Why cannot I set DbContextOptions with = operator?

c# entity-framework-core


Why cannot I use the following

IServiceCollection service= new ServiceCollection();
var opt = new DbContextOptionsBuilder<Application>().UseSqlite("Data Source=MyDatabase.db");
service.AddDbContextPool<ApplicationContext>(options => options = opt);

instead of the following ?

IServiceCollection service= new ServiceCollection();
service.AddDbContextPool<ApplicationContext>(options => options.UseSqlite("Data Source=MyDatabase.db"));


Error message:

  Message=No database provider has been configured for this DbContext. A provider can be configured by overriding the DbContext.OnConfiguring method or by using AddDbContext on the application service provider. If AddDbContext is used, then also ensure that your DbContext type accepts a DbContextOptions<TContext> object in its constructor and passes it to the base constructor for DbContext.
   at Microsoft.EntityFrameworkCore.Internal.DbContextServices.Initialize(IServiceProvider scopedProvider, IDbContextOptions contextOptions, DbContext context)
   at Microsoft.EntityFrameworkCore.DbContext.get_InternalServiceProvider()
   at Microsoft.EntityFrameworkCore.DbContext.Microsoft.EntityFrameworkCore.Infrastructure.IInfrastructure<System.IServiceProvider>.get_Instance()
   at Microsoft.EntityFrameworkCore.Infrastructure.DatabaseFacade.Microsoft.EntityFrameworkCore.Infrastructure.IInfrastructure<System.IServiceProvider>.get_Instance()
   at Microsoft.EntityFrameworkCore.RelationalDatabaseFacadeExtensions.GetRelationalService[TService](IInfrastructure`1 databaseFacade)
   at Microsoft.EntityFrameworkCore.RelationalDatabaseFacadeExtensions.Migrate(DatabaseFacade databaseFacade)
2/5/2018 9:08:23 AM

Accepted Answer

Here, (options => options = opt) just works like a regular method:

private void Method(TypeOfParameter options)
    options = opt;

Here, the options variable is just overwritten. Nothing happens outside of the method. The variable that was used to call the method is not changed. That is why the assignment is useless and has no effect whatsoever.

2/5/2018 9:16:27 AM

Popular Answer

It's because AddDbContextPool accepts an Action as parameter. This action in turn accepts an object of type DbContextOptionsBuilder. This action will be called by framework at some point and the framework will pass to this action an instance of the Builder that it created with some internal rules and tricks. You are supposed to further work with this object, not redefine it.

If you do the assignement you effectively overwrite the object created by framework with your own, which the framework knows nothing about. So all the internal logic of object creation, internal plumbing etc. is lost.

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