如何将参数从查询字符串传递到Autofac中的组件注册？

Question

Im building a web application and I'm using AutoFac for dependency injection.

For this project, when I resolve the dependencies for DbContext of Entity Framework, I need to pass a custom parameter, because the connection string is dynamic and can change depending of this parameter.

This parameter comes from query string (from another application). I need to somehow intercept it, take this value before the initialization of the Injector, but I don't know the best approach for doing this, since the initialization of the Injector is in the Application_Start() method.

So, I have the following code:

//Entity Framework DbContext
public class MyContext : DbContext
{
    public MyContext(int portalCode){...}
}

//AutoFac registering:
container.Register<MyContext>(c => new MyContext(how to pass the parameter here ??)).InstancePerRequest();

Where container is the default ContainerBuilder

And then, calling the AutofacInitialize in Application_Start() just like it is in the AutoFac Doc.

How do I achieve that during the Dependency Resolver with Autofac?

Answer 1

Long story short, you may simply do like this:

builder.Register<MyContext>(c => 
    new MyContext(int.Parse(HttpContext.Current.Request.QueryString["PortalCode"])))
    .InstancePerRequest();

Or a more verbose solution using a provider class

public interface IPortalCodeProvider
{
    int GetPortalCode();
}

public class PortalCodeProvider : IPortalCodeProvider
{
    public const string PortalCodeQueryStringKey = "PortalCode";
    public const int DefaultPortalCode = 123;

    public int GetPortalCode()
    {
        var portalCodeString = HttpContext.Current.Request.QueryString[PortalCodeQueryStringKey];
        int portalCode;
        if (int.TryParse(portalCodeString, out portalCode)) return portalCode;
        else return DefaultPortalCode;
    }
}

MyContext ctor signature would be MyContext(IPortalCodeProvider portalCodeProvider)

Autofac class registration:

builder.RegisterType<PortalCodeProvider>().AsImplementedInterfaces().InstancePerRequest();
builder.RegisterType<MyContext>().InstancePerRequest();

Answer 2

Pass in a variable for the parameter value:

var parameter = "my value";
container.Register<MyContext>(c => new MyContext(parameter)).InstancePerRequest();