Plzz Solve In this code error

Question

SqlCommand cmd = new SqlCommand("insert into tblProduct(ProductID,Name,UnitPrice,QtyAvl)Values(@ProductID,@Name,@UnitPrice,@QtyAvl)", con);

cmd.Connection = con;

cmd.Parameters.Add("@ProductID", SqlDbType.Int).Value = TextBox1.Text;

cmd.Parameters.Add("@Name", SqlDbType.VarChar, 50).Value = TextBox2.Text;

cmd.Parameters.Add("@UnitPrice", SqlDbType.NVarChar, 50).Value = TextBox3.Text;

cmd.Parameters.Add("@QtyAvl", SqlDbType.NVarChar, 50).Value = TextBox4.Text;

cmd.ExecuteNonQuery();

SqlDataAdapter ds = new SqlDataAdapter();

DataTable dt = new DataTable();

ds.Fill(dt);

GridView2.DataSource = dt;

GridView2.DataBind();

cmd.Dispose();

con.Close();

what's error in code