Passing connection string to Entity framework at runt time for each call

Question

My Entity framework context is as following

 public partial class MyContext : DbContext, IMyContext
{
    static MyContext()
    {
        System.Data.Entity.Database.SetInitializer<MyContext>(null);
    }

    public MyContext()
        : base("Name=MyContext")
    {
    }

I am resolving it through autofac in the following way

builder.RegisterType(typeof(MainContext)).As(typeof(DbContext)).InstancePerLifetimeScope();
        builder.RegisterType<MainContext>().As<IMainContext>().InstancePerRequest();

This db context gets called in repository layer

#region Fields

    private readonly IMyContext _context;

    #endregion

    #region Constructors and Destructors

    public EmployeeRepository(IMyContext context)
    {
        _context = context;
    }

    #endregion

    public void Create(Employee emp)
    {
        this._context.Employee.Add(emp);
    }

Now my issue is , I want to set the connection string dynamically per call. The connection string will be passed through a webapi which i want to pass on to this context. Can anyone help me how can i do that? I am confused about autofac here. Secondly how can i make sure each call sets connection string and does not cache it.

Answer 1

You can use a factory that will build the context and set the connectionstring for you.

public interface IContextFactory
{
    IContext GetInstance();
}

public class MyContextFactory : IContextFactory 
{
    public IContext GetInstance()      
    {
        String connectionString = this.GetConnectionString(HttpContext.Current); 
        return new MyContext(connectionString); 
    }

    private String GetConnectionString(HttpContext context)
    {
        // do what you want
    }
}


builder.RegisterType<MyContextFactory>()
       .As<IContextFactory>()
       .InstancePerRequest(); 
builder.Register(c => c.Resolve<IContextFactory>().GetInstance())
       .As<IContext>()
       .InstancePerRequest(); 

If you can't get connectionstring based on HttpContext, you can change contextFactory implementation to expect initialization by WebAPI before creating the instance. For example :

public interface IContextFactory
{
    IContext GetInstance();
    void Initialize(String connectionString);
}

public class MyContextFactory : IContextFactory 
{

    private String _connectionString; 

    public void Initialize(String connectionString)
    {
        this._connectionString = connectionString; 
    }

    public IContext GetInstance()      
    {
        if (this._connectionString == null)
        {
            throw new Exception("connectionString not initialized"); 
        }
        return new MyContext(this._connectionString); 
    }
}

At the beginning of your web API call (through attribute for example), you can call the Initialize method. Because the factory is InstancePerRequest you will have one instance for the duration of the request.

---

By the way, I'm not sure to understand this registration

builder.RegisterType(typeof(MainContext)).As(typeof(DbContext)).InstancePerLifetimeScope();
builder.RegisterType<MainContext>().As<IMainContext>().InstancePerRequest();

It looks buggy because you will have 2 different registration of the same type and not for the same scope, is it intended ? Furthermore, it doesn't sound a good idea to register a DbContext, do you need this registration ?

The following registration looks better :

builder.RegisterType<MainContext>()
       .As<IMainContext>()
       .As<DbContext>()
       .InstancePerRequest();