ASP.NET Core Dependency Injection

ASP.NET Core now has a built in default DI container. Here’s a simple example of using this that isn’t in a Controller class (all the examples I’ve seen so far are how to inject objects into a controller) …

Mapping our interfaces

First we need to map our interfaces to the implementations we want the app to use in Startup.ConfigureServices()

public void ConfigureServices(IServiceCollection services)
{
    services.AddTransient<IHelloMessage, HelloMessage>();
}

This code tells the DI container that when IHelloMessage is requested, return a new instance of HelloMessage.

The code for IHelloMessage and HelloMessage is below:

public interface IHelloMessage
{
    string Text { get; set; }
}

public class HelloMessage : IHelloMessage
{
    public string Text { get; set; }

    public HelloMessage()
    {
        Text = "Hello world at " + DateTime.Now.ToString();
    }
}

Requesting the implementation

Controller classes do this for us but we use IApplicationBuilder.ApplicationServices.GetRequiredService() to get the implementation for other classes.

The code below injects IHelloMessage into ResponseWriter during each http request.

public void Configure(IApplicationBuilder app)
{
    app.Run(async context =>
    {
        await Task.Run(() =>
        {
            new ResponseWriter(app.ApplicationServices.GetRequiredService<IHelloMessage>()).Write(context);
        });
    });
}

The code for ResponseWriter is below:

public class ResponseWriter
{
    public IHelloMessage HelloMessage { get; set; }

    public ResponseWriter(IHelloMessage helloMessage)
    {
        HelloMessage = helloMessage;
    }
    public void Write(HttpContext HttpContext)
    {
        HttpContext.Response.WriteAsync(HelloMessage.Text);
    }
}

If we run the app, we get something like below and everytime you refresh the browser, the time within the message should change.

Other mapping options

If we wanted to only use a single instance when IHelloMessage is requested, we could have used:

public void ConfigureServices(IServiceCollection services)
{
    services.AddScoped<IHelloMessage, HelloMessage>();
}

Now when we refresh the browser, the time should stay static.

If we wanted a specific instance to be used, we could have used:

public void ConfigureServices(IServiceCollection services)
{
    services.AddInstance<IHelloMessage>(new HelloMessage() {Text="My instance"});
}

Now when the run the app, we should see “My instance” in the web page.