Webservice – Return Generic Result Type or Purposed Result Type

As I can't use true multiple inheritance in C#, how would I design this to be able to share the methods and properties?

This is the right way to share the methods and properties. But:

The trouble now is that in my extendedTextBox class I can't cast my extendedTextBox as a 'BaseExtendedControl' to use the extension methods, as the extendedTextBox inherits from the standard TextBox class like this:

public class ExtendedTextBox : TextBox, IExtendedControl

so there's no common base to cast ExtendedTextBox as 'BaseExtendedControl' as it doesn't inherit from it.

Yes, you're right there is no common class, but you can solve the problem by casting to the IExtentedControl interface which is common to your classes and contains the methods that you need. So what about:

public static class ExtendedControlHelper
{
    public static void AddRequiredFieldValidator(this IExtendedControl control)
    {
        control.rfv = new RequiredFieldValidator();
        control.rfv.ErrorMessage = control.RequiredMessage;
        ConfigureAndAddValidator(control, control.rfv);
    }

    public static void ConfigureAndAddValidator(this IExtendedControl control, BaseValidator validator)
    {
        validator.ControlToValidate = control.ID;
        validator.Display = ValidatorDisplay.Dynamic;
        validator.CssClass = "validationMessage ";
        validator.CssClass += control.ValidatorCssClass;
        validator.EnableClientScript = control.ClientScript;
        control.Controls.Add(validator);
    }
}

I don't think you need to implement the IExtendedControl interface in the ExtendedControlHelper class. Moreover I corrected the grammar of the interface name from IExtendendControl to IExtendedControl.

EDIT :

Another solution could be to use the dynamic keyword. The dynamic keyword is like object but the type is checked at run time instead of compile time:

public static class ExtendedControlHelper
{
    public static void AddRequiredFieldValidator(this dynamic control)
    {
        //...
    }

    public static void ConfigureAndAddValidator(this dynamic control, BaseValidator validator)
    {
         //...
    }
}

The dynamic type doesn't exist at run time because it's solved by the compiler, so if you write this: myExtendedTextBox.AddRequiredFieldValidator(); the method is solved as

//...
public static void AddRequiredFieldValidator(this ExtendedTextBox control)
{
    //...
}

and so on. So the type can change continuously at run time.

For more informations give a look here.

Basically, what you're asking is: should I make the search results statically typed? I think the answer depends on what you're going to do with them and how much do you like static typing.

Regarding what you're going to do: if you're just going to display the results to the user, or something like that that treats all the fields in the search result equally, then using a Dictionary (which is basically the same as dynamic typing) makes sense. On the other hand, if you're often going to write things like searchHist.Address (or searchHit["Address"]), then that points more to static typing.

Regarding how much you like static typing: to me, as a proponent of static typing, using dynamic typing gives you small short term advantage (e.g. you don't have to write those dozen lines declaring each TSearchHit) for significant long term disadvantages (no compiler checking for typos, no IntelliSense). This means that I think static typing is almost always worth the up-front cost. Proponents of dynamic typing would tell you otherwise, but C# is mostly statically typed language, and presumably you're using that for a reason.