You Must Declare the scaler variable @fullname while insert the data

Question

using System;
using System.Collections.Generic;
using System.Data;
using System.Data.SqlClient;
using System.Linq;
using System.Text;

namespace Trading
{
    class DBConnection
    {
       private  SqlConnection _con;
        public SqlCommand cm;
        public DBConnection()
        {
            _con = new SqlConnection(@"Data Source=(LocalDB)\MSSQLLocalDB;AttachDbFilename=C:\Users\Public\TTMS\dbTSIS.mdf;Integrated Security=True;Connect Timeout=30");
        }
        public void SqlQuery(string queryText)
        {
           
             cm = new SqlCommand(queryText, _con);
            if (_con.State == ConnectionState.Closed)
            {
                _con.Open();
            }          
            cm.ExecuteNonQuery();
            _con.Close();
        }
       

    }
}

 

 

 public partial class FrmUserEntry : Form
    {
        private DBConnection con;
           // SqlConnection con = new SqlConnection(@"Data Source=(LocalDB)\MSSQLLocalDB;AttachDbFilename=C:\Users\Public\TTMS\dbTSIS.mdf;Integrated Security=True;Connect Timeout=30");
           //SqlCommand cm = new SqlCommand();
        public FrmUserEntry()
        {
           
            InitializeComponent();
            rsBtnUpdate.Enabled = false;
            
           
        }
       
        private void rsBtnSave_Click(object sender, EventArgs e)
        {
            try
            {
                con = new DBConnection();
                if (MessageBox.Show("Are you sure you want to save this user?", "Saving Record", 
                MessageBoxButtons.YesNo, MessageBoxIcon.Question) == DialogResult.Yes)
                con.SqlQuery("INSERT INTO tbUser(fullname,username,password,contactno)VALUES(@fullname,@username,@password,@contactno)");       
                con.cm.Parameters.AddWithValue("@fullname", mTxtFullname.Text);
                con.cm.Parameters.AddWithValue("@username", mTxtUserName.Text);
                con.cm.Parameters.AddWithValue("@password", mTxtPassword.Text);
                con.cm.Parameters.AddWithValue("@contactno", mTxtContact.Text);
                MessageBox.Show("User has been Successfully saved.");
                Clear();
            
            }
            catch(Exception ex)
            {
                MessageBox.Show(ex.Message);
            }
        }

 

when i save the data error like You must declare the scaler variable @fullname.

pls help