Is there a way to set an implicit conversion from a string to an interface type in C#

Question

1.00/5 (1 vote)

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I have a library that I'm using that has the following interface:

C#

public interface IStringType
{
    string GetValue();
    bool IsSet();
    void SetValue(string val);
}

So, when I go to use this, I need to do checks like so:

C#

public void ReadName(IStringType i)
{
    string Name;

    if(i != null)
    {
        Name = i.GetValue();
    }
    else
    {
        Name = string.Empty;
    }
}

Ideally, I'd like to just be able to do this:

C#

public void ReadName(IStringType i)
{
    string Name = i;
}

I found some information on doing implicit conversions, but it seems like it more geared for when you are creating your own interfaces/classes, not trying to use others existing ones.

Posted 21-Mar-14 8:33am

hpjchobbes

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Comments

PIEBALDconsult 21-Mar-14 15:08pm

For that particular example you might want to try the ?? operator.

string Name = i ?? string.Empty ;

OriginalGriff 21-Mar-14 15:34pm

Except...that doesn't work, because string is not derived from IStringType.
The compiler will complain.

PIEBALDconsult 21-Mar-14 16:00pm

Yes, but eliminate the Interface; it's a false-start.

Sergey Alexandrovich Kryukov 21-Mar-14 16:45pm

It makes no sense at all. If you explained you ultimate purpose, you could get some practical advice.
—SA

hpjchobbes 21-Mar-14 18:34pm

It's really just trying to simplify using a third party library. It's the QuickBooks SDK, and they do this with almost all their types; IQBStringType, IQBAmountType, IQBAddress, etc. I end up coding a lot of "if( i != null)" checks before I can use anything, and it just feels like redundant code.

Sergey Alexandrovich Kryukov 21-Mar-14 19:03pm

You did not properly formulate your problem. You can start with at least one legitimate and working code sample, point out what seems to be redundant and explain why. Strictly speaking, "if (something != null)" is not redundant at all. If you want to maintain the technique when "something" is never null, you could explain the idea. Also, nothing in your comment is related to your really weird idea to convert as string to interface.

Are you going to say that calculation out of some interface instance from some string would be beneficial? But if you could succeed doing so, it would create a real disaster. Imaging that you misspell some string. A compiler would never be able to detect a problem...

—SA

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OriginalGriff · Accepted Answer · 2014-03-21T09:36:00

Trouble is that you can't implement any code in an interface, so you can't create a implicit cast operator!

There is no real solution: Except to declare a "dummy" derived class or a static instance of a derived class that is empty:

C#

public interface IStringType
    {
    string GetValue();
    bool IsSet();
    void SetValue(string val);
    }
public class ConcreteStringType : IStringType
    {
    public string GetValue() { return string.Empty; }
    }
public void ReadName(IStringType i)
    {
    string Name = (i ?? new ConcreteStringType()).GetValue();
    }

Or

C#

public interface IStringType
    {
    string GetValue();
    bool IsSet();
    void SetValue(string val);
    }
public class DerivedStringType : IStringType
    {
    }
public static DerivedStringType inst = new DerivedStringType();
public void ReadName(IStringType i)
    {
    string Name = (i ?? inst).GetValue();
    }

Nasty...