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using System;
using System.Collections;
using System.Configuration;
using System.Reflection;
using System.Web;
[assembly: PreApplicationStartMethod(typeof(EnvSettings.SettingsProcessor), "Start")]
namespace EnvSettings
public static class SettingsProcessor
private const string AppSettingPrefix = "APPSETTING_";
private const string SqlServerPrefix = "SQLCONNSTR_";
private const string MySqlServerPrefix = "MYSQLCONNSTR_";
private const string SqlAzureServerPrefix = "SQLAZURECONNSTR_";
private const string CustomPrefix = "CUSTOMCONNSTR_";
public static void Start()
// Go through all the environment variables and process those that start
// with one of our prefixes
foreach (DictionaryEntry entry in Environment.GetEnvironmentVariables())
var name = (string)entry.Key;
var val = (string)entry.Value;
if (name.StartsWith(MySqlServerPrefix, StringComparison.OrdinalIgnoreCase))
name = name.Substring(MySqlServerPrefix.Length);
SetConnectionString(name, val, "MySql.Data.MySqlClient");
else if (name.StartsWith(SqlAzureServerPrefix, StringComparison.OrdinalIgnoreCase))
name = name.Substring(SqlAzureServerPrefix.Length);
SetConnectionString(name, val, "System.Data.SqlClient");
else if (name.StartsWith(SqlServerPrefix, StringComparison.OrdinalIgnoreCase))
name = name.Substring(SqlServerPrefix.Length);
SetConnectionString(name, val, "System.Data.SqlClient");
else if (name.StartsWith(CustomPrefix, StringComparison.OrdinalIgnoreCase))
name = name.Substring(CustomPrefix.Length);
SetConnectionString(name, val);
else if (name.StartsWith(AppSettingPrefix, StringComparison.OrdinalIgnoreCase))
name = name.Substring(AppSettingPrefix.Length);
ConfigurationManager.AppSettings[name] = val;
private static void SetConnectionString(string name, string connString, string providerName = null)
// Set the value of the connection string if it exists. Note that we don't
// create a new one if it doesn't, as the app must already have some logic
// that tries to use this connection string.
ConnectionStringSettings settings = ConfigurationManager.ConnectionStrings[name];
if (settings != null)
settings.SetData(connString, providerName);
private static void SetData(this ConnectionStringSettings settings, string connString, string providerName)
// Note: we need to use reflection to be able to modify connection strings
// This is an unfortunate framework limitation, and we are working with
// the ASP.NET team to relax that restriction in the next framework
FieldInfo readOnlyField = typeof(ConfigurationElement).GetField(
"_bReadOnly", BindingFlags.Instance | BindingFlags.NonPublic);
readOnlyField.SetValue(settings, value: false);
settings.ConnectionString = connString;
// we should leave the original provider name if a new provider name is not specified
if (providerName != null)
settings.ProviderName = providerName;

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@fhtino fhtino commented Dec 28, 2018

Not clear why you don't allow to add non-existing connection strings. For appSettings you don't worry about that and you add non-existing items. The following code "unlocks" the collection and adds a new item. Do you see any issue?

FieldInfo readOnlyField = typeof(ConfigurationElementCollection).GetField("bReadOnly", BindingFlags.Instance | BindingFlags.NonPublic);
readOnlyField.SetValue(ConfigurationManager.ConnectionStrings, value: false);

settings = new ConnectionStringSettings()
    Name = name,
    ConnectionString = connstring,
    ProviderName = provider

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