Indexer notation requires a reference to this. Since static methods don't have an reference to a particular instance of the class, you can't use this with them, and consequently you can't use indexer notation on static methods

Example of an indexer:
class ParentClass
{
    private string[] range = new string[5];
    public string this[int indexrange]
    {
        get
        {
            return range[indexrange];
        }
        set
        {
            range[indexrange] = value;
        }
    }
}

/* The Above Class just act as array declaration using this pointer */

class childclass
{
    public static void Main()
    {
        ParentClass obj = new ParentClass();

        /* The Above Class ParentClass create one object name is obj */

        obj[0] = "ONE";
        obj[1] = "TWO";
        obj[2] = "THREE";
        obj[3] = "FOUR ";
        obj[4] = "FIVE";

        Console.WriteLine("{0}\n,{1}\n,{2}\n,{3}\n,{4}\n", obj[0], obj[1], obj[2], obj[3], obj[4]);
    }
}