app.config migration for .net framework 4.x assemblies referenced in Core 2.0 Web API project?

Question

I have a .net core web api project that references 4.x assemblies that have app.config settings. Can I convert the app.config settings to appsettings.json without modifying the old assemblies or do I just drop in an app.config file with the settings for the older assemblies? If I just drop in app.config files, how can I change those settings on publish?

Answer 1

In Core 2.0 Web API you can point your configuration to old app.config as well this way:

IConfigurationRoot configuration = new ConfigurationBuilder()
    .AddJsonFile("appsettings.json", optional: true)
    .AddXmlFile("app.config")
    .Build();

Then you will be able to access for instance your connection string (and other app settings) by the keys (looks like a colon separated XML path). For instance for the connection string section you can get the value for connection name MyConnectionName this way:

key: "connectionStrings:add:MyConnectionName:connectionString" 
value: "...your connection string..."

Or you can implement your own LegacyConfigurationProvider as described here in details and have something like this:

IConfigurationRoot configuration = new ConfigurationBuilder()
    .AddJsonFile("appsettings.json", optional: true)
    .Add(new LegacyConfigurationProvider())
    .Build();

See also Configuration in ASP.NET Core .

Answer 2

So what I ended up doing is just adding app.config files. I also added an app.release.config and rename it to app.config as part of my publish process. I could not use config transformations. I could not get that to work in Azure CI at all.