.NET 5 vNext使用参数解析依赖项

Question

I have been able to resolve dependencies with parameterless constructors using vNext DI .

But when I tried to specify a parameter for one of my dependencies, it gives runtime error:

System.InvalidOperationException Unable to resolve service for type 'System.String' while attempting to activate 'Namespace.MyService'

Constructor:

public MyService(string name)
{
    // initialize
}

Usage:

private readonly IMyService _myService;
public Consumer(IMyService myService)
{
        // initialize
        _myService = myService;
}

I updated this dependency registration by adding:

services.AddInstance(new MyService("Hello"));

It only works if I update the Consumer class constructor parameter to use type MyService

My initial registration was:

services.AddTransient<IMyService, MyService>(); // Todo: configure constructor injection

I really want to use the interfaces not the concrete classes for this process. How can I get this to work?

Answer 1

It's as simple as explicitly specifying the type of the service:

services.AddInstance<IMyService>(new MyService("Hello"));

The compiler would normally infer the generic parameter type, but you can always explicitly specify generic type arguments.

Answer 2

another thing to consider though there is not much documentation on it yet, is instead of taking a string in your constructor take an IOptions class and put the string as a property of the options

http://shazwazza.com/post/using-aspnet5-optionsmodel/

then later if needed you can add properties to pass in without changing the constructor

Answer 3

There is another approach, you can add config class:

public class MyServiceConfig
{
    public MyServiceConfig(string name)
    {
        // initialize
    }
}

public class MyService
{
    public MyService(MyServiceConfig myServiceConfig)
    {
        // initialize
    }
}

services.AddInstance(new MyServiceConfig("Hello"));
services.AddTransient<IMyService, MyService>();