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C# Connection string Error C# to Mysql insert button code


New Coder
I want to insert table in db and make connection to mysql to vs.

Error for below code is:
System.NullReferenceException: 'Object reference not set to an instance of an object.'

System.Configuration.ConnectionStringSettingsCollection.this[string].get returned null.

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Configuration;
using System.Windows.Forms;
using System.Data;
using System.Drawing;
using System.Threading.Tasks;
using System.Data.SqlClient;

private void button6_Click(object sender, EventArgs e)
string id = textBox6.Text;
string name = textBox7.Text;
string salary = textBox8.Text;
textBox6.Text = "";
textBox7.Text = "";
textBox8.Text = "";
string query = "INSERT INTO employee VALUES(@employee_id, @employee_name, @employee_salary)";
string constr = ConfigurationManager.ConnectionStrings["constr"].ConnectionString;
using (SqlConnection con = new SqlConnection(constr))
using (SqlCommand cmd = new SqlCommand(query))
cmd.Parameters.AddWithValue("@employee_id", id);
cmd.Parameters.AddWithValue("@employee_name", name);
cmd.Parameters.AddWithValue("@employee_salary", salary);
cmd.Connection = con;

Error for below code is:

System.Data.SqlClient.SqlException: 'A network-related or instance-specific error occurred while establishing a connection to SQL Server. The server was not found or was not accessible. Verify that the instance name is correct and that SQL Server is configured to allow remote connections. (provider: Named Pipes Provider, error: 40 - Could not open a connection to SQL Server)'

private void button8_Click(object sender, EventArgs e)
string connetionString;
SqlConnection cnn;
connetionString = @"Data Source=LAPTOP-NDOAR92R;Initial Catalog=testDB;User ID=root;Password=mysql";
cnn = new SqlConnection(connetionString);
MessageBox.Show("Connection Open!");


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