C# List of inherited classes

Question

I have the following classes:

public class BaseClass() {}
public class Class1 : BaseClass {}
public class Class2 : BaseClass {}

public class BaseClassList : List<BaseClass> {}
public class Class1List : List<Class1> {}
public class Class2List : List<Class2> {}

The data is in JSON format are is loaded as a list of BaseClass objects. Once loaded and want to populate the specific lists with the relevant objects:

public void Setup()
{
    Class1List list1 = new Class1List();

    var query = from x in BaseClassList
                where x.Type = MyType.Class1
                select x;

    list1.AddRange(query);
}

However AddRange doesn't compile because I'm trying to added BaseClass objects to a Class1 list.

How can I successfully add the inherited classes?

BaseClassList is the name of a type - or do you have some property of that type too? It's also not clear why you have subclasses of List<T> at all - that's usually not a great idea, IME. — Jon Skeet
– Jon Skeet, Commented Feb 10, 2014 at 9:23

csteinmueller · Accepted Answer · 2014-02-10 11:15:07Z

1

Try this code snippet,

public void Setup()
{
  Class1List list1 = new Class1List();

  var query = from x in BaseClassList
            where x.GetType() == typeof(Class1)
            select x as Class1;

  list1.AddRange(query);
}

The as operator casts your type BaseClassto type Class1

Reference: MSDN as Operator

As mentioned in a comment, you can do it a little bit more save with this snippet:

public void Setup()
{
  Class1List list1 = new Class1List();

  var query = from x in BaseClassList
        where x.GetType() == typeof(Class1)
        select x;

   foreach(var item in query){
      var temp = item as Class1;
      if(temp  != null)
         list1.Add(temp);
   }
}

After casting to Class1 you can check for null, as the asoperator returns null if unable to cast. I would prefer to make a type compare with where x.GetType() == typeof(Class1)

edited Feb 10, 2014 at 11:15

answered Feb 10, 2014 at 9:22

csteinmueller

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7 Comments

Jon Skeet Over a year ago

Personally I'd use a cast rather than as - do you really want it to silently proceed (and add a null reference) if there's a non-Class1 element with a Type property of MyType.Class1?

Ivan-Mark Debono Over a year ago

The casting is always returning a null

csteinmueller Over a year ago

@Ivan-MarkDebono why does it?

Ivan-Mark Debono Over a year ago

don't know. temp is always null.

csteinmueller Over a year ago

So the objects are not of type Class1. What is MyType.Class1 ? Seems like a enum?!

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