Return a generic data type (int, float, string)

Question

_{Friday, May 30, 2008 8:27 PM}

Have a look at this function:

        public T Add<T>(string varName)  where T: struct
        {

             // Where dt is a class that can have int, float, string as values
            return (T) dt.Value ;

        }

Now this wouldn't work if T is of type String. And If I remove where T: struct, it wouldn't work at all. I need to be able to return int, float, and string data types. How can I achieve that?

TIA,
Kumar

Kumar

All replies (9)

_{Tuesday, June 3, 2008 9:17 AM ✅Answered}

In that case, it cannot infer the parameter type from the argument.

But you can specify the type at the call site, like this:

class Program
{
    static void Main(string[] args)
    {
        string s = Test<string>("string");
        int i = Test<int>("int");
        double d = Test<double>("double");
    }

    static object LookupThing(string thing)
    {
        switch (thing)
        {
            case "string": return "string";
            case "int": return 1;
            case "double": return 1.0;
            default: throw new InvalidOperationException("pop");
        }
    }

    static T Test<T>(string param)
    {
        return (T) LookupThing(param);
    }
}

_{Saturday, May 31, 2008 8:34 AM}

Perhaps you need to send a clearer example, because i have a hard time trying to understand what you're trying to achieve.

• System.String is a reference type, thus the constraint T: struct would not be satisfied.

• Removing the constraint would work, but leaves more options than only int, float and string.

• Since you have only a limited range of allowed returntypes (and thus no need to be generic), you could implement three methods: AddInt, AddFloat and AddString.

Tim Van Wassenhove - Please remember to mark the replies as answers if they help.

_{Saturday, May 31, 2008 9:11 AM}

OK; what us dt.Value?

If you are sure about the type, you can get around it (after removing the constraint) by casting to object as an intermediary:

return (T)(object)dt.Value;

The problem is that this will box and unbox any value-types. I have some more direct conversion code (that uses on-the-fly compilation and a few other tricks to avoid boxing), but it is more complex. So; if you tell us what "dt.Value" is, I might be able to suggest something more appropriate.

Marc

_{Monday, June 2, 2008 1:56 PM}

What I am trying to achieve here is to write a generic function that returns both numeric types and string values. Is that possible?

TIA,
Kumar

Kumar

_{Monday, June 2, 2008 2:08 PM}

Well yes:

class Program
{
    static void Main(string[] args)
    {
        string s = Test("string");
        int i = Test(1);
        double d = Test(1.0);
    }

    public static T Test<T>(T param)
    {
        return param;
    }
}

_{Monday, June 2, 2008 2:27 PM}

See, my problem is the argument is not of T type. It is of string type. And finally, I need to type cast an object to T type.

In the example you gave here, the argument is of T type and so no issues there.

Thank you,
Kumar

Kumar

_{Tuesday, June 3, 2008 10:32 AM}

Hi. Unless I misunderstand, you want to retrieve an object of the type that you specify by name.

Johan van Rhyn - Please remember to mark the replies as answers if you found them helpful.

_{Tuesday, June 3, 2008 11:14 AM}

return default(Type);

That will always return null, the default for any reference-type (Type).

If I understand what you are trying to do, you'd have to use generics and MakeGenericMethod to do what you are trying to describe with that line... but I don't think it is what the OP is after.

_{Tuesday, June 3, 2008 1:09 PM}

Thank you, Matthew. That will work.

Thank you,
Kumar

Kumar