Auto activation readme (#4681)

* Add auto-activation README

* Typos

* Activate

Author: sebastienros

src/Libraries/Microsoft.Extensions.DependencyInjection.AutoActivation/AutoActivationExtensions.Keyed.cs

@@ -12,7 +12,7 @@ namespace Microsoft.Extensions.DependencyInjection;
 public static partial class AutoActivationExtensions
 {
     /// <summary>
-    /// Enforces keyed singleton activation at startup time rather then at runtime.
+    /// Enforces keyed singleton activation at startup time rather than at runtime.
     /// </summary>
     /// <typeparam name="TService">The type of the service to activate.</typeparam>
     /// <param name="services">The service collection containing the service.</param>
@@ -49,7 +49,7 @@ public static IServiceCollection ActivateKeyedSingleton<TService>(
     }
 
     /// <summary>
-    /// Enforces keyed singleton activation at startup time rather then at runtime.
+    /// Enforces keyed singleton activation at startup time rather than at runtime.
     /// </summary>
     /// <param name="services">The service collection to add the service to.</param>
     /// <param name="serviceType">The type of the service to activate.</param>

src/Libraries/Microsoft.Extensions.DependencyInjection.AutoActivation/AutoActivationExtensions.cs

@@ -16,7 +16,7 @@ namespace Microsoft.Extensions.DependencyInjection;
 public static partial class AutoActivationExtensions
 {
     /// <summary>
-    /// Enforces singleton activation at startup time rather then at runtime.
+    /// Enforces singleton activation at startup time rather than at runtime.
     /// </summary>
     /// <typeparam name="TService">The type of the service to activate.</typeparam>
     /// <param name="services">The service collection containing the service.</param>
@@ -50,7 +50,7 @@ public static IServiceCollection ActivateSingleton<TService>(this IServiceCollec
     }
 
     /// <summary>
-    /// Enforces singleton activation at startup time rather then at runtime.
+    /// Enforces singleton activation at startup time rather than at runtime.
     /// </summary>
     /// <param name="services">The service collection containing the service.</param>
     /// <param name="serviceType">The type of the service to activate.</param>

src/Libraries/Microsoft.Extensions.DependencyInjection.AutoActivation/README.md

@@ -1,6 +1,8 @@
 # Microsoft.Extensions.DependencyInjection.AutoActivation
 
-Extensions to auto-activate registered singletons in the dependency injection system.
+This provides the ability to instantiate registered singletons during startup instead of during the first time it is used.
+
+A singleton is typically created when it is first used, which can lead to higher than usual latency in responding to incoming requests. Creating the instances on startup helps prevent the service from exceeding its SLA for the first set of requests it processes.
 
 ## Install the package
 
@@ -18,6 +20,84 @@ Or directly in the C# project file:
 </ItemGroup>
 ```
 
+## Usage Example
+
+### Registering Services
+
+The services to auto-activate can be registered using the following methods:
+
+```csharp
+static IServiceCollection ActivateSingleton<TService>(this IServiceCollection services);
+static IServiceCollection ActivateSingleton(this IServiceCollection services, Type serviceType);
+static IServiceCollection AddActivatedSingleton<TService, TImplementation>(this IServiceCollection services, Func<IServiceProvider, TImplementation> implementationFactory);
+static IServiceCollection AddActivatedSingleton<TService, TImplementation>(this IServiceCollection services);
+static IServiceCollection AddActivatedSingleton<TService>(this IServiceCollection services, Func<IServiceProvider, TService> implementationFactory);
+static IServiceCollection AddActivatedSingleton<TService>(this IServiceCollection services);
+static IServiceCollection AddActivatedSingleton(this IServiceCollection services, Type serviceType);
+static IServiceCollection AddActivatedSingleton(this IServiceCollection services, Type serviceType, Func<IServiceProvider, object> implementationFactory);
+static IServiceCollection AddActivatedSingleton(this IServiceCollection services, Type serviceType, Type implementationType);
+static void TryAddActivatedSingleton(this IServiceCollection services, Type serviceType);
+static void TryAddActivatedSingleton(this IServiceCollection services, Type serviceType, Type implementationType);
+static void TryAddActivatedSingleton(this IServiceCollection services, Type serviceType, Func<IServiceProvider, object> implementationFactory);
+static void TryAddActivatedSingleton<TService>(this IServiceCollection services);
+static void TryAddActivatedSingleton<TService, TImplementation>(this IServiceCollection services);
+static void TryAddActivatedSingleton<TService>(this IServiceCollection services, Func<IServiceProvider, TService> implementationFactory);
+
+static IServiceCollection ActivateKeyedSingleton<TService>(this IServiceCollection services, object? serviceKey);
+static IServiceCollection ActivateKeyedSingleton(this IServiceCollection services, Type serviceType, object? serviceKey);
+static IServiceCollection AddActivatedKeyedSingleton<TService, TImplementation>(this IServiceCollection services, object? serviceKey, Func<IServiceProvider, object?, TImplementation> implementationFactory);
+static IServiceCollection AddActivatedKeyedSingleton<TService, TImplementation>(this IServiceCollection services, object? serviceKey);
+static IServiceCollection AddActivatedKeyedSingleton<TService>(this IServiceCollection services, object? serviceKey, Func<IServiceProvider, object?, TService> implementationFactory);
+static IServiceCollection AddActivatedKeyedSingleton<TService>(this IServiceCollection services, object? serviceKey);
+static IServiceCollection AddActivatedKeyedSingleton(this IServiceCollection services, Type serviceType, object? serviceKey);
+static IServiceCollection AddActivatedKeyedSingleton(this IServiceCollection services, Type serviceType, object? serviceKey, Func<IServiceProvider, object?, object> implementationFactory);
+static IServiceCollection AddActivatedKeyedSingleton(this IServiceCollection services, Type serviceType, object? serviceKey, Type implementationType);
+static void TryAddActivatedKeyedSingleton(this IServiceCollection services, Type serviceType, object? serviceKey);
+static void TryAddActivatedKeyedSingleton(this IServiceCollection services, Type serviceType, object? serviceKey, Type implementationType);
+static void TryAddActivatedKeyedSingleton(this IServiceCollection services, Type serviceType, object? serviceKey, Func<IServiceProvider, object?, object> implementationFactory);
+static void TryAddActivatedKeyedSingleton<TService>(this IServiceCollection services, object? serviceKey);
+static void TryAddActivatedKeyedSingleton<TService, TImplementation>(this IServiceCollection services, object? serviceKey);
+TryAddActivatedKeyedSingleton<TService>(this IServiceCollection services, object? serviceKey, Func<IServiceProvider, object?, TService> implementationFactory)
+```
+
+For example:
+
+```csharp
+var builder = WebApplication.CreateBuilder(args);
+
+builder.Services.AddActivatedSingleton<MyService>();
+
+var app = builder.Build();
+
+app.Run();
+
+public class MyService
+{
+    public MyService()
+    {
+        Console.WriteLine("MyService is created");
+    }
+}
+```
+
+Result:
+
+```
+MyService is created
+info: Microsoft.Hosting.Lifetime[14]
+      Now listening on: http://localhost:5297
+info: Microsoft.Hosting.Lifetime[0]
+      Application started. Press Ctrl+C to shut down.
+```
+
+Services that are already registered can also be auto-activated:
+
+```csharp
+
+builder.Services.AddSingleton<OtherService>();
+// ...
+builder.Services.ActivateSingleton<OtherService>();
+```
 
 ## Feedback & Contributing