How to add commas to a string?

String.Join takes multiple strings in some form for the second argument, so you need to read all your results into a string list of some sort and then pass that in once to String.Join

List<string> contentResults = new List<string>();

SqlDataReader reader = command.ExecuteReader();

while (reader.Read())
{
    contentResults.Add(reader.GetString(0));
}

result = string.Join(", ", contentResult);
</string></string>

In addition to above 2 solution, here is the another way of adding comma's

StringBuilder result = new StringBuilder();
 while (reader.Read())
        {
            string content = (reader.GetString(0));
            result.Append(content + ","); ;
        }
        return result.ToString().TrimEnd(',');