ASP.NET

Read appSettings values from web.config in ASP.NET (C#)

ASP.NET · By the Postback Notes editorial team · Published 2026-08-14

The <appSettings> section holds simple key-value settings: a page size, a feature switch, an e-mail address. Reading them is one line; turning the strings into typed values with sensible defaults is where a small helper pays off.

The settings

<configuration>
  <appSettings>
    <add key="PageSize" value="25" />
    <add key="EnableNewCheckout" value="true" />
    <add key="AdminEmails" value="ops@example.org;billing@example.org" />
    <add key="SupportPhone" value="" />
  </appSettings>
</configuration>

Reading them

using System.Configuration;   // add a reference to System.Configuration

string raw = ConfigurationManager.AppSettings["PageSize"];   // "25", or null if the key is missing

A typed wrapper keeps conversions and defaults in one place:

public static class AppConfig
{
    public static int PageSize => GetInt("PageSize", 20, min: 5, max: 200);
    public static bool EnableNewCheckout => GetBool("EnableNewCheckout", false);
    public static string[] AdminEmails =>
        (ConfigurationManager.AppSettings["AdminEmails"] ?? "")
        .Split(new[] { ';' }, StringSplitOptions.RemoveEmptyEntries)
        .Select(s => s.Trim()).ToArray();

    private static int GetInt(string key, int fallback, int min, int max) =>
        int.TryParse(ConfigurationManager.AppSettings[key], NumberStyles.Integer, CultureInfo.InvariantCulture, out int v)
            && v >= min && v <= max ? v : fallback;

    private static bool GetBool(string key, bool fallback) =>
        bool.TryParse(ConfigurationManager.AppSettings[key], out bool v) ? v : fallback;
}

// usage
gvOrders.PageSize = AppConfig.PageSize;
if (AppConfig.EnableNewCheckout) { /* ... */ }

Parse numbers with the invariant culture so that a server set to a culture with different formats reads the file the same way.

In markup

<asp:Literal runat="server" Mode="Encode" Text="<%$ AppSettings:SupportPhone %>" />

The <%$ … %> expression builder only works in server control property values, not as free text in the page.

Changing a value on a live site

Saving web.config restarts the application domain: in-process session state is lost and the first request after the change is slower. Plan changes accordingly, or move frequently changed settings to a database table.

Secrets

An API key in appSettings ends up in source control unless you prevent it. Use the file attribute to merge values from a file that is not committed:

<appSettings file="App_Data\secrets.config">
  <add key="PageSize" value="25" />
</appSettings>
<!-- App_Data\secrets.config (not in source control) -->
<appSettings>
  <add key="PaymentApiKey" value="..." />
</appSettings>

If the external file is missing, ASP.NET simply ignores it, so the application still starts in development. The section can also be encrypted with aspnet_regiis, as explained in appSettings vs connectionStrings.

Common mistakes

Official documentation