IConfiguration.Get throwing "Cannot create instance of type 'System.String' because it is missing a public parameterless constructor."

[Daniel15]

I'm trying to upgrade a site from Beta 6 to RC 1. The site uses Entity Framework 6 so I need to manually load the connection string from the config file. I have a line of code that looks like this:

config.Get("Data:DefaultConnection:ConnectionString")

This is throwing Cannot create instance of type 'System.String' because it is missing a public parameterless constructor.

Stack:

   at Microsoft.Extensions.Configuration.ConfigurationBinder.CreateInstance(Type type)
   at Microsoft.Extensions.Configuration.ConfigurationBinder.BindInstance(Type type, Object instance, IConfiguration config)
   at Microsoft.Extensions.Configuration.ConfigurationBinder.Get(IConfiguration configuration, Type type)
   at Microsoft.Extensions.Configuration.ConfigurationBinder.Get[T](IConfiguration configuration, T defaultValue)
   at Daniel15.Data.DatabaseContext..ctor(IConfiguration config) in C:\Users\danlo\Documents\Visual Studio 2015\Projects\dan.cx\Daniel15.Data\DatabaseContext.cs:line 40

Config is a .json file that looks like:

{
  "Data": {
    "DefaultConnection": {
      "ConnectionString": "Server=192.168.146.1; Database=daniel15_site; Uid=dev; Pwd=password;"
    }
  }
}

[Daniel15]

Looks like I need to use config["Data:DefaultConnection:ConnectionString"] now. Was this change documented? I didn't see it in any of the release notes.

[kalch]

I got the same as you Daniel. Couldn't find it in the release notes.

[MichaCo]

Yes this has been documented here: aspnet/Announcements#55
The interfaces of IConfiguration changed, you might want to use GetSection now, to retrieve a section of your configuration by path, or use the indexer.

Get<T> was added as an extension of the ConfigurationBinder to directly bind the given section to an object, like config.Get<MyClass>() or config.Get<MyClass>("section")

That being said, lately an overload Get<T>(T default) has been added where you can pass in a default value of T which causes your problem because the T gets implicitly resolved.
@divega, @HaoK maybe the default value overloads should be removed or renamed, because it will cause a lot of confusion?

[divega]

Yeah. I have a very clear expectation of what config.Get("Data:DefaultConnection:ConnectionString") will do, and it is not what is happening. We have two overloads that could match that call:

public static T Get<T>(this IConfiguration configuration, T defaultValue)
...
public static T Get<T>(this IConfiguration configuration, string key)
...

But in order to the right one (the second one) to match you would need to specify the generic argument as string in the call. So yes, maybe we need to get rid of the first overload.

[divega]

BTW, the other overload that takes a default value doesn't seem to be affected:

public static T Get<T>(this IConfiguration configuration, string key, T defaultValue)

[HaoK]

Yes I don't really like the default value overloads. Nuking then works for me too.

[divega]

@HaoK we will only nuke one of them, just to annoy you 😄

[MichaCo]

Yup

public static T Get<T>(this IConfiguration configuration, string key, T defaultValue)

is not ambiguous as the other overloads.
Although you could change the first one to

public static T Get<T>(this IConfiguration configuration, object defaultValue)

Not very nice (potential type mismatch) but would work, too

[HaoK]

ba1d9b4 Get has been split into GetValue/Bind