what is the c# code concatenate two different datatype column in single column with bind dropdownlist ?

That will really depend on the data types.

So, look at this example where I have a list of strings and a list of integers that I want to connect.
I can simply do a foreach (or something else) to match them one-to-one.
It really depends on how the data fits together, the type of data and the data type of the result.

using System.Collections.Generic;
using System.Linq;

namespace DW_409612_CS_CON
   class Program
      static void Main(string[] args)
         List<string> lst_str = new List<string> { "a", "b", "c" };
         List<int> lst_int = new List<int> { 17, 52, 64 };

         //List<string> lst_str2 =
            from str in lst_str
            let i_strOffset = lst_str.IndexOf(str)
            from i in lst_int
            let i_intOffset = lst_int.IndexOf(i)
            where i_strOffset.Equals(i_intOffset)
            select (str + '-' + i.ToString())
         ).ToList().ForEach(s => System.Diagnostics.Debug.WriteLine(s));

output (in Debug Window):