ASP.NET Core Web API - ConnectionString in project reference

It seems ConfigurationManager failed to read json file,have you tried add a .config file in your WebApi project? I tried two solutions as below :

public  class Class1
    {
        public  string? con1 { get; set; }

        public   string? con2 { get; set; }
        public readonly IConfiguration Config;

        public Class1()
        {
            //read connectstring from .config file
            con1 = System.Configuration.ConfigurationManager.ConnectionStrings["ReadConnectStringContext"].ConnectionString;
            Config = new ConfigurationBuilder().SetBasePath(Directory.GetCurrentDirectory()).AddJsonFile("appsettings.json").Build();
            //read connectstring from appsettings.json
            con2 = Config.GetConnectionString("ReadConnectStringContext");
        }

    }

The structure:

The Result:

Here is the documentation from MS:

Class libraries can access configuration settings in the same way as executable apps, however, the configuration settings must exist in the client app's App.config file. Even if you distribute an App.config file alongside your library's assembly file, the library code will not read the file.

A quick fix for you is to place the correct DB connection string section in the Web/WinForm Apps settings file.

If you really want to use the app settings from the library project, using a custom setting to reference the app settings from in the library project. Here is the reference.