How to Add Dependency Injection for a Class Library Used by a Web API Project

I'm trying to use dependency injection across a Web API project and a Class Library (CL) so that when a controller in the Web API calls on a class in the CL to do some work, all the dependencies are instantiated by the CL.

I'm thinking I can add a project dependency in the Web API to the CL, but I've read that I also have to add in assemblies so that I can compile the CL in the Web API in order for the dependency injection to work. Is this true? Anyways, it doesn't sound as seamless as the Web API calling the class in the CL and the CL just "makes it work".

Is what I'm asking for been done already? I'm new to dependency injection in .NET and it's been difficult navigating to the correct tutorials/blogs for this kind of approach, so I'm open to any resources that I can research. I know Clean Architecture suggests using a completely separate project to handle the dependency injection across other projects, but I plan to separate these projects into a Microservices architecture in the future (currently using a Layered architecture until I do that) and find it hard to justify to myself the scalability of that approach.

If it at all matters, I'm using .NET 6.0 (where there's no Startup.cs and only the Program.cs without the function signatures, not sure what to call this) on VS Code for Mac OS.

Using services located in different projects for dependency injection is not too difficult.

There are only a few things you need to do:

1. Add the used library to your project references in Visual studio:
   

2. After you have done this, you can register the service to the container as usual - here is a simple example Program.cs (without function signatures, as you mentioned):

NOTE: You need to decide yourself if your service should be injected as a singleton, transient or scoped, see the Microsoft Docs for further information on that topic.

using MachineData.Domain;

var builder = WebApplication.CreateBuilder(args);

// Add services to the container.

builder.Services.AddControllers().AddNewtonsoftJson();
// Learn more about configuring Swagger/OpenAPI at https://aka.ms/aspnetcore/swashbuckle
builder.Services.AddEndpointsApiExplorer();
builder.Services.AddSwaggerGen();
builder.Services.AddSwaggerGenNewtonsoftSupport();
builder.Services.AddSingleton<HttpClient>() // used in ServiceInLibrary
builder.Services.AddSingleton<ServiceInLibrary>(); // TODO: insert your service here
var app = builder.Build();

// Configure the HTTP request pipeline.
if (app.Environment.IsDevelopment())
{
    app.UseSwagger();
    app.UseSwaggerUI();
}

app.UseCors(opts =>
{
    opts.AllowAnyHeader().AllowAnyMethod().AllowAnyOrigin();
});

app.UseHttpsRedirection();

app.UseAuthorization();

app.MapControllers();

app.Run();

3. You can also use injected objects in the service located in your second project as usual. Just add a constructor parameter, as you already know it:

Of course, you need to register the injected class as well in your Program.cs.

public class ServiceInLibrary
{
    ...
    public ServiceInLibrary(HttpClient http)
    {

    }
    ...
}

I hope this answers your question. If not, please let me know.