dotnet-csharp-dependency-injection

Name: dotnet-csharp-dependency-injection
Author: wshaddix

65stars10forks

How to install dotnet-csharp-dependency-injection?

wshaddix/dotnet-skills/dotnet-csharp-dependency-injection

$npx -y skills add wshaddix/dotnet-skills --skill dotnet-csharp-dependency-injection

Installs into the current project.

Summary

Registering or resolving services with MS DI. Keyed services, scopes, decoration, hosted services.

Files · 1

View on GitHub

SKILL.md

1	# dotnet-csharp-dependency-injection
2
3	Advanced Microsoft.Extensions.DependencyInjection patterns for .NET applications. Covers service lifetimes, keyed services (net8.0+), decoration, factory delegates, scope validation, and hosted service registration.
4
5	Cross-references: [skill:dotnet-csharp-async-patterns] for `BackgroundService` async patterns, [skill:dotnet-csharp-configuration] for `IOptions<T>` binding.
6
7	---
8
9	## Service Lifetimes
10
11	\| Lifetime \| Registration \| When to Use \|
12	\|----------\|-------------\|-------------\|
13	\| Transient \| `AddTransient<T>()` \| Lightweight, stateless services. New instance per injection. \|
14	\| Scoped \| `AddScoped<T>()` \| Per-request state (EF Core `DbContext`, unit of work). \|
15	\| Singleton \| `AddSingleton<T>()` \| Thread-safe, stateless, or shared state (caches, config). \|
16
17	### Lifetime Mismatches (Captive Dependencies)
18
19	Never inject a shorter-lived service into a longer-lived one:
20
21	```csharp
22	// WRONG -- scoped DbContext captured in singleton = same context for all requests
23	builder.Services.AddSingleton<OrderService>(); // singleton
24	builder.Services.AddScoped<AppDbContext>(); // scoped -- CAPTIVE!
25
26	// CORRECT -- use IServiceScopeFactory in singletons
27	public sealed class OrderService(IServiceScopeFactory scopeFactory)
28	{
29	public async Task ProcessAsync(CancellationToken ct = default)
30	{
31	using var scope = scopeFactory.CreateScope();
32	var db = scope.ServiceProvider.GetRequiredService<AppDbContext>();
33	await db.Orders.Where(o => o.IsPending).ToListAsync(ct);
34	}
35	}
36	```
37
38	### Enable Scope Validation (Development)
39
40	```csharp
41	var builder = WebApplication.CreateBuilder(args);
42	// In Development, ValidateScopes is already true by default.
43	// For non-web hosts:
44	var host = Host.CreateDefaultBuilder(args)
45	.UseDefaultServiceProvider(options =>
46	{
47	options.ValidateScopes = true;
48	options.ValidateOnBuild = true; // Validates all registrations at startup
49	})
50	.Build();
51	```
52
53	---
54
55	## Registration Patterns
56
57	### Interface-Implementation Pair
58
59	```csharp
60	builder.Services.AddScoped<IOrderRepository, SqlOrderRepository>();
61	```
62
63	### Multiple Implementations
64
65	```csharp
66	// Register multiple implementations
