Store / Retrieve ConnectionString from appSettings.json in ASP.net Core 2 MVC app

Define your connection string(s) in appsettings.json

{
    "connectionStrings": {
        "appDbConnection": "..."
    }
}

Read its value on Startup

If you follow the convention and define your connection string(s) under connectionStrings, you can use the extension method GetConnectionString() to read its value.

public class Startup
{
    public IConfiguration Configuration { get; private set; }

    public Startup(IConfiguration configuration)
    {
        Configuration = configuration;
    }

    public void ConfigureServices(IServiceCollection services)
    {
        // Since you said you're using Dapper, I guess you might want to
        // inject IDbConnection?
        services.AddTransient<IDbConnection>((sp) => 
            new SqlConnection(this.Configuration.GetConnectionString("appDbConnection"))
        );

        // ...
    }
}

Use IDbConnection within the repository?

public interface ISpecificationRepository
{
    Specification GetById(int specificationId);
}

public SpecificationRepository : ISpecificationRepository
{
    private readonly IDbConnection _dbConnection;

    public SpecificationRepository(IDbConnection dbConnection)
    {
        _dbConnection = dbConnection;
    }

    public Specification GetById(int specificationId)
    {
        const string sql = @"SELECT * FROM [YOUR_TABLE]
                             WHERE Id = @specId;";

        return _dbConnection
            .QuerySingleOrDefault<Specification>(sql,
                new { specId = specificationId });
    }
}

Just need the connection string in a POCO?

You might use the Options Pattern.

1. Define a class that exactly matches the JSON object structure in appsettings.json

   public class ConnectionStringConfig
   {
       public string AppDbConnection { get; set; }
   }
   

2. Register that configuration on Startup

   public void ConfigureServices(IServiceCollection services)
   {
      // ...
   
      services.Configure<ConnectionStringConfig>(
          this.Configuration.GetSection("connectionStrings")
      );
   
      // ...
   }
   

3. Receive the accessor in your POCO

   public class YourPoco
   {
       private readonly ConnectionStringConfig _connectionStringConfig;
   
       public YourPoco(IOptions<ConnectionStringConfig> configAccessor)
       {
           _connectionStringConfig = configAccessor.Value;
   
           // Your connection string value is here:
           // _connectionStringConfig.AppDbConnection;
       }
   }
   

Notes:

1. See my sample codes on how to read values from appsettings.json both on Core 1.x and 2.0.
2. See how I setup if you have more than 1 connection string.