# Dependency Injection

ASP.NET Core supports the **dependency injection (DI)** software design pattern, which is a technique for achieving **Inversion of Control (IoC)** between classes and their dependencies.

### Registration

Register your dependencies in `ConfigureServices`.

```csharp
public void ConfigureServices(IServiceCollection services)
{
    services.AddMvc().SetCompatibilityVersion(CompatibilityVersion.Version_2_2);

    services.AddScoped<IMyDependency, MyDependency>();
    services.AddTransient<IOperationTransient, Operation>();
    services.AddScoped<IOperationScoped, Operation>();
    services.AddSingleton<IOperationSingleton, Operation>();
    services.AddSingleton<IOperationSingletonInstance>(new Operation(Guid.Empty));

    // OperationService depends on each of the other Operation types.
    services.AddTransient<OperationService, OperationService>();
}
```

#### Transient <a href="#transient" id="transient"></a>

Transient lifetime services (`AddTransient`) are created each time they're requested from the service container. This lifetime works best for lightweight, stateless services.

#### Scoped <a href="#scoped" id="scoped"></a>

Scoped lifetime services (`AddScoped`) are created once per client request (connection).

#### Singleton <a href="#singleton" id="singleton"></a>

Singleton lifetime services (`AddSingleton`) are created the first time they're requested (or when `Startup.ConfigureServices` is run and an instance is specified with the service registration). Every subsequent request uses the same instance.

### Usage

```csharp
public class SomeClass
{
    private readonly IMyDependency _myDependency;

    // injection through constructor
    public SomeClass(IMyDependency myDependency)
    {
        _myDependency = myDependency;
    }
    
    public void DoSomeWork()
    {
        // some logic here that can use _myDependency
    }
}
```


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