67	builder.Services.AddScoped<INotifier, EmailNotifier>();
68	builder.Services.AddScoped<INotifier, SmsNotifier>();
69	builder.Services.AddScoped<INotifier, PushNotifier>();
70
71	// Inject all -- order matches registration order
72	public sealed class CompositeNotifier(IEnumerable<INotifier> notifiers)
73	{
74	public async Task NotifyAsync(string message, CancellationToken ct = default)
75	{
76	foreach (var notifier in notifiers)
77	{
78	await notifier.NotifyAsync(message, ct);
79	}
80	}
81	}
82	```
83
84	### Factory Delegates
85
86	```csharp
87	builder.Services.AddScoped<IOrderService>(sp =>
88	{
89	var repo = sp.GetRequiredService<IOrderRepository>();
90	var logger = sp.GetRequiredService<ILogger<OrderService>>();
91	var options = sp.GetRequiredService<IOptions<OrderOptions>>();
92	return new OrderService(repo, logger, options.Value.MaxRetries);
93	});
94	```
95
96	### `TryAdd` for Library Registrations
97
98	Libraries should use `TryAdd` so applications can override:
99
100	```csharp
101	// Library code -- won't overwrite app registrations
102	builder.Services.TryAddScoped<IOrderRepository, DefaultOrderRepository>();
103
104	// Application code -- takes precedence if registered first
105	builder.Services.AddScoped<IOrderRepository, CustomOrderRepository>();
106	```
107
108	---
109
110	## Keyed Services (net8.0+)
111
112	Register and resolve services by a key, replacing the need for named service patterns.
113
114	```csharp
115	// Registration
116	builder.Services.AddKeyedScoped<ICache, RedisCache>("distributed");
117	builder.Services.AddKeyedScoped<ICache, MemoryCache>("local");
118
119	// Injection via attribute
120	public sealed class OrderService(
121	[FromKeyedServices("distributed")] ICache distributedCache,
122	[FromKeyedServices("local")] ICache localCache)
123	{
124	public async Task<Order?> GetAsync(int id, CancellationToken ct = default)
125	{
126	// Check local cache first, then distributed
127	return await localCache.GetAsync<Order>(id.ToString(), ct)
128	?? await distributedCache.GetAsync<Order>(id.ToString(), ct);
129	}
130	}
131
132	// Manual resolution
133	var cache = sp.GetRequiredKeyedService<ICache>("distributed");
134	```
135
136	> net8.0+ only. On earlier TFMs, use factory patterns or a dictionary-based approach.
137
138	---
139
140	## Decoration Pattern
141
142	The built-in container does not natively support decoration. Use one of these approaches:
143
144	### Manual Decoration
145
146	```csharp
147	builder.Services.AddScoped<SqlOrderRepository>();
148	builder.Services.AddScoped<IOrderRepository>(sp =>
149	{
150	var inner = sp.GetRequiredService<SqlOrderRepository>();
151	var logger = sp.GetRequiredService<ILogger<LoggingOrderRepository>>();
152	return new LoggingOrderRepository(inner, logger);
153	});
154
155	public sealed class L

