Question

I wish to extend a base class in C# with some additional functionality. I have existing code which returns an array of the base class objects (Account) which I need to convert into the extended version.

So I have the extended version:

class AccountXtra : Account {     public int Period { get; set; }     public int Visitors { get; set; }     public string ContactName { get; set; } } 

All is good.

BUT how do I create a new instance of AccountXtra when I have a instance of Account?

I have tried:

//This doesn't work AccountXtra newInstance = (AccountXtra)accountInstance; //This also doesn't work AccountXtra newInstance = new AccountXtra(); newInstance = accountInstance; 

Answer 1

You need to be generating new objects of the derived class now, not the base. Replace your old calls to new Account with new AccountXtra. Or, you need a constructor for AccountXtra that takes an Account object and makes a new derived-class version of it:

public AccountXtra(Account baseInstance) : this() {     this.Field1 = baseInstance.Field1;     this.Field2 = baseInstance.Field2;     ... } 

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Explanation: You can’t cast a base class to a derived class unless it is of the derived class type. This will work:

AccountXtra accountXtra = new AccountXtra(); Account xtraAsBase = (Account)accountXtra; AccountXtra xtraCastBack = (AccountXtra)xtraCastAsBase; 

But this will not:

Account Base = new Account(); AccountXtra baseAsDerived = (AccountXtra)Base;  //Cast base to derived class