⚡ 10 Performance and Query Optimization Interview Questions for .NET Developers

🧪 1. How do you measure and monitor performance in a .NET application?

📌 Why this is asked:
To check your practical experience with diagnosing and profiling performance issues.

✅ Answer:
I use tools like:

• Application Insights or Elastic APM for real-time telemetry

• dotTrace, PerfView, or Visual Studio Profiler for CPU/memory profiling

• BenchmarkDotNet for micro-benchmarking methods
  I monitor key metrics: CPU, memory usage, response time, database duration, and exceptions.

🗃️ 2. What’s the difference between IQueryable and IEnumerable? When would you use each?

📌 Why this is asked:
To assess understanding of deferred execution and query efficiency.

✅ Answer:

• IQueryable executes on the database side, translating LINQ into SQL — ideal for data filtering and pagination.

• IEnumerable brings all data into memory first, then filters in .NET.
  I use IQueryable for database operations and IEnumerable for in-memory processing (e.g., business logic after query execution).

📉 3. How do you identify and fix slow SQL queries?

📌 Why this is asked:
To test how well you optimize database-heavy .NET applications.

✅ Answer:
I analyze the execution plan, index usage, and query stats using SQL Server Management Studio (SSMS). Common fixes include:

• Adding proper indexes

• Avoiding SELECT *

• Breaking large joins into smaller queries

• Using EXISTS instead of IN I also use DbCommandInterceptor in EF Core to log slow queries.

🧵 4. What are connection pooling and its performance impact?

📌 Why this is asked:
To verify database efficiency knowledge and resource management skills.

✅ Answer:
Connection pooling reuses existing DB connections instead of opening new ones, significantly improving performance. In .NET, it's enabled by default for SQL Server. I always ensure connections are closed properly (using statement or await using) to return them to the pool.

🧠 5. How do you reduce memory pressure in long-running .NET applications?

📌 Why this is asked:
To check your strategies for optimizing memory and avoiding leaks.

✅ Answer:

• Reuse objects where possible (e.g., StringBuilder, MemoryStream)

• Use value types or Span<T> when applicable

• Release unused references

• Use ArrayPool<T> or ObjectPool<T> for frequent allocations

• Profile heap usage and GC frequency with diagnostic tools

🔁 6. How do you implement caching in .NET to improve performance?

📌 Why this is asked:
To see how you reduce redundant operations and increase API speed.

✅ Answer:
I use:

• In-memory cache (IMemoryCache) for short-lived, frequent lookups

• Distributed cache (e.g., Redis) for shared, scalable caching

• Set expiration with sliding/absolute time

• For EF Core, I cache expensive queries (e.g., read-heavy reports) and cache the DTOs, not EF entities

📦 7. What are EF Core tracking vs. no-tracking queries? How do they affect performance?

📌 Why this is asked:
To test knowledge of ORM internals and memory efficiency.

✅ Answer:

• Tracking queries monitor entities for changes (used for updates)

• No-tracking queries skip change tracking, reducing overhead — ideal for read-only operations
  I use .AsNoTracking() in read-heavy services to improve query speed and lower memory usage.

📍 8. How do you paginate data efficiently in SQL and .NET APIs?

📌 Why this is asked:
To assess your ability to handle large datasets in a performant way.

✅ Answer:
I use Skip() and Take() with proper OrderBy() in LINQ:

dbContext.Customers
    .OrderBy(c => c.Id)
    .Skip((page - 1) * size)
    .Take(size);

In SQL Server, I use OFFSET and FETCH. I avoid deep pagination (e.g., page 1000+) and offer cursor-based pagination if needed.

🔍 9. What’s the impact of lazy vs. eager loading in Entity Framework?

📌 Why this is asked:
To test your understanding of data access and performance trade-offs.

✅ Answer:

• Lazy loading loads related data on demand, causing N+1 query issues if misused.

• Eager loading (Include) fetches related data in a single query, which is better for known use cases. I use eager loading for known navigations and disable lazy loading by default to avoid surprises.

🔄 10. How do you optimize loops and LINQ queries for performance?

📌 Why this is asked:
To test your ability to write high-performance code in .NET.

✅ Answer:

• Minimize LINQ chains that enumerate multiple times

• Use ToList() only when necessary

• Avoid closures and boxing in loops

• Favor foreach over for for readability unless you need index

• Profile lambda-heavy logic and refactor hot paths
  I also use Span<T> and array pooling in high-throughput scenarios.