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How can I add a dbContext after Startup in .Net Core?

I am using .Net Core, using the built-in dependency injection. In my login screen, I need the user to also choose a departmental database - we have different databases with the same structure to use the same application with different data. However, I can't figure out how to add/modify the dbContext that late. Startup.cs has the DI, but I don't know which connection string to read from the config until the user has chosen the department. It is a small database, and the company is not concerned about the management of the duplicate databases.
How can I add the service late
services.AddDbContext<my_accountingContext>(options =>
options.UseMySQL(Configuration.GetConnectionString("CorrectDepartmentConfig")));
when I actually know what CorrectDepartmentConfig is?
Or, if that can't be done, how can I do a smelly change of the my_accountingContext after Startup.cs?

You can use an implementation factory overload of IServiceCollection in ConfigureServices method form Startup class:
//First register a custom made db context provider
services.AddTransient<ApplicationDbContextFactory>();
//Then use implementation factory to get the one you need
services.AddTransient(provider => provider.GetService<ApplicationDbContextFactory>().CreateApplicationDbContext());
The implementation of CreateApplicationDbContext depends on your specific needs, but a base implementation should look like the following:
public ApplicationDbContext CreateApplicationDbContext(){
//TODO Something clever to create correct ApplicationDbContext with ConnectionString you need.
}
After this implementation, you can inject the correct ApplicationDbContext in your controller, action...
public MyController(ApplicationDbContext dbContext)
{
_dbContext = dbContext;
}
public IActionResult([FromServices] ApplicationDbContext dbContext){
}

You can always set the connection string from inside the protected OnConfiguring method. You can get access to the IConfiguration instance from there (the DbContext class has a service locator, Instance property), retrieve the connection string, and then call UseMySql extension method with the appropriate connection.
Something like this:
protected virtual void OnConfiguring(DbContextOptionsBuilder builder)
{
var configuration = (this as IInfrastructure<IServiceProvider>).GetService<IConfiguration>();
var connectionString = configuration.GetConnectionString("<name>");
builder.UseMySql(connectionString);
base.OnConfiguring(builder);
}
For the strongly-typed version of GetService do not forget to reference namespace Microsoft.Extensions.DependencyInjection.

How to Implement DbContextOptionsBuilder in .NET Core

I am new to ASP.Net Core and I am trying to implement ASP.NET Core DI.
I configured like below in ConfigureServices Method in Startup.cs
services.AddScoped<DbContext, AutomationDbContext>();
services.AddSingleton<IHttpContextAccessor, HttpContextAccessor>();
services.AddTransient<IUserService, UserService>();
In UserService Constructor, I am trying to use DI. I think below is NOT the right way to implement this.
public UserService(IHttpContextAccessor httpContextAccessor, AutomationDbContext automationDbContext, IConfiguration configuration)
{
this.configuration = configuration;
this.optionsBuilder = new DbContextOptionsBuilder<AutomationDbContext>();
var connectionString = this.configuration.GetConnectionString("Automation");
this.optionsBuilder.UseSqlServer(connectionString);
this.automationDbContext = new AutomationDbContext(this.optionsBuilder.Options);
this.httpContext = httpContextAccessor.HttpContext;
}
I don't like building optionsbuilder in constructor and get connectionstring.
What would be the better place to build these optionsBuilder and pass in constructor.

You need to use services.AddDbContext<TContext> instead:
services.AddDbContext<AutomationDbContext>(o =>
o.UseSqlServer(Configuration.GetConnectionString("Automation")));
Then, just inject your context:
public UserService(IHttpContextAccessor httpContextAccessor, AutomationDbContext automationDbContext)
As for IHttpContextAccessor, you should simply use:
services.AddHttpContextAccessor();
However, I would encourage you to strongly consider whether you actually need this in your service or not. If you need something like the current user's id, that should be passed into the method that needs it, not retrieved from within your service.
UPDATE
Since it was brought up, let me elucidate the reasons why adding your context in the way you currently are is incorrect, since it will shed a little light on how DI works in general.
First, you're binding DbContext directly to AutomationDbContext, which means you can then only use that one context. Maybe you don't need more than one context... now. That could change later. Second, when you register a service in that way, you can only inject the abstract type, i.e. DbContext here. The service registration literally means "when you see DbContext, inject an instance of AutomationDbContext". If you try to inject AutomationDbContext directly, as you're currently doing in your controller, that will actually throw an exception because that type is not actually registered as service: DbContext is. Third, AddScoped provides no real ability to configure the context, which is of course the part your were missing. There's ways to work around this such as using the factory overload of AddScoped or defining OnConfiguring on your context, but both of those are substandard to just using the right method in the first place: AddDbContext<TContext>
For what it's worth, there's also somewhat of a fourth reason, in that you can opt to use AddDbContextPool<TContext> instead of AddDbContext<TContext>, for connection pooling. There's no other way to set that up, so if you did want/need connection pooling, you'll never get there with AddScoped.

IdentityServer 4, Create Panel to CRUD Clients

Currently I Have configured Identityserver4 as separated project + My WebAPI and store in DB Credentials in IdentityServer.
Now i have problem how to make CRUD(In my frontend API) to IdentityServer(I want from my API add Clients to IdentityServer)
How to make property?

