.Net Core passing connection string to DBContext class

Instead of passing connection string to DbContext, configuring DbContext in Startup.cs(if possible) is better way. See official docs to understand how to configure DbContext and use it via Dependency Injection.

EDIT : Below code is not good way

However, if you want to pass connection string to DbContext you can use options pattern.

Here is an example how to pass connection string with options pattern:

First you need an options class which accessible from Data Layer and MVC layer

public class ConnectionStringOption
{
     public string ConStr { get ; set; }
}

Then set option value

public void ConfigureServices(IServiceCollection services)
{
     services.AddOptions();
     services.Configure<ConnectionStringOption>(options=>
     {
         // set connection string from configuration  
         options.ConStr = Configuration.GetConnectionString("Default");
     });      
}

appsetting.json

{
  "ConnectionStrings": {
    "Default": "<your connection string>"
  }
}

Finally DbContext

    private readonly IOptions<ConnectionStringOption> _conStrOptions;

    protected YourDbContext()
    {
    }
    public YourDbContext(IOptions<ConnectionStringOption> conStrOptions, DbContextOptions options) 
        : base(options)
    {
        _conStrOptions= conStrOptions;
    }

Edit for another way

Using Static Service Locator may be a solution:

Create a DependencyResolver in Data Layer

public static class DependencyResolver
{
    private static IServiceProvider _provider;
    public static IServiceProvider ServiceProvider
    {
        get
        {
            return _provider;
        }
        set
        {
            if(_provider == null)
            {
                _provider = value;
            }
        }
    }
}

In ConfigureServices method

    public void ConfigureServices(IServiceCollection services)
    {
        // other stuff
        services.AddOptions();
        services.Configure<ConnectionStringOption>(options=>
        {
            // set connection string from configuration  
            options.ConStr = Configuration.GetConnectionString("Default");
        }); 
        DependencyResolver.ServiceProvider = services.BuildServiceProvider();
    }

And finally get option:

protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
{            
    var conStr = DependencyResolver.ServiceLocator.GetService<IOptions<ConnectionStringOption>>().Value.ConStr;
    optionsBuilder.UseSqlServer();
}

Final Edit for previous stupid way

public static class ConnectionStringGetter
{
    public static string ConStr{get;set;}
}

public Startup(IHostingEnvironment env)
{
    //...
    Configuration = builder.Build();
    ConnectionStringGetter.ConStr =  Configuration.GetConnectionString("Default");
}

protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
{            
    optionsBuilder.UseSqlServer(ConnectionStringGetter.ConStr);
}