ASP.NET Core中的连接字符串DI

Question

I am not sure what I am doing wrong here, I want to use the connection string from appsettings.json and use DI to setup my DbContext for use.

{
  "ConnectionStrings": {
    "DarkwinterDatabase": "server=localhost;userid=user;pwd=pass;port=3306;database=Darkwinter;sslmode=none;"
  },
  ...
}

In the ConfigureServices method in Startup.cs I have the following code.

services.AddDbContext<PostContext>(options =>
    options.UseMySQL(Configuration.GetConnectionString("DarkwinterDatabase")));

My DbContext has one DbSet and accepts DbContextOptions to pass to base to be initialized. This is where I think I am wrong but should DI pass options to this class?

public class PostContext : DbContext
{
    public PostContext(DbContextOptions options) : base(options) { }
    public DbSet<Post> Post { get; set; }
}

How ever when I try to use my Context I get an error: There is no argument given that corresponds to the required formal parameter 'options' of 'PostContext.PostContext(DbContextOptions)

using(var db = new PostContext())
{
    var posts = db.Post.ToList();
    return View(posts);
}

This makes sense since I didn't pass one in but I thought that the service should do that? I know I am thinking about this the wrong way and was hoping someone would take the time to explain it to me.

Answer 1

You are thinking the right way. However, there are a couple of issues.

Let me explain: according to the Dependency Injection documentation the AddDbContext is a registered extension method for .NET Core Dependency Injection, that is right.

What's more important is to understand what you are injecting. It's actually very easy: with AddDbContext<PostContext> you are telling the DI framework " inject the PostContext as the application Db context everywhere where it's needed ".

Now let's check what you have in PostContext (that's where you want stuff injected into ):

public PostContext(DbContextOptions options)

You are trying to use constructor injection, that's a great start. You are also expecting DbContextOptions to be injected. However, the DI framework has no idea what to inject for DbContextOptions - you only told it about ApplicationDbContext and it does not know which one to inject ( you might have registered a bazillion of different contexts ).

You can change it to ApplicationDbContext (otherwise everything written below makes zero sense):

AddDbContext<ApplicationDbContext>(//...)

Now, your original DbContext service definition is done properly but it's still not working. There are good news for you though: you are not the only one who was shockazzeled by this behavior: " I'm registering an ApplicationDbContext and then asking for DbContextOptions and I really expect for the magic to happen " (ref. Entity Framework Issue #4558 ).

I have never actually tried this but according to the Entity Framework Issue #4668 the DbContextOptions<TContext> was added to the application's service provider. Now, you probably already guessed the trick to try: make the constructor ask for DbContextOptions<TContext> (if you have changed your registration to use ApplicationDbContext instead of PostContext ), like this:

public class PostContext : DbContext
{
    public PostContext(DbContextOptions<ApplicationDbContext> options) : base(options) { }
    public DbSet<Post> Post { get; set; }
}

Or like this if you have decided to stay with AddDbContext<PostContext> :

public class PostContext : DbContext
{
    public PostContext(DbContextOptions<PostContext> options) : base(options) { }
    public DbSet<Post> Post { get; set; }
}

Again, not sure if it will work, never tried it. If it doesn't - well, you'll have to implement a context options factory and inject it instead...

Answer 2

您可以在startup.cs文件中尝试一下吗

services.AddDbContext<PostContext>(options => options.UseSqlServer(Configuration.GetConnectionString("DarkwinterDatabase")));