⏳ The 3 Lifetimes
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🔵 Singleton → stateless global services
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🟢 Scoped → per-request (web default)
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🟠 Transient → lightweight, multi-instance
❌ Absolute Rule
Never inject a Scoped service into a Singleton.
Bad:
builder.Services.AddSingleton<MyService>(); // depends on DbContext ❌
✔ Correct
Use factory:
builder.Services.AddSingleton<MyService>(sp =>
new MyService(sp.GetRequiredService<IDbContextFactory<AppDb>>()));
🧠 Why?
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Prevents stale data
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Eliminates threading issues
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Perfect for background workers