ArgumentException when trying to open a SqlConnection - asp.net

Question

This code is for the button:
But every time i run it say's ArgumentException was unhandled by user code.
It's a site made to look up in at Database to see if there is enough ink.
using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Data.SqlClient;
using System.Configuration;
public partial class _Default : System.Web.UI.Page
{
SqlConnection con = new SqlConnection(ConfigurationManager.ConnectionStrings["ConnectionString"].ConnectionString);
protected void Page_Load(object sender, EventArgs e)
{
con.Open();
}
protected void Button1_Click(object sender, EventArgs e)
{
SqlCommand cmd = new SqlCommand("insert into Toner Values('"+txtFname.Text+"','"+txtLname.Text+"','"+TxtCity.Text+"')",con);
cmd.ExecuteNonQuery();
con.Close();
Label1.Visible = true;
Label1.Text = "Indsætning succesfuld!!!";
TxtCity.Text = "";
txtFname.Text = "";
txtLname.Text = "";
}
protected void TextBox3_TextChanged(object sender, EventArgs e)
{
}
}

Answer 1

My best guess is that TxtCity should be txtCity.
Try this:
using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Data.SqlClient;
using System.Configuration;
public partial class _Default : System.Web.UI.Page
{
protected void Button1_Click(object sender, EventArgs e)
{
string query = "insert into Toner Values (#Fname, #Lname, #City)";
SqlConnection con = new SqlConnection(ConfigurationManager.ConnectionStrings("ConnectionString").ConnectionString);
SqlCommand cmd = new SqlCommand(query, con);
//Use parameterized query to prevent SQL injection
cmd.Parameters.Add("Fname", SqlDbType.VarChar, 50).Value = txtFname.Text;
cmd.Parameters.Add("Lname", SqlDbType.VarChar, 50).Value = txtLname.Text;
//C# is case-sensitive... is it txtCity or TxtCity?
cmd.Parameters.Add("City", SqlDbType.VarChar, 50).Value = TxtCity.Text;
con.Open();
cmd.ExecuteNonQuery();
con.Close();
Label1.Visible = true;
Label1.Text = "Indsætning succesfuld!!!";
TxtCity.Text = "";
txtFname.Text = "";
txtLname.Text = "";
}
}