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Home Interviews What is the difference between IEnumerable and IQueryable? – MindStick
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1 Answers

ICSM Computer
ICSM Computer 11 Jun 2025
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IEnumerable<T>

  • Namespace: System.Collections.Generic
  • Purpose: Represents a sequence of elements that can be enumerated (iterated) over in memory.
  • Execution: Deferred execution in memory.
  • Use case: Best for in-memory collections like arrays, lists, or already-fetched data.
  • Query execution: LINQ to Objects – the query is executed in the application, after fetching data.

Example:

List<int> numbers = new List<int> { 1, 2, 3, 4 };
IEnumerable<int> evens = numbers.Where(n => n % 2 == 0); // Executed in memory

Characteristics:

  • Supports only LINQ to Objects.
  • Filters are applied after retrieving all data into memory.
  • Less suitable for large data sources like databases.

IQueryable<T>

  • Namespace: System.Linq
  • Purpose: Enables querying a data source (e.g., database) with expression trees that can be translated to the native query language (e.g., SQL).
  • Execution: Deferred execution on the data source.
  • Use case: Best for querying remote data sources (e.g., Entity Framework, LINQ to SQL).
  • Query execution: LINQ to Entities, SQL, or other providers.

Example:

IQueryable<User> users = dbContext.Users.Where(u => u.Age > 30); // Translates to SQL

Characteristics:

  • Converts LINQ queries into SQL and executes them on the server.
  • Efficient for databases because it can perform filtering, joining, paging, etc., on the server.
  • Ideal for large datasets or when you want to avoid loading unnecessary data.

Key Differences

Feature IEnumerable<T> IQueryable<T>
Namespace System.Collections.Generic System.Linq
Query execution In memory (client-side) At data source (server-side)
Use with databases Not suitable Yes (e.g., Entity Framework)
Deferred execution Yes Yes
Query translation N/A Translates expression trees to SQL
Performance May fetch all records Efficient due to server-side filtering

Summary

  • Use IEnumerable<T> when working with in-memory collections.
  • Use IQueryable<T> when working with databases or external data sources to avoid pulling too much data into memory.
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