向实现已知接口的未知类的特定实例添加扩展方法

Question

I am using a library called SpreadsheetGear. With this I work with "IWorkbook" objects that I initialize by a method SpreadsheetGear.Factory.GetWorkbook() whose contract returns an IWorkbook.

I want to derive implement an interface and write extension methods, for example I define

 public partial interface IClientWorkbook : IWorkbook

so I can do something like

 IClientWorkbook workbook = SpreadsheetGear.Factory.GetWorkbook;
 workbook.ChangeClientName("NewName");

and define the ChangeClientName method somewhere else, but not have it available to IWorkbooks that aren't IClientWorkbook.

I'm not sure how to approach this as can't see the actual class that the GetWorkbook() method returns. Hopefully the question makes sense.

Answer 1

As I understand your question, you want to define a custom interface (IClientWorkbook) that you then want to be implemented by objects returned from a 3rd-party API. You cannot do this. You can implement extension methods for the existing class or interface, but you cannot add a new interface to an existing class without editing the code for that class. Also, you cannot create an extension method that doesn't apply to all instances of the extended type. That is, you cannot create an extension method that only applies to some IWorkbook instances -- you either don't have an extension method, or you have an extension method that applies to all IWorkbook instances.

You could write wrapper classes for all the bits of the 3rd-party API, and you can then implement your classes however you choose. This is sometimes worth it, but more often not.

Answer 2

It must be as easy as

public static class ClientWorkbookExtensions {
    public static void ChangeClientName(this IClientWorkbook workbook, String newName) {
        workbook.Name = newName;    //  Or any other code you like
    }
}