using System;

using System.IO;

using System.Xml.Linq;

public class Program

{

    public static void Main()

    {

        var s = new StringReader(@"

        <x:y xmlns:x=""https://foo.example.com/x"">

        </x:y>

        ");

        var x = XElement.Load(s);

        Console.WriteLine(x.GetPrefixOfNamespace(x.Name.Namespace));

    }

}

​