Question

So for example I have two classes:

Class A
{
  string property1;
  string property2;
}

Class B : A
{
  string property3;
  string property4;
  ....
} 

So B inherits class A’s properties. They are sitting in a list, that is sitting in a dictionary

Dictionary <string, List<A>> myDictionary = new Dictionary<string, List<A>>();

List<A> myList = new List<A>();

There is one Dictionary, containing many List’s, that all contain a mix of Class A & B objects.
While looping through, I am trying to access some properties from Class B objects, I have an if statement to find them but the program still thinks they are of type Class A and throws an error when I try and use a property3 or property4. For example:

string key = string key in dictionary;
string index = object position in list;

myDictionary[key][index].property3.someMethod();

Is there a way to tell the program that this is a class B object and allow the properties 3 & 4 to be used?

Answer 1

Cast the object safely as a B-type object, then check for null

var obj = myDictionary[key][index];

var bObj = obj as B;
if (bObj != null)
{
     bObj.someMethod();
}

Although, I would also probably say it seems like your design is off. Ordinarily, I wouldn’t expect something like this. Normally, if you’re using inheritance, you’d want a design that allows them to be used interchangeably. For example, you might implement the behavior on A as a no-op, but override it on B to actually do something. This would make it so that consuming classes need not care whether the “A” thing is really an A or a B instance.