Here is the clearest real-time explanation of when to use

AddSingleton, AddScoped, and AddTransient — with practical scenarios you can give directly in interviews.

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✅ Real-Time Scenarios for AddSingleton, AddScoped, AddTransient

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🟦 1. AddSingleton — One instance for the entire application

✔ Real-Time Scenarios

Use Singleton when the data is shared and does NOT change per request.

Examples

a) Caching data

services.AddSingleton<ICacheService, MemoryCacheService>();

• Stores frequently used data (e.g., reference data, master data)

b) Configuration / settings

services.AddSingleton<IConfigProvider, ConfigProvider>();

c) Logging service

services.AddSingleton<ILoggerService, LoggerService>();

d) Background job manager

• Quartz scheduler

• Hangfire manager

✔ When to use:

• When service is stateless

• When service is thread-safe

• When you want performance improvements

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🟩 2. AddScoped — One instance per HTTP request

✔ Real-Time Scenarios

Use Scoped for things that should be unique per request (API call).

Examples

a) DbContext

services.AddScoped<AppDbContext>();

Each request gets a new DbContext → avoids cross-request data issues.

b) Repository services

services.AddScoped<IUserRepository, UserRepository>();

c) Business services

Example: OrderService, ProductService, UserService

✔ When to use:

• Anything that works with database

• Per-request logic (validation, authentication)

• Services that should not share state across users

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🟥 3. AddTransient — New instance every time it is requested

✔ Real-Time Scenarios

Use Transient for lightweight, stateless objects.

Examples

a) Utility classes

services.AddTransient<IEmailFormatter, EmailFormatter>();

b) Helper classes

• Password generator

• Token builder

• String utilities

c) External API call builders

services.AddTransient<IRequestBuilder, RequestBuilder>();

d) Mappers

services.AddTransient<IObjectMapper, ObjectMapper>();

✔ When to use:

• When service is very lightweight

• When a fresh object is required every time

• When the service is not shared and not dependent on state

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🎯 Interview-Friendly Summary

Singleton → Use for caching, logging, config — shared for entire app
Scoped → Use for DbContext, repository, business logic — per request
Transient → Use for utility/helper classes — new instance every time

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🔥 Quick 15-Second Answer

AddSingleton for shared, long-lived services like cache or logging.
AddScoped for per-request services like DbContext, repositories, and business services.
AddTransient for lightweight, stateless utility services like mappers, formatters, token generators.

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