Use custom serialization using ICollection interface implementation, and mark Hashtable as [NonSerialized], instead, use custom collection instead of Hashtable or use it internally, for collection of elements, as in this example:
using System;
using System.IO;
using System.Collections;
using System.Xml.Serialization;

public class Test{
static void Main(){
Test t = new Test();
t.SerializeCollection(“coll.xml”);
}

 private void SerializeCollection(string filename){
 Employees Emps = new Employees();
 // Note that only the collection is serialized -- not the 
 // CollectionName or any other public property of the class.
 Emps.CollectionName = "Employees";
 Employee John100 = new Employee("John", "100xxx");
 Emps.Add(John100);
 XmlSerializer x = new XmlSerializer(typeof(Employees));
 TextWriter writer = new StreamWriter(filename);
      x.Serialize(writer, Emps);
 }

}
public class Employees:ICollection{
public string CollectionName;
private ArrayList empArray = new ArrayList();

 public Employee this[int index]{
 get{return (Employee) empArray[index];}
 }
 public void CopyTo(Array a, int index){
      empArray.CopyTo(a, index);
 }
 public int Count{
 get{return empArray.Count;}
 }
 public object SyncRoot{
 get{return this;}
 }
 public bool IsSynchronized{
 get{return false;}
 }
 public IEnumerator GetEnumerator(){
 return empArray.GetEnumerator();
 }
 public void Add(Employee newEmployee){
      empArray.Add(newEmployee);
 }

}

public class Employee{
public string EmpName;
public string EmpID;
public Employee(){}
public Employee(string empName, string empID){
EmpName = empName;
EmpID = empID;
}
}

Now you can check here :
https://weblogs.asp.net/avnerk/Serilalizing-Dictionaries