From IdentityServer4.EntityFramework and IdentityServer4.EntityFramework.Storage, you have access to IConfigurationDbContext (once you've added the required services in ConfigureServices using e.g. AddConfigurationStore). Because this is registered as part of the Dependency Injection system, you can take a dependency on it in one of your controllers. e.g.:
public class ClientsController : ControllerBase
{
private readonly IConfigurationDbContext _configurationDbContext;
public ClientsController(IConfigurationDbContext configurationDbContext)
{
_configurationDbContext = configurationDbContext;
}
// ...
}
IConfigurationDbContext is an abstraction of a standard DbContext, with the following DbSet<T> properties:
Clients
IdentityResources
ApiResources
It also includes both SaveChanges and SaveChangesAsync - Everything one might expect from a DbContext. Because of all of this, you can CRUD each of these entities just like any other Entity Framework Core driven database.
One final thing to note is that there are both Models (in IdentityServer4.Storage) and Entities (in IdentityServer4.EntityFramework.Storage). There are also a few extension methods for mapping between these (e.g. ClientMappers.ToEntity).
Given all of this, you can create a Model inside of your controller (or perhaps somewhere much better encapsulated than directly there). Here's a basic example for creating a new Client:
var clientModel = new Client
{
ClientId = "",
ClientName = "",
// ...
};
_configurationDbContext.Clients.Add(clientModel.ToEntity());
await _configurationDbContext.SaveChangesAsync();
The Client class here comes from IdentityServer4.Models and is then converted to an Entity using a ToEntity extension method I hinted at above. Working with a Model and converting to an Entity is simpler than trying to manipulate an Entity directly - If you're interested, you can see the mapping that takes place here.
This works in the same way for ApiResources, IdentityResources, etc. Use the source code links I've provided if you want to find out more about those specifically, but the information I've provided here should have you covered.
In order to use IdentityServer4 and IdentityServer4.EntityFramework in your API project, you can just add the two references to your API project. After that, you can configure the DI in the same way (using AddIdentityServer in ConfigureServices), but you don't need to add the middleware (using UseIdentityServer in Configure). You can even just use AddIdentityServer().AddConfigurationStore(...) to set up the relevant services, as you don't need a signing key, etc.

One way you can do this is by bootstrapping the ID4 Quickstart (tutorial located here):
http://docs.identityserver.io/en/release/quickstarts/3_interactive_login.html
Other option is to use their quickstart seeds located here to speed this up:
https://github.com/IdentityServer/IdentityServer4.Samples
Now if you want to implement restfull login there are constraints around it (i wanted to find out as well) check out this question:
IdentityServer 4 Restfull Login/Logout

StructureMap and HTTP request-scoped services - why is my service created twice in a single scope?

I have an ASP.NET MVC application using StructureMap.
I have created a service called SecurityContext which has a static Current property. A simplified version looks like this:
public class SecurityContext : ISecurityContext
{
public bool MyProperty { get; private set; }
public static SecurityContext Current
{
get
{
return new SecurityContext() { MyProperty = true };
}
}
}
I've hooked this up in my StructureMap registry as follows:
For<ISecurityContext>().Use(() => SecurityContext.Current);
My understanding of this Linq expression overload of the Use method is that the returned concrete object is the same for the entire HTTP request scope.
However, I've set up a test case where my context interface is injected in two places, once in the controller's constructor and again using the SetterProperty attribute in the base class my view inherits from.
When debugging I observe the Current static method being hit twice so clearly my assumptions are wrong. Can anyone correct what I'm doing here? The reason I want this request-scoped is because I'm loading certain data into my context class from the database so I don't want this to happen multiple times for a given page load.
Thanks in advance.

The default lifecycle for a configuration is Transient, thus each request for an ISecurityContext will create a new instance of SecurityContext. What I think you want is to use the legacy HttpContext lifecycle.
Include the StructureMap.Web nuget package. Then change your configuration to the following:
For<ISecurityContext>()
.Use(() => SecurityContext.Current)
.LifeCycleIs<HttpContextLifecycle>();
More information on lifecyles can be found here.
The HttpContextLifecycle is obsolete, however I do not know if or when it will be removed. The StructureMap team does recommend against using this older ASP.Net lifecycle. They state in the documentation that most modern web frameworks use a nested container per request to accomplish the same scoping. Information about nested containers can be found here.
I don't know if the version of ASP.Net MVC you are using is considered a modern web framework. I doubt it is because ASP.Net Core 1.0 is the really the first in the ASP.Net line to fully embrace the use of DI. However, I will defer to #jeremydmiller on this one.

How do I associate my ICacheClient with a separate database in ServiceStack?

We are using ServiceStack with an OrmLiteCacheClient. We are using PostgreSQL and two different schemas within one database. I created custom interfaces for both connections (one for each schema in the db), and they both inherit from IDbConnectionFactory. How do I make certain that my cache is using the connection I want it to use?

You can't, they both use the same IDbConnectionFactory that's registered in your IOC.

I did something very similar to this in a recent website. I have a main OLTP database which shares data & processing with back-end systems, and a second database just to support Caching & front-end related information. The main thing is that the second DB can NOT be used to auto-wire into services, not without some additional jiggery-pokery.
Here's what I put in my AppHost Configure:
IDbConnectionFactory dbFactory = new OrmLiteConnectionFactory(
ConfigurationManager.ConnectionStrings["Database"].ConnectionString,
SqlServerDialect.Provider
);
// Register the main database as the default singleton
container.Register<IDbConnectionFactory>(dbFactory);
IDbConnectionFactory dbCacheFactory = new OrmLiteConnectionFactory(
// Create a second factory, but ONLY use it to instantiate the Cache
ConfigurationManager.ConnectionStrings["BottleDropCache"].ConnectionString,
SqlServerDialect.Provider
);
var cache = new OrmLiteCacheClient();
cache.DbFactory = dbCacheFactory;
cache.InitSchema();
container.Register<ICacheClient>(cache);