How to set comma after reading a email from a table?

SqlConnection conn=new SqlConnection("conn string");
conn.Open();
SqlCommand cmd = new SqlCommand("select FirstName from Employees", conn);
SqlDataReader dr = cmd.ExecuteReader();
string str = "";
string com = ",";
while (dr.Read())
{
str = dr["name"] + com + str;
}

output:
shakeer@gmail.com,hussain@rediffmail.com,Rama@yahoo.com,