The first step is to define the basic converter, with minimal validation which converts to and from an integer. This is done with the code below.

using System;
using Ant.ContentEntry;
namespace HowTos
{
    /// <summary>
    /// Converter which checks that the value is an integer and is divisible by 3, it also checks that the Int is between 0 and 600
    /// </summary>
    public class IntegerDivisibleConverter : Converter
    {
        ContentDefinitionField def;
        /// <summary>
        /// Init method is called by the content entry framework immediately after construction
        /// </summary>
        /// <param name="def">Field which is represented by this converter</param>
        public void Init(ContentDefinitionField def)
        {
            this.def = def;
        }
        /// <summary>
        /// Gets the regular expression that represents the validation of this converter. In this case it is not relevant
        /// </summary>
        public string GetRegEx()
        {
            return null;
        }
        /// <summary>
        /// Returns the .net type which this converter converts to
        /// </summary>
        /// <returns>Int32 in this case</returns>
        public Type GetSupportedType()
        {
            return typeof(Int32);
        }
        /// <summary>
        /// Allows for the XmlSchema that represents the entire content to be modified to represent specific validation.
        /// </summary>
        /// <param name="user">Logged on user</param>
        /// <param name="type">Simple type to modify</param>
        public void ModifyXmlSchemaType(Ant.Security.AuthenticatedUser user, System.Xml.Schema.XmlSchemaSimpleType type)
        {
            // TODO:  Add IntegerDivisibleConverter.ModifyXmlSchemaType implementation
        }
        /// <summary>
        /// Gets the integer which represents the string
        /// </summary>
        /// <param name="str">string which represents an integer</param>
        /// <param name="context">Context in which we are editing</param>
        /// <returns>Integer</returns>
        public object GetObject(string str, System.Collections.IDictionary context)
        {
            try
            {
                if (str == null || str.Length == 0)
                {
                    if (this.def.IsRequired)
                    {
                        throw new ConverterValidationException("Field "+def.FieldName+" is not specified but is marked as required");
                    }
                    else 
                        return null;
                }
                // TODO: validation
                Int32 val = Int32.Parse(str);
                return val;
            }
            catch (Exception e)
            {
                throw new ConverterValidationException("Can't convert value "+str+" using converter IntegerDivisibleConverter", e);
            }
        }
        /// <summary>
        /// Gets the string which represents this value
        /// </summary>
        /// <param name="obj">Object which is the value</param>
        /// <param name="context">Context in which we are editing</param>
        /// <returns>string which represents the integer</returns>
        public string GetString(object obj, System.Collections.IDictionary context)
        {
            if (obj is Int32)
            {
                Int32 i = (Int32)obj;
                // TODO: validation
                return i.ToString();
            }
            else if (obj == null)
            {
                if (this.def.IsRequired)
                {
                    throw new ConverterValidationException("Field "+def.FieldName+" is not specified but is marked as required");
                }
                else 
                    return null;
            }
            else
                throw new ConverterValidationException("IntegerDivisibleConverter converter can not convert "+obj.GetType().ToString());
        }
    }
}