C#•2y ago

✅ When using Dependency Injection, how can I pass other parameters into the constructor?

I'm just starting to get my head around the basics of DI. I've got a number of instances that were being manually created, and I'm working to get them shifted over to using DI. Having references to other classes in the constructor makes some kind of sense to me now. However, I'm now wondering if there's a way to pass other values into the constructor. For example, if I have the following, how could I define what version is in the constructor, and is that even possible? If it's not possible, then if it's a nullable type

public class MyService : IHostedService
{
  private readonly SemVersion _version;
  private readonly ILogger _logger;
  
  public MyService(ILogger<MyService> logger, SemVersion version)
  {
    _logger = logger;
    _version = version;
  }

  public async static Task Main()
  {
    SemVersion serviceVersion = new SemVersion(0, 1, 0);

    IHostBuilder builder = Host.CreateDefaultBuilder();
    builder.ConfigureServices(services =>
    {
      services.AddHostedService<MyService>();
    });

    IHost host = builder.Build();
    await host.StartAsync();
  }

  // Other stuff, including StartAsync() and StopAsync()
}

public class MyService : IHostedService
{
  private readonly SemVersion _version;
  private readonly ILogger _logger;
  
  public MyService(ILogger<MyService> logger, SemVersion version)
  {
    _logger = logger;
    _version = version;
  }

  public async static Task Main()
  {
    SemVersion serviceVersion = new SemVersion(0, 1, 0);

    IHostBuilder builder = Host.CreateDefaultBuilder();
    builder.ConfigureServices(services =>
    {
      services.AddHostedService<MyService>();
    });

    IHost host = builder.Build();
    await host.StartAsync();
  }

  // Other stuff, including StartAsync() and StopAsync()
}

Having played a bit more while writing this question, this seems to work, but it only allows one value per type, and it doesn't feel like it's the right way to do it...

services.Add(new ServiceDescriptor(typeof(SemVersion), serviceVersion));

services.Add(new ServiceDescriptor(typeof(SemVersion), serviceVersion));

What's the best approach to this kind of thing? Thanks

9 Replies

Hugh•2y ago

I've just found this SO post which I think answers my question... https://stackoverflow.com/questions/34834295/dependency-injection-inject-with-parameters

Stack Overflow

Dependency injection, inject with parameters

I'm using vNext implementation of DI. How to pass parameters to constructor? For example, i have class: public class RedisCacheProvider : ICacheProvider { private readonly string _connectionS...

Hugh•2y ago

I'll leave this open for a bit in case anyone else has any comments on this approach I've now got:

builder.ConfigureServices(services =>
{
  services.AddHostedService<MyService>(provider => new MyService(provider.GetService<ILogger<MyService>>()!, serviceVersion);
});

builder.ConfigureServices(services =>
{
  services.AddHostedService<MyService>(provider => new MyService(provider.GetService<ILogger<MyService>>()!, serviceVersion);
});

What I don't like about this is that GetService() returns a T?, and so I need to either manually check, accept an ILogger? in MyService or add a ! at the end, none of which feel great

Anton•2y ago

you have to use a factory or a provider GetRequiredService returns a T

Hugh•2y ago

ahh - awesome thanks - didn't spot that one I think I'll go with this then. Thanks! It's a bit of a shame that I can't just add new things to the MyService contstructor and have them appear - I have to add them in 2 places, but that's fine

Anton•2y ago

if it's gonna be just a single instance of the version, you can register that with DI also

Hugh•2y ago

Would that be using:

services.Add(new ServiceDescriptor(typeof(SemVersion), serviceVersion));

services.Add(new ServiceDescriptor(typeof(SemVersion), serviceVersion));

Because it would need to be a specific instance or, I guess, I could use AddSingleton with a provider...

services.AddSingleton<SemVersion>(provider => serviceVersion);

services.AddSingleton<SemVersion>(provider => serviceVersion);

Anton•2y ago

just AddSingleton(version) should work

Hugh•2y ago

ah - very cool I think that'll be my approach then As it will just be the one instance of that type appreciate it! (always helpful)

Anton•2y ago

Gaming

Programming

✅ When using Dependency Injection, how can I pass other parameters into the constructor?