I'm creating a factory method that determines the object to return based on the generic type in the method call

public IDbStrategy<T> GetDefaultStrategy<T>()
{
    return typeof(T).IsAssignableFrom(typeof(DbModel))
        ? new DefaultModelDbStrategy<T>()
        : new DefaultDbStrategy<T>();
}

the DefaultModelDbStrategy as a type constraint of where T : DbModel but the DefaultDbStrategy does not. I am getting a compiler error when trying to use T as the generic type for DefaultModelDbStrategy because the compiler doesn't know that T is a DbModel. Is there a way to ensure my compiler that the generic argument passed in does indeed derive from DbModel as the reflection statement above asserts?

Thanks

CodePudding user response：

Not without reflection. Because T is statically resolved at compile time, the compiler would need to instantiate type DefaultModelDbStrategy<object> (if T is object), but this is an illegal type. The types used within a method need to be resolved when the method is JITed, and not only when a particular line is executed.

There's a workaround using reflection though: You can do

(IDbStrategy<T>)Activator.CreateInstance(typeof(DefaultModelDbStrategy<>).MakeGenericType(typeof(T)));

This creates an instance of DefaultModelDbStrategy, with the given T. It will throw a runtime error if T doesn't satisfy the constraints.