How to bind a string to a method of an object when it is injected into another?

Question

I have a NHibernate.Cfg.Configuration provider.

ConfigurationProvider

public class ConfigurationProvider : Provider<Configuration> {
    public class ConfigurationProvider(string connectionString
        , Assembly mappings) {
        ConnectionString = connectionString;
        Mappings = mappings;
    } 

    protected override Configuration CreateInstance(IContext context) {
        var c = new Configuration().Configure();
        c.AddAssembly(Mappings);
        c.Properties["connection.connection_string"] = ConnectionString;
        return c;
    }

    private readonly string ConnectionString;
    private readonly Assembly Mappings;
}

Which the connection string comes from the Configurationservice.BuildConnectionString() of my application.

ConfigurationService

public class ConfigurationService {
    public ConfigurationService(ICurrentUser user) { CurrentUser = user; }

    public string BuildConnectionString() {
        return string.Format(DefaultConnectionString
            , CurrentUser.DatabaseInstanceName
            , CurrentUser.Login
            , CurrentUser.Password);
    }

    private readonly ICurrentUser CurrentUser;
}

That in turn depends on the CurrentUser to impersonate.

CurrentUser

public class CurrentUser : ICurrentUser {
    public CurrentUser(string dbinstance, string login, string password) { 
        DatabaseInstanceName = dbinstance;
        Login = login;
        Passwword = password;
    }

    public readonly string DatabaseInstanceName;
    public readonly string Login;
    public readonly string Password;
}

So manually injecting my dependency would look like:

string dbInstance = authenticationDialog.DatabaseInstanceName;
string login = authenticationDialog.Login;
string password = authenticationDialog.Password;

var nHibernateConfigurationProvider = 
    new ConfigurationProvider(
        new ConfigurationService(
            new CurrentUser(dbInstance, login, password)).BuildConnectionString()
        , Assembly.GetExecutingAssembly());

The problem is that I don't know how to bind, using Ninject, the ConfigurationService.BuildConnectionString() method to resolve the string used as constructor argument in the ConfigurationProvider.ctor().

Pushing it even further, how do I tell Ninject to actually bind these types only once the user is authenticated?

Here's what I have so far

// Wait until user is authenticated before continuing to bind???
Bind<string>().ToMethod(ctx => ctx.Request.???.BuildConnectionString()).WhenInjectedInto<ConfigurationProvider>().InSingletonScope();
Bind<Assembly>().ToConstant<Assembly>(Assembly.GetExecutingAssembly().WhenInjectedInto<ConfigurationProvider>();

Is this a web application? I mean, once the `CurrentUser` exists, can we assume it as a singleton? (no other user authenticated at the same time for the same process) — cvbarros, Apr 11 '14 at 13:27
It is not a web application, and we shall consider the current user as a singleton, apart otherwise mentioned, as the current user is required to authenticate upon application startup. Only one application is to be run per application domain/windows process. — Will Marcouiller, Apr 11 '14 at 14:21

score 2 · Accepted Answer · answered Apr 11 '14 at 14:53

2

Here is my suggestion on how you can achieve this.

Since you don't have the data beforehand and only when your ICurrentUser is created via the credentials and parameters you need, it means that you need to "save it" into a globally accessible location once it has been created.

If we think about a web .NET application, that would tipically involve binding the ICurrentUser with InRequestScope() using a static reference to a HttpContext.

As this is not your case, I've chosen to bind ICurrentUser as a singleton and initialize it passing the values. When you need your other classes that depend on it, the container will pass the already-initialized instance, since it is bound with InSingletonScope().

I've also tried to mimic your current dependencies. For instance, instead of passing in the connectionString to ConfigurationProvider, my way of doing it would be declare a direct dependency on ConfigurationService in order to obtain the information.

// Do your stuff to collect credentials

// User now authenticated, initialize ICurrentUser instance singleton
// You could have used a ICurrentUserFactory or static member, this is just a sample implementation
kernel.Get<ICurrentUser>(new ConstructorArgument("dbInstance", dbInstance),
    new ConstructorArgument("login", login),
    new ConstructorArgument("password", password));

// You can now access your NHibernate configuration properly
var config = kernel.Get<Configuration>();

And the bindings:

Bind<Configuration>().ToProvider<ConfigurationProvider>();
Bind<ConfigurationProvider>()
    .ToMethod(
        ctx => new ConfigurationProvider(ctx.Kernel.Get<ConfigurationService>().BuildConnectionString()));

Bind<ICurrentUser>().To<CurrentUser>().InSingletonScope();

Hope this helps.

answered Apr 11 '14 at 14:53

cvbarros

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Aside, as the composition root is located in the `Program.Main()` method, once the bindings are done and the kernal is intantiated, will the connecting be built only once it's required, that is, only the first time a `Configuration` is required, or will it be initiliazed upfront with then the null string values coming from an empty `ICurrentUSer` instance not yet initialized? My guess is that Ninject tries to instantiate everything at startup, thus letting me no chance at all to provide the user credentials, am I getting it wrong? – Will Marcouiller Apr 11 '14 at 16:28
In order to know whether the user is authenticated, the action occurs in my main application form, let's name it `MainForm`. So, upon `MainForm` initialization, the user is forced to authenticate. It is only then that I know the user credentials. Because of it, I would require to have a reference to the `IKernel` class, which could then be seen/used as a `ServiceLocator`, and this is an anti-pattern. To be honest, I kind of lost here... – Will Marcouiller Apr 11 '14 at 17:16
+1 The solution you brought forward allowed me to get through multiple concerns, though not implemented totally the way you suggest. My concern now is that one cannot access the kernel itself, as it would be, as far as I know, and I may be wrong about that, just like the use of the `ServiceLocator` anti-pattern. Thank you very much for your precious time. =) – Will Marcouiller Apr 11 '14 at 19:30
I agree with you on the concerns regarding the `ServiceLocator`. But you can't escape the authenticate -> inject the NHibernate config issue. You need to save your `ICurrentUser` somewhere when authenticated. You can avoid the service locator using factories and other patterns to not expose your `IResolutionRoot`. I'll update the answer later to provide more details. – cvbarros Apr 11 '14 at 20:24
Furthermore, when binding `ToMethod()`, how the heck can you access the kernel? I have been looking around `ctx.Kernel` for the whole afternoon, going to the `ParentRequest`, `ParentContext`, etc. and no `Kernel` has the `Get()` method. All I can get is `ctx.Kernel.GetBindings()`... =( – Will Marcouiller Apr 11 '14 at 20:30
Forget my last comment... I just found out that I had not imported the proper namespace into my module, hence I was not able to access the proper methods... =P – Will Marcouiller Apr 11 '14 at 20:33

How to bind a string to a method of an object when it is injected into another?

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