ASP.NET Core: How to pass connection string to DBContext?

Question

There is the working example: https://www.codeproject.com/Articles/3132485/CRUD-Operation-using-ASP-NET-CORE-2-2-and-React-Re

I'd like to replace the hardcoded connection string in assembly with string from config. It is in the original example:

public partial class ContactDBContext : DbContext
{
    public ContactDBContext()
    {
    }

    public ContactDBContext(DbContextOptions<ContactDBContext> options)
        : base(options)
    {
    }

    public virtual DbSet<Contacts> Contacts { get; set; }

    protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
    {
        if (!optionsBuilder.IsConfigured)
        {
            //#warning To protect potentially sensitive information 
            //in your connection string, you should move it out of source code. 
            //See http://go.microsoft.com/fwlink/?LinkId=723263 for guidance 
            //on storing connection strings.
            optionsBuilder.UseSqlServer("Server=yourservername ;
                                         Database=ContactDB;Trusted_Connection=True;");
       }
    }
}

I have added the code:

public void ConfigureServices(IServiceCollection services)
{
    //...

    services.AddDbContext<ContactDBContext>(options => options.UseSqlServer(Configuration.GetConnectionString(nameof(ContactDBContext))));//<------?

    //...
}

The string is read ok, but it is not used. The hardcoded string still used (see the first piece of code).

I use the context like

public class ContactService : IContactService
{
    public async Task<List<ContactModel>> GetContacts()
    {
        using (ContactDBContext db = new ContactDBContext())
        {

        //...

How to pass the connection string from app to EF context?

Remove the code from within the OnConfiguring which is overriding the connection set at startup — Nkosi
– Nkosi, Commented May 30, 2019 at 13:00
You're overwriting the connection string by setting it in OnConfiguring, since that occurs after ConfigureServices. — Heretic Monkey
– Heretic Monkey, Commented May 30, 2019 at 13:00
Ah, remove the code or remove function OnConfiguring at all? — ZedZip
– ZedZip, Commented May 30, 2019 at 13:03
Also it is possible that because you have a default constructor that the options are not being passed to the context and as such it will use the OnConfiguring because the builder options are not configured in that case — Nkosi
– Nkosi, Commented May 30, 2019 at 13:04
I have made 2 tests: removed the code in OnConfiguring , removed the function, run app - the same problem. — ZedZip
– ZedZip, Commented May 30, 2019 at 13:06

Nkosi · Accepted Answer · 2019-05-30 13:32:10Z

Because you are manually creating the instance of the context, the configuration applied at startup is not being injected into the context and instead it is using the OnConfiguring method.

That is why your configuration at start is not being applied.

Refactor the context to remove the default constructor and OnConfiguring

public partial class ContactDBContext : DbContext {
    public ContactDBContext(DbContextOptions<ContactDBContext> options)
        : base(options) {
    }

    public virtual DbSet<Contacts> Contacts { get; set; }

    }
}

Next ensure that the context is injected into the desired dependent class

public class ContactService : IContactService {
    ContactDBContext db;

    public ContactService (ContactDBContext db) { 
        this.db = db
    } 

    public async Task<List<ContactModel>> GetContacts() {
        var contacts = db.Contacts;

        //...convert to models

        //...
    }
}

The configuration applied at startup should now be included when the container resolves the context for injection.

Reference Configuring a DbContext

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