Q:

How to get connection string in class library project

In my .net solution, I have two different projects: An MVC core web application project and a class library project. In web application project, database connection string is in appsettings.json file. I would like to access that connection string from class library project. Is it possible? If yes, how? If there is any better approach to get connection string, please let me know.

A:

To make simple and trivial example you could create a class in your class library that will represent singleton or ambient context to provide connection string:
public static class ConnectionString
{
    public static string Value {get; set;}
}

And then in you app Startup class you set it's value:
var builder = new ConfigurationBuilder()
...
var configuration = builder.Build();
ConnectionString.Value = configuration["connectionString"]; // Or how you do it in your code.

And then you will be able to use it in your class library.
In real-world applications it is better to inject either connection string or settings that exposes connection string via constructor using Dependency injection technics.

