What are the practical scenarios to use IServiceCollection.AddTransient, IServiceCollection.AddSingleton and IServiceCollectionAddScoped Methods?

Question

After reading this post I can understand the differences between AddTransient,AddScoped and AddSingleton however, I am unable to see the p

Answer 1

I've seen the "just use AddTransient()" view, but I don't agree.

Think about memory allocation

I hate allocating things when I don't have to, so if I know I'm creating something that's thread-safe, or that I have explicit documentation that having a singleton instance is the expected usage, then I'm creating a singleton.

AddSingleton()

Here's the ApplicationInsights TelemetryClient instance as a singleton. Their documentation says this works.

telemetryClient = new TelemetryClient(TelemetryConfiguration.Active);
services.AddSingleton(telemetryClient);

In this project I use Azure Table Storage as well, and I've found that creating a CloudTableClient as a singleton works just fine. I don't need to keep creating instances of it for every request.

CloudStorageAccount storageAccount = CloudStorageAccount.Parse(Configuration.GetValue("storageAccountConnectionString"));
CloudTableClient someTableClient = storageAccount.CreateCloudTableClient();
services.AddSingleton(someTableClient);

In some sense, it's the equivalent of a class's readonly static property, for instance:

public static readonly CloudTableClient cloudTableClient = new CloudTableClient(...);

... there's only one instance of it in the whole application, but by using services.AddSingleton() we get direct access to it using Dependency Injection.

AddScoped()

An example of AddScoped() for me is that I want to embed the JavaScript that I need to get Application Insights into a web page, but I use Content-Security-Policy, so I need to put a nonce on any on-page JavaScript. I have a little bit of code that helps me do it.

services.AddScoped();

AddTransient()

I haven't found a need to use AddTransient() for anything yet. It could be that I don't think of things that I have to create, every time I need them, as "services"... they're just variables that I new up. In some sense, AddTransient() is a hidden use of the Factory pattern... instead of calling a static MyServiceFactory.Create() function, you have Dependency Injection (effectively) do the same thing for you.