
{
    List<T> LoadValues();
}

Or

interface ILoader
{        
    List<T> LoadValues<T>();
}        

Or even, possibly:

interface ILoader 
{        
    List<dynamic> LoadValues();
}

^Isn't the best idea but usable.