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I have been teaching myself generics and I wanted to try it out with a list but I struggled upon a problem I cant figure out how to "feed" the generic list to my method. What is the proper way to make a generic method "eat" my list? :)

Heres my code:

 class Program<AnyDataType>
{
    static void Main(string[] args)
    {
        jdtlist.Add("something");
        jdtlist.Add("something");
        jdtlist.Add("something");
        Console.WriteLine(countlist(jdtlist));
        Console.ReadKey();
    }
    static List<AnyDataType> jdtlist = new List<AnyDataType>();

    public static int countlist(List<AnyDataType> list) // Yes I know this is practically useless but thats not why I am here :)
    {
        int listcount = 0;
        for (int i = 0; i < list.Count; i++)
        {
            listcount++;
        }
        return listcount;
    }
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  • 1
    What's wrong with the code you have? How is it not working? Your question is not that clear. Also, don't anthropomorphize code. Commented May 8, 2015 at 14:50
  • 2
    What do you mean by "feed" and "eat"? Commented May 8, 2015 at 14:50
  • Im trying to make a generic method that returns listcount no matter what variable type the list is, but I cant seem to figure out how to do it correctly. Heres a screenschot of my code in VS hope it helps: puu.sh/hFVLW/62cf4c2a03.png puu.sh/hFVOG/986185a223.jpg Commented May 8, 2015 at 14:54
  • Your screen shot just shows the code you've already given to us except with some errors underlined. That's of no use without the text of the actual errors. Come on, man....help us help you. Commented May 8, 2015 at 14:55
  • @HolyFish any generic list has a property Count? Commented May 8, 2015 at 14:55

4 Answers 4

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If you are writing generic method, then it should have generic parameter

public static int CountList<T>(List<T> list)
{
    int listcount = 0;

    for (int i = 0; i < list.Count; i++)        
        listcount++;

    return listcount;
}

Then you can call it with any generic list

var list = new List<AnyDataType>();
// ..
Foo.CountList(list);

Same goes to classes. If you want to parametrize class with some generic type, you should provide generic argument

public class Foo<T>

As @DStanley stated, you don't need to parametrize individual methods in that case

public class Foo<T>
{
    public static int CountList(List<T> list)
    {
        int listcount = 0;

        for (int i = 0; i < list.Count; i++)        
            listcount++;

        return listcount;
    } 
}

But you need to parametrize class

Foo<int>.CountList(list)

Suggested reading: Generics (C# Programming Guide)

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2 Comments

the class is generic, so the generic parameter on the method is implied.
@DStanley yes, I know. Just wanted to show separately generic method definition and generic class definition
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Your problem is not passing the list to your method - that part is fine. Your problem is that you're trying to fill a "generic" list with a "specific" type (namely strings). Making a class generic means "I am not specifying the data type here - the consumer of the class will do that". So a better use case for your class would be:

class Program
{
    static void Main(string[] args)
    {
        MyList<string>.Add("something");
        MyList<string>.Add("something");
        MyList<string>.Add("something");
        Console.WriteLine(MyList<string>.countlist(MyList<string>.jdtlist));
        Console.ReadKey();
    }
}
public class MyList<AnyDataType>
{
    public static List<AnyDataType> jdtlist = new List<AnyDataType>();

    public static void Add(AnyDataType item)
    {
        jdtList.Add(item);
    }

    public static int countlist(List<AnyDataType> list) 
    {
        int listcount = 0;
        for (int i = 0; i < list.Count; i++)
        {
            listcount++;
        }
        return listcount;
    }

This is the minimum needed to get your program to work - there are several improvements that can be made (not using static so much, etc.) but hopefully it helps you understand generics better.

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0

You need to call the static method of the generic class via class-name including the type of the parameter:

so instead of

Console.WriteLine(countlist(jdtlist));

this:

Console.WriteLine(Program<string>.countlist(jdtlist));

Another way is to make the method generic, not the class:

public static int countlist<AnyDataType>(List<AnyDataType> list) {}

Then you can call it in these ways(with explicit type or inferred from parameter):

Program1.countlist<string>(jdtlist)
Program1.countlist(jdtlist)
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0

Okay you have:

public static int countlist(List<AnyDataType> list)
{
    int listcount = 0;
    for (int i = 0; i < list.Count; i++)
    {
        listcount++;
    }
    return listcount;
}

And well, it works fine, but only if you have List<AnyDataType> to begin with.

Well, you could do:

public static int Countlist<T>(List<T> list)
{
    int listcount = 0;
    for (int i = 0; i < list.Count; i++)
    {
        listcount++;
    }
    return listcount;
}

Now the method itself is generic. What's more overloads will happen automatically in that you can call CountList(new List<string>()) rather than having to explicitly call CountList<strong>(new List<string>()).

But this combines with simple matters of inheritance. Consider that your CountList could work just as well with any other IEnumerable<T> implementation:

public static int Count<T>(IEnumerable<T> source)
{
  int tally = 0;
  foreach(var item in source)
    ++tally;
  return tally;
}

So just as generics mean you don't have to restrict yourself to a particular type of list, so normal inheritance and interface implementation means you can work with the most general case applicable.

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