output.xml

<x y="uh, 5">
  <child1 />hi. 2×4=8<child2 c2a1="non-interpolated $tring{}" /></x>

XElementDemo.cs

using System;
using System.Xml.Linq;

class XElementDemo
{
    static void Main(string[] args)
    {
        var tree = new XElement(
            "x",
            new XAttribute("y", $"uh, {1+4}"),
            new XElement("child1"),
            $"hi. 2×4={2*4}",
            new XElement(
                "child2",
                new XAttribute("c2a1", "non-interpolated $tring{}")));
        Console.WriteLine(tree);
    }
}