.NET 5 Multiple projects in the same assembly

Yes, you are absolutely able to run both an API and Web App on the same Assembly even if they are from different projects. Not only can you do it but it is very quick and easy to do and can do it for many projects.

Since we are using .NET5 in this example we are going to assume you have two console applications. One for your WebApp and one for your API.

Step 1: Choose your Entry point

Choose the project you want to Luanch from. In this case lets choose the WebApp. For all the other projects convert them from console application to class library and delete all program.cs files.

Step 2: Add References to these class files

Self explanitory.

Step 3: Call all the Startup.cs files in the class library from both the ConfigureServices and Configure functions.

See code below:

Add a variable to your WebApp's Startup.cs folder for the other Startup.cs files and Initialize:

    private MyWebApi.Startup ApiStartUp { get; } 
    public IConfiguration Configuration { get; }

    public Startup(IConfiguration configuration)
    {
        Configuration = configuration;
        ApiStartUp = new MyWebApi.Startup(Configuration);
    }
    

Now call the registration functions from their counterpart functions.

    public void ConfigureServices(IServiceCollection services)
    {
        ApiStartUp.ConfigureServices(services);

        // ... Rest of code
    }

    public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
    {
        ApiStartUp.Configure(app, env);
        
        // ... Rest of code
    }