Try This
string strConn = ConfigurationManager.ConnectionStrings["connection"].ConnectionString.ToString();
connection = new SqlConnection(strConn);
SqlCommand sqlcmd = new SqlCommand();
connection.Open();
sqlcmd.CommandType = CommandType.StoredProcedure;
sqlcmd.CommandText = "insert_Post";
sqlcmd.Connection = connection;
SqlCommand cmd = new SqlCommand("select userid from user where username ='" + lblusername.Text + "'", connection);
SqlDataReader dr = cmd.ExecuteReader();
if (dr.Read())
a = dr["userid"].ToString();
dr.Close();
SqlParameter userid = new SqlParameter();
Reg_Id.ParameterName = "@userid";
Reg_Id.Value = a;
sqlcmd.Parameters.Add(userid);
SqlParameter post = new SqlParameter();
Category.ParameterName = "@post";
Category.Value = Txtpost.Text;
sqlcmd.Parameters.Add(post);
SqlParameter postdate = new SqlParameter();
postdate.ParameterName = "@postdate";
postdate.Value = "txtdate";
sqlcmd.Parameters.Add(postdate);
or
http://www.c-sharpcorner.com/Forums/Thread/153165/