ASP.NET

Get the connection string from web.config and connect to SQL Server in ASP.NET

ASP.NET · By the Postback Notes editorial team · Published 2026-08-31

Every data-driven Web Forms page starts the same way: read the connection string from web.config, open a SqlConnection, run a command, close it. Doing these steps correctly once, in a helper, saves a lot of trouble later.

1. Store it in web.config

<configuration>
  <connectionStrings>
    <!-- Windows authentication: the IIS application pool identity logs in -->
    <add name="Db" providerName="System.Data.SqlClient"
         connectionString="Data Source=SQL01;Initial Catalog=Shop;Integrated Security=True;Application Name=ShopWeb" />
  </connectionStrings>
</configuration>

Common variants of the connectionString value:

With Windows authentication, grant the application pool's identity (for example IIS APPPOOL\Shop) a login and only the database permissions it needs. With a SQL login, protect the password: see appSettings vs connectionStrings for encrypting the section.

2. Read it once

using System.Configuration;

public static class Db
{
    public static string ConnectionString { get; } =
        ConfigurationManager.ConnectionStrings["Db"]?.ConnectionString
        ?? throw new ConfigurationErrorsException("Connection string 'Db' is missing from web.config.");
}

Failing early with a clear message beats a NullReferenceException deep in a page.

3. Open, use, dispose

using (var con = new SqlConnection(Db.ConnectionString))
using (var cmd = new SqlCommand("SELECT COUNT(*) FROM dbo.Products", con))
{
    con.Open();
    int count = (int)cmd.ExecuteScalar();
    lblCount.Text = count.ToString();
}

Open late, close early: using returns the connection to the pool even if an exception is thrown. Creating a new SqlConnection per operation is cheap thanks to connection pooling; keeping one open in a static field or in session is a classic source of errors under load.

Errors you may see

Next steps: running commands and keeping them safe with parameters.

Official documentation

• Microsoft Learn: Connection string syntax, Connection strings and configuration files
• Microsoft Learn: SQL Server connection pooling
• Microsoft Learn: Introduction to Microsoft.Data.SqlClient