Question
I have setup .net core project and db context also. But i cant start using dbContext yet due this error-
"there is no argument given that corresponds to the required formal parameter 'options'"
Controller:
public IActionResult Index()
{
using (var db = new BlexzWebDb())
{
}
return View();
}
Dbcontext Code:
public class BlexzWebDb : DbContext
{
public BlexzWebDb(DbContextOptions<BlexzWebDb> options)
: base(options)
{ }
public DbSet<User> Users { get; set; }
public DbSet<Role> Roles { get; set; }
public DbSet<AssignedRole> AssignedRoles { get; set; }
}
error picture attached. Whats the possible fix for that issue? Thanks in advance
Accepted Answer
update
These days the prefered method to add a DbContext to the service collection is to use the AddDbContextPool method:
//hey, options!
//hey, DbContextPool
services.AddDbContextPool<BlexzWebDb>(options =>
options.UseSqlServer(Configuration.GetConnectionString("BlexzWebConnection")));
//etc
for more info, see msdn
In EF Core it's common to pass some DbContextOptions to the constructor.
So in general, a constructor looks like this:
public BlexzWebDb(DbContextOptions<BlexzWebDb> options) : base(options)
As you can see there, there is no valid overload in the form of a parameter-less constructor:
Thus, this does not work:
using (var db = new BlexzWebDb())
Instead
.Net Core has IoC implemented in it's roots. Okay, this means; you don't create a context, you ask the framework to give you one, based on some rules you defined before.
Example: somewhere you will register your dbcontext, (Startup.cs):
//typical configuration part of .net core
public void ConfigureServices(IServiceCollection services)
{
//some mvc
services.AddMvc();
//hey, options!
services.AddDbContextPool<BlexzWebDb>(options =>
options.UseSqlServer(Configuration.GetConnectionString("BlexzWebConnection")));
//...etc
Now the registering part is done, you can retrieve your context from the framework. E.g.: inversion of control through a constructor in your controller:
public class SomeController : Controller
{
private readonly BlexzWebDb _db;
//the framework handles this
public SomeController(BlexzWebDb db)
{
_db = db;
}
//etc.
why?
So, why not just provide the arguments and new it?
Obviously that will work as well. But, Inversion Of Control is considered to be a good practice. When doing asp dotnet core you should use it quite often because most libraries provide extension methods to use it.
Therefore, instead of providing "just a way to instantiate" the object, I'll try to get you onto the right track - inline with the framework. It will save you some hassle afterwards. Besides, otherwise "use an activator's CreateInstance" would just be as valid as an answer ;-)
Some links:
Popular Answer
Instantiate new object of DbContext from ConnectionString
var connectionstring = "Connection string";
var optionsBuilder = new DbContextOptionsBuilder<ApplicationDbContext>();
optionsBuilder.UseSqlServer(connectionstring);
ApplicationDbContext dbContext = new ApplicationDbContext(optionsBuilder.Options);
// Or you can also instantiate inside using
using(ApplicationDbContext dbContext = new ApplicationDbContext(optionsBuilder.Options))
{
//...do stuff
}
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