C# interface inheritance

Shane5555

Hi, I have a basic enough problem with implementing inheritance in an interface. Here is an example of what I would LIKE to do:

    static void Main(string\[\] args)
    {
        List<ISuperString> myStrings = new List<ISuperString>();
        myStrings.Add(...SOMETHING...);

        Console.WriteLine(myStrings\[0\].containsCapitalLetters());
        Console.WriteLine(myStrings\[0\].ToLower());
    }

    public interface ISuperString : string
    {
        bool containsCapitalLetters();
    }

I have seen (and implemented) some workarounds for this, however they are a bit messy for my taste. Basically, is it possible to either have an interface inherit from a class or to derive an interface from a class (i.e. an IString interface)? thanks

realJSOP

What I think you really want is an extension method.

public class ExtensionMethods
{
public static bool ContainsCapitalLetters(this String)
{
bool result = false;
// ...do your code here...
return result;
}
}

Usage would be

String myString = "AbcdefG";
bool containsCapitalLetters = myString.ContainsCapitalLetters();

.45 ACP - because shooting twice is just silly
-----
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DaveyM69

As the string class is sealed you cannot subclass it to force it to implement your interface. If it's just methods you want to add and you're using 3.5 or 4.0 then you can use extension methods as John suggested. If you want to add more than methods or are using 2.0 then you will need to create a wrapper around the string class - something like this will get you started

public class SuperString : ISuperString
{
private string wrappedString;

public SuperString(string stringToWrap)
{
    wrappedString = stringToWrap;
}

public static implicit operator string(SuperString superString)
{
    return superString.wrappedString;
}
public static implicit operator SuperString(string stringToWrap)
{
    return new SuperString(stringToWrap);
}

public bool containsCapitalLetters()
{
    throw new NotImplementedException();
}

}

public interface ISuperString
{
bool containsCapitalLetters();
}

Dave
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Shane5555

Thanks guys, however the problem goes a bit deeper than my string example (and btw, I didn't realise a string was sealed so I suppose that turned my oversimplification on its head) What I need is an interface to use within a function (like in the list above) A wrapper class would work fine (and at the moment I am using a wrapper interface) but inheriting from a class to an interface would be a LOT cleaner.

AspDotNetDev

Not really sure what your question is. Sounds like you just want to create an interface and implement it with a class. That's pretty basic C#... that's all I've gathered from your question (now that you've added the disclaimer about not using string as an example anymore because it's sealed). You might want to take a look at my StringBuilderPlus Improves Upon StringBuilder article, which makes use of interfaces and implementing those interfaces. The diagram at the bottom of the article shows the interface IString and the two classes, PlainString and StringsWrapper, that implement IString. Download the code if you want to see how that's done.

Shane5555 wrote:

inheriting from a class to an interface would be a LOT cleaner

An interface cannot inherit from a class, but a class can inherit from a class. And a class can implement an interface. And an interface can inherit from an interface.

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Lost User

DaveyM69 wrote:

and you're using 3.5 or 4.0

Or 2.0! Just add

namespace System.Runtime.CompilerServices
{
[AttributeUsage(AttributeTargets.Method | AttributeTargets.Class | AttributeTargets.Assembly)]
public sealed class ExtensionAttribute : Attribute
{
public ExtensionAttribute() { }
}
}

And set the compiler to C#3

AspDotNetDev

So you're saying extension methods can be used in Visual Studio 2005? Sounds like a really good tip/trick!

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Lost User

AFAIK no, it does require the new compiler (and it would probably mess up IntelliSense on VS05), but with this trick you can target .NET 2.0 (with VS08 and higher) and still use extension methods

PIEBALDconsult

Shane5555 wrote:

would be a LOT cleaner.

No, I'm pretty sure not. At most you may be looking for duck-typing, but maybe not. You may need to present your concept better.