c# - Using app.config in .Net Core

Question

I have problem. I need to write a program in .Net Core(C#) which use app.config like this:

<?xml version="1.0" encoding="utf-8" ?>
<configuration>
  <configSections>
    <section name="custom" type="ConfigurationSample.CustomConfigurationSection, ConfigurationSample"/>
  </configSections>
  <connectionStrings>
    <add name="sampleDatabase" connectionString="Data Source=localhostSQLExpress;Initial Catalog=SampleDatabase;Integrated Security=True"/>
  </connectionStrings>
  <appSettings>
    <add key="sampleApplication" value="Configuration Sample"/>
  </appSettings>
  <custom>
    <customConfigurations>
      <add key="customSample" name="Mickey Mouse" age="83"/>
    </customConfigurations>
  </custom>
</configuration>

and I write:

string connectionString = ConfigurationManager.ConnectionStrings["sampleDatabase"].ConnectionString;
Console.WriteLine(connectionString);

// read appSettings configuration
string appSettingValue = ConfigurationManager.AppSettings["sampleApplication"];
Console.WriteLine(appSettingValue);

and it is example from the internet so I thought would work, but I am getting exceptions:

System.Configuration.ConfigurationErrorsException: 'Error Initializing the configuration system.'
Inner Exception
TypeLoadException: Could not load type 'System.Configuration.InternalConfigurationHost' from assembly 'CoreCompat.System.Configuration, Version=4.2.3.0, Culture=neutral, PublicKeyToken=null' because the method 'get_bundled_machine_config' has no implementation (no RVA).

I downloaded via NuGet - Install-Package CoreCompat.System.Configuration -Version 4.2.3-r4 -Pre and still don't work. Maybe someone can help me?

Answer 1

It is possible to use your usual System.Configuration even in .NET Core 2.0 on Linux. Try this test example:

1. Created a .NET Standard 2.0 Library (say MyLib.dll)
2. Added the NuGet package System.Configuration.ConfigurationManager v4.4.0. This is needed since this package isn't covered by the meta-package NetStandard.Library v2.0.0 (I hope that changes)
3. All your C# classes derived from ConfigurationSection or ConfigurationElement go into MyLib.dll. For example MyClass.cs derives from ConfigurationSection and MyAccount.cs derives from ConfigurationElement. Implementation details are out of scope here but Google is your friend.
4. Create a .NET Core 2.0 app (e.g. a console app, MyApp.dll). .NET Core apps end with .dll rather than .exe in Framework.
5. Create an app.config in MyApp with your custom configuration sections. This should obviously match your class designs in #3 above. For example:

---

<?xml version="1.0" encoding="utf-8"?>
<configuration>
  <configSections>
    <section name="myCustomConfig" type="MyNamespace.MyClass, MyLib" />
  </configSections>
  <myCustomConfig>
    <myAccount id="007" />
  </myCustomConfig>
</configuration>

That's it - you'll find that the app.config is parsed properly within MyApp and your existing code within MyLib works just fine. Don't forget to run dotnet restore if you switch platforms from Windows (dev) to Linux (test).

Additional workaround for test projects

If you're finding that your App.config is not working in your test projects, you might need this snippet in your test project's .csproj (e.g. just before the ending </Project>). It basically copies App.config into your output folder as testhost.dll.config so dotnet test picks it up.

  <!-- START: This is a buildtime work around for https://github.com/dotnet/corefx/issues/22101 -->
  <Target Name="CopyCustomContent" AfterTargets="AfterBuild">
    <Copy SourceFiles="App.config" DestinationFiles="$(OutDir)esthost.dll.config" />
  </Target>
  <!-- END: This is a buildtime work around for https://github.com/dotnet/corefx/issues/22101 -->