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dotnet-csharp-dependency-injection

65stars10forks

View on GitHub

How to install dotnet-csharp-dependency-injection?

wshaddix/dotnet-skills/dotnet-csharp-dependency-injection

$npx -y skills add wshaddix/dotnet-skills --skill dotnet-csharp-dependency-injection

Installs into the current project.

Summary

Registering or resolving services with MS DI. Keyed services, scopes, decoration, hosted services.

Files · 1

View on GitHub

SKILL.md

1	# dotnet-csharp-dependency-injection
2
3	Advanced Microsoft.Extensions.DependencyInjection patterns for .NET applications. Covers service lifetimes, keyed services (net8.0+), decoration, factory delegates, scope validation, and hosted service registration.
4
5	Cross-references: [skill:dotnet-csharp-async-patterns] for `BackgroundService` async patterns, [skill:dotnet-csharp-configuration] for `IOptions<T>` binding.
6
7	---
8
9	## Service Lifetimes
10
11	\| Lifetime \| Registration \| When to Use \|
12	\|----------\|-------------\|-------------\|
13	\| Transient \| `AddTransient<T>()` \| Lightweight, stateless services. New instance per injection. \|
14	\| Scoped \| `AddScoped<T>()` \| Per-request state (EF Core `DbContext`, unit of work). \|
15	\| Singleton \| `AddSingleton<T>()` \| Thread-safe, stateless, or shared state (caches, config). \|
16
17	### Lifetime Mismatches (Captive Dependencies)
18
19	Never inject a shorter-lived service into a longer-lived one:
20
21	```csharp
22	// WRONG -- scoped DbContext captured in singleton = same context for all requests
23	builder.Services.AddSingleton<OrderService>(); // singleton
24	builder.Services.AddScoped<AppDbContext>(); // scoped -- CAPTIVE!
25
26	// CORRECT -- use IServiceScopeFactory in singletons
27	public sealed class OrderService(IServiceScopeFactory scopeFactory)
28	{
29	public async Task ProcessAsync(CancellationToken ct = default)
30	{
31	using var scope = scopeFactory.CreateScope();
32	var db = scope.ServiceProvider.GetRequiredService<AppDbContext>();
33	await db.Orders.Where(o => o.IsPending).ToListAsync(ct);
34	}
35	}
36	```
37
38	### Enable Scope Validation (Development)
39
40	```csharp
41	var builder = WebApplication.CreateBuilder(args);
42	// In Development, ValidateScopes is already true by default.
43	// For non-web hosts:
44	var host = Host.CreateDefaultBuilder(args)
45	.UseDefaultServiceProvider(options =>
46	{
47	options.ValidateScopes = true;
48	options.ValidateOnBuild = true; // Validates all registrations at startup
49	})
50	.Build();
51	```
52
53	---
54
55	## Registration Patterns
56
57	### Interface-Implementation Pair
58
59	```csharp
60	builder.Services.AddScoped<IOrderRepository, SqlOrderRepository>();
61	```
62
63	### Multiple Implementations
64
65	```csharp
66	// Register multiple implementations
67	builder.Services.AddScoped<INotifier, EmailNotifier>();
68	builder.Services.AddScoped<INotifier, SmsNotifier>();
69	builder.Services.AddScoped<INotifier, PushNotifier>();
70
71	// Inject all -- order matches registration order
72	public sealed class CompositeNotifier(IEnumerable<INotifier> notifiers)
73	{
74	public async Task NotifyAsync(string message, CancellationToken ct = default)
75	{
76	foreach (var notifier in notifiers)
77	{
78	await notifier.NotifyAsync(message, ct);
79	}
80	}
81	}
82	```
83
84	### Factory Delegates
85
86	```csharp
87	builder.Services.AddScoped<IOrderService>(sp =>
88	{
89	var repo = sp.GetRequiredService<IOrderRepository>();
90	var logger = sp.GetRequiredService<ILogger<OrderService>>();
91	var options = sp.GetRequiredService<IOptions<OrderOptions>>();
92	return new OrderService(repo, logger, options.Value.MaxRetries);
93	});
94	```
95
96	### `TryAdd` for Library Registrations
97
98	Libraries should use `TryAdd` so applications can override:
99
100	```csharp
101	// Library code -- won't overwrite app registrations
102	builder.Services.TryAddScoped<IOrderRepository, DefaultOrderRepository>();
103
104	// Application code -- takes precedence if registered first
105	builder.Services.AddScoped<IOrderRepository, CustomOrderRepository>();
106	```
107
108	---
109
110	## Keyed Services (net8.0+)
111
112	Register and resolve services by a key, replacing the need for named service patterns.
113
114	```csharp
115	// Registration
116	builder.Services.AddKeyedScoped<ICache, RedisCache>("distributed");
117	builder.Services.AddKeyedScoped<ICache, MemoryCache>("local");
118
119	// Injection via attribute
120	public sealed class OrderService(
121	[FromKeyedServices("distributed")] ICache distributedCache,
122	[FromKeyedServices("local")] ICache localCache)
123	{
124	public async Task<Order?> GetAsync(int id, CancellationToken ct = default)
125	{
126	// Check local cache first, then distributed
127	return await localCache.GetAsync<Order>(id.ToString(), ct)
128	?? await distributedCache.GetAsync<Order>(id.ToString(), ct);
129	}
130	}
131
132	// Manual resolution
133	var cache = sp.GetRequiredKeyedService<ICache>("distributed");
134	```
135
136	> net8.0+ only. On earlier TFMs, use factory patterns or a dictionary-based approach.
137
138	---
139
140	## Decoration Pattern
141
142	The built-in container does not natively support decoration. Use one of these approaches:
143
144	### Manual Decoration
145
146	```csharp
147	builder.Services.AddScoped<SqlOrderRepository>();
148	builder.Services.AddScoped<IOrderRepository>(sp =>
149	{
150	var inner = sp.GetRequiredService<SqlOrderRepository>();
151	var logger = sp.GetRequiredService<ILogger<LoggingOrderRepository>>();
152	return new LoggingOrderRepository(inner, logger);
153	});
154
155	public sealed class L

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