C#

I want to write 1 extension method,it's purpose is convert 1 IEnumerable<T> to 1 ObservableCollection<T>, but i've error :
Error 5 The type or namespace name 'T' could not be found (are you missing a using directive or an assembly reference?)
Here is my code :

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Collections.ObjectModel;
using System.Data;

namespace BookStore.Helper
{
    public static class ExtensionMethods
    {
        public static ObservableCollection<T> ToObservableCollection(this IEnumerable<T> source)
        {
            ObservableCollection<T> target = new ObservableCollection<T>();
            foreach (T item in source)
                target.Add(item);
            return target;
        }

    }
}

ProcessStartInfo p1 = new ProcessStartInfo();
p1.UseShellExecute = true;
p1.Verb = "runas";
p1.FileName = "cscript";
p1.Arguments = "I:\\WPKG\\wpkg.js /synchronize /quiet /nonotify";
Process p = new Process();
p.StartInfo = p1;
p.Start();