using System;

using System.Xml;

using System.Linq;

​

​

public class Program

{   

    public static void Main()

    {

        var str = "<verticalTab>\k</verticalTab>";

        Console.WriteLine(RemoveInvalidXmlChars(str));

        Console.WriteLine(CheckValidXmlChars(str));

    }

    public static string RemoveInvalidXmlChars(string content)

    {

       return new string(content.Where(ch => System.Xml.XmlConvert.IsXmlChar(ch)).ToArray());

    }

    public static bool CheckValidXmlChars(string content)

    {

       return content.All(ch => System.Xml.XmlConvert.IsXmlChar(ch));

    }

}