how to add a method or region in end of a class with parse cs file

hello

i wanna read a cs file and add a method or region in end of a class .

this operation is located in a visual studio add-in project (extensibility project).

cs file sample:

using System;
using System.Collections.Generic;
using System.Text;
using System.Runtime.Serialization;
using Tools.Cmn;
using Common.Cmn;
namespace Life.Cmn
{
    [Serializable]
    [LocalCache(false)]
    [Table(BaseTableName = "lf.Config", TableName = "lf.Config")]
    public partial class LifeConfig : CommonConfig
    {
        public LifeConfig()
        {
        }

        public LifeConfig(SerializationInfo info, StreamingContext context)
            : base(info, context)
        {
        }

        [Association(OtherKey = "Subject,Name", ThisKey = "Subject,Name")]
        public EntityList<LifeConfigHistory> LifeConfigHistoryList
        {
            get { return GetNavChildList<LifeConfigHistory>("LifeConfigHistoryList"); }
        }

    }
}

and i want add below code in end of class:

 #region UniversalLifePrm

        public static T UniversalLifePrm<T>(string configName)
        {
            return LifeCmn.GetConfigVal("UniversalLifePrm", configName, default(T));
        }

        #endregion

in finally my class will be this:

using System;
using System.Collections.Generic;
using System.Text;
using System.Runtime.Serialization;
using Tools.Cmn;
using Common.Cmn;
namespace Life.Cmn
{
    [Serializable]
    [LocalCache(false)]
    [Table(BaseTableName = "lf.Config", TableName = "lf.Config")]
    public partial class LifeConfig : CommonConfig
    {
        public LifeConfig()
        {
        }

        public LifeConfig(SerializationInfo info, StreamingContext context)
            : base(info, context)
        {
        }

        [Association(OtherKey = "Subject,Name", ThisKey = "Subject,Name")]
        public EntityList<LifeConfigHistory> LifeConfigHistoryList
        {
            get { return GetNavChildList<LifeConfigHistory>("LifeConfigHistoryList"); }
        }

        #region UniversalLifePrm

        public static T UniversalLifePrm<T>(string configName)
        {
            return LifeCmn.GetConfigVal("UniversalLifePrm", configName, default(T));
        }

        #endregion

    }
}

please help me.

thanks.