Try using this conversion.
public static class Serializa
{
    public static string SerializaParaString<T>(this T valor)
    {
        XmlSerializer xml = new XmlSerializer(valor.GetType());
        StringWriter retorno = new StringWriter();
        xml.Serialize(retorno, valor);
        return retorno.ToString();
    }
    public static object DeserializaParaObjeto(string valor, Type tipo)
    {
        XmlSerializer xml = new XmlSerializer(tipo);
        var valorSerealizado = new StringReader(valor);
        return xml.Deserialize(valorSerealizado);
    }
}
Call for conversion methods:
class Program
{
    static void Main(string[] args)
    {
        var stringParaSerializar = "Frase Qualquer";
        var valor = Serializa.SerializaParaString(stringParaSerializar);
        Console.WriteLine(valor);
        Console.ReadKey();
    }
}
Upshot:
<?xml version="1.0" encoding="utf-16"?>
<string>Frase Qualquer</string>
More information about serializing and deserializar XML objects to string: C# -  Serializing Objects for String and vice versa
							
							
						 
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– Igor Carreiro