using System;

using System.Collections.Generic;

public class Program

{

    public static void Main()

    {

        string parameterValue1 = "Value1";

        string parameterValue2 = ""; // purposefully providing an empty value

        string parameterValue3 = "Value3";

​

        var listParamIn = new List<string> { parameterValue1, parameterValue2, parameterValue3 };

        var listParamOut = new List<string>();

​

        foreach (string s in listParamIn){

            if (string.IsNullOrEmpty(s))

                continue;

            listParamOut.Add(s);

        }

        string newValue = String.Join(", ", listParamOut);

        Console.WriteLine(newValue);

​

        /* continuw ith your transaction

        using (Transaction t = new Transaction(revitDoc, "Set Parameter name")){

            t.Start();

            duct.LookupParameter("New").Set(newValue);

            t.Commit();

        }

        */

    }

}

​