What is the difference between IEnumerable and IQueryable?

💡 Concept: IEnumerable vs IQueryable

IEnumerable and IQueryable are interfaces used for querying data collections in C#, but they differ in execution and capabilities.

📘 Quick Intro

IEnumerable executes queries in memory and is suitable for in-memory collections, whereas IQueryable allows query translation for remote data sources like databases.

🧠 Analogy

Think of IEnumerable as reading a book already in your hands, while IQueryable is like sending a query to a librarian to fetch specific pages.

🔧 Technical Explanation

🧮 IEnumerable performs LINQ queries in-memory, after data is loaded.
🖥️ IQueryable translates LINQ queries to expressions executed on the data source.
⚙️ IQueryable supports deferred execution with query translation.
🔍 IEnumerable is suitable for collections; IQueryable for remote queries (e.g., databases).
💡 IQueryable can optimize queries and reduce data transferred.

🎯 Use Cases

✅ Use IEnumerable for local, in-memory collection filtering.
✅ Use IQueryable for querying databases and remote sources.
✅ IQueryable helps reduce data load by pushing filters to data source.
✅ Choose based on data source and performance needs.

💻 Code Example


using System;
using System.Collections.Generic;
using System.Linq;

class Program {
    static void Main() {
        List<int> numbers = new List<int> {1, 2, 3, 4, 5};

        // IEnumerable example
        IEnumerable<int> enumerable = numbers.Where(n => n > 2);
        foreach (var num in enumerable) {
            Console.WriteLine(num);
        }

        // IQueryable example with Entity Framework (example)
        // IQueryable<int> queryable = dbContext.Numbers.Where(n => n > 2);
    }
}

❓ Interview Q&A

Q1: Where does IEnumerable execute queries?
A: In memory after data is loaded.

Q2: What does IQueryable do?
A: Translates queries for remote execution.

Q3: Can IQueryable optimize queries?
A: Yes, by pushing filters to data source.

Q4: Is IEnumerable suitable for databases?
A: No, mainly for in-memory collections.

Q5: What is deferred execution?
A: Queries execute when enumerated.

Q6: Which namespace contains these interfaces?
A: System.Linq.

Q7: Can IQueryable cause performance gains?
A: Yes, by reducing data transfer.

Q8: What is the difference in query syntax?
A: IQueryable queries can be translated to SQL.

Q9: Can IEnumerable be used with LINQ?
A: Yes.

Q10: How to convert IQueryable to IEnumerable?
A: Using ToList() or AsEnumerable().

📝 MCQs

Q1. Where does IEnumerable execute queries?

In memory
Remote
Database
Server

Q2. What does IQueryable do?

Executes locally
Translates queries
Deletes data
Creates data

Q3. Can IQueryable optimize queries?

No
Yes
Sometimes
Never

Q4. Is IEnumerable suitable for databases?

Yes
No
Sometimes
Always

Q5. What is deferred execution?

Queries execute immediately
Queries execute when enumerated
Queries never execute
Queries are compiled

Q6. Namespace for IEnumerable and IQueryable?

System.Collections
System.Linq
System.Text
System.IO

Q7. Can IQueryable improve performance?

No
Yes
Sometimes
Never

Q8. Difference in query syntax?

No difference
IQueryable queries translate to SQL
Only IEnumerable supports LINQ
None

Q9. Can IEnumerable be used with LINQ?

No
Yes
Sometimes
Never

Q10. How to convert IQueryable to IEnumerable?

ToArray()
ToList() or AsEnumerable()
Cast()
Convert()

💡 Bonus Insight

Choosing between IEnumerable and IQueryable wisely improves performance and resource usage in LINQ queries.

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