Trying To Test Extension Method That Converts A List Of Strings In A String Comma Separated?

Im trying to test my extension method that converts a list of strings in a string comma separated:

public static class Extensions
{
public static string ToCommaString<T>(this IList<T> input)
{
StringBuilder sb = new StringBuilder();
foreach (T value in input)
{
sb.Append(value);
sb.Append(",");
}
return sb.ToString();
}
public void TestExtension()
{
IList test=new List<string>();
//test.ToCommaString doesnt appear
}
}

The issue is that in the method TestExtension i cant use ToCommaString method. Do you know what's happening?

Could i make available for all my web application this extension method registering in web.config or something similar?