Add example to correspond to text. Fixes #41220 (#41222)

Author: IEvangelist

docs/core/extensions/dependency-injection.md

@@ -3,7 +3,7 @@ title: Dependency injection
 description: Learn how to use dependency injection within your .NET apps. Discover how to registration services, define service lifetimes, and express dependencies in C#.
 author: IEvangelist
 ms.author: dapine
-ms.date: 03/15/2024
+ms.date: 06/03/2024
 ms.topic: overview
 ---
 
@@ -289,7 +289,19 @@ The framework provides service registration extension methods that are useful in
 
 For more information on type disposal, see the [Disposal of services](dependency-injection-guidelines.md#disposal-of-services) section.
 
-Registering a service with only an implementation type is equivalent to registering that service with the same implementation and service type. This is why multiple implementations of a service cannot be registered using the methods that don't take an explicit service type. These methods can register multiple *instances* of a service, but they will all have the same *implementation* type.
+Registering a service with only an implementation type is equivalent to registering that service with the same implementation and service type. For example, consider the following code:
+
+```csharp
+services.AddSingleton<ExampleService>();
+```
+
+This is equivalent to registering the service with both the service and implementation of the same types:
+
+```csharp
+services.AddSingleton<ExampleService, ExampleService>();
+```
+
+This equivalency is why multiple implementations of a service can't be registered using the methods that don't take an explicit service type. These methods can register multiple *instances* of a service, but they will all have the same *implementation* type.
 
 Any of the above service registration methods can be used to register multiple service instances of the same service type. In the following example, `AddSingleton` is called twice with `IMessageWriter` as the service type. The second call to `AddSingleton` overrides the previous one when resolved as `IMessageWriter` and adds to the previous one when multiple services are resolved via `IEnumerable<IMessageWriter>`. Services appear in the order they were registered when resolved via `IEnumerable<{SERVICE}>`.