从未知类调用已知属性

Question

I have a BaseClass from which I have derived multiple other Classes using Inheritance. Elsewhere I want to be able to retrieve Properties from anyone of these inherited classes even though the type of Class isn't known until Runtime.

For example lets say I have two classes derived from BaseClass

FirstClass : BaseClass
SecondClass : BaseClass

My BaseClass has a property called ID which is an int (amongst many others)

At runtime my app will receive 'either' a FirstClass object or a SecondClass object, but either way I need to retrieve the ID property.

public int MyMethod(object unknownClass)
{
    int myID = unknownClass.ID; 
    return myID                    //...does not compile
}

public int MyMethod(object unknownClass)
{
    BaseClass tryCasting = (BaseClass)unknownClass;
    int myID = tryCasting.ID;           
    return myID                    //...does not compile either
}

I'm not sure what else to try. How can I get a Property which I know exists without knowing the Object type first?

Answer 1

Don't use object and use BaseClass :

public int MyMethod(BaseClass unknownClass)
{
    int myID = unknownClass.ID; 
    return myID;                    
}

Answer 2

See the answer from Darren Davies for your specific case of ID. The approaches in this answer are useful when the property is defined in the derived classes, not the base class.

---

Use dynamic instead of object . That will use the runtime to retrieve the actual object type.

public int MyMethod(dynamic unknownClass)
{
    int myID = (int)unknownClass.ID; 
    return myID;
}

Alternatively, you can use Reflection to retrieve the property value:

var myID = (int)unknownClass.GetType().GetProperty("ID").GetValue(unknownClass);

Answer 3

Polymorfism is the answer. You should know that when you create a class derived from another, the derived class acquires inmediately the properties from the base class. Thus, if you don't redefine the method (override), when you call the method MyMethod on a derived class it will always call the base method. So the next code should work well :

BaseClass { 
    private : int id ; 
    public  : int getId { return id ; }
} ;

FirstClass : BaseClass { DO NOT OVERRIDE getID ...} ; 

int main (void) { 
    BaseClass base ; 
    FirstClass first ; 
    base = first ; 
    base.getId() ; //Gets the id of first.
}

What does this program? When you assign first to base you upcast first type to BaseClass . Then when you get to the execution of base.getId() the program lookups if there is a redefinition (overriding) of this method in FirstClass, if there isn't it calls the base method.

Hope that you find this information useful. You should read the first chapters of "Thinking in C++" of Bruce Eckels, it's a good introduction to Object Oriented Programming.