If you're trying to actually extend a class, you do it like this:
public class BaseClass
{
//.. No Editable Access
}
public class Extended : BaseClass
{
public bool FlaggedAsDeleted { get; set; }
}
If you're trying to add data to an existing class, you have two options:
Inheritance - as seen above.
Encapsulation - create a new object that holds an instance of the type you're adding to.
C# provides a feature called Extension Methods, which allows you to seemingly add methods to existing classes. However, these are really just syntactic sugar, as you're still constrained to the class's public API.
public class BaseClass
{
public int Value { get; set; }
}
public static class ExtensionMethods
{
public static void Increment(this BaseClass b)
{
b.Value += 1;
}
}
Extension methods do not allow you to add data to an existing class though.