IQueryable vs. IEnumerable: When Should I Use Which?

IQueryable vs. IEnumerable: A Practical Guide

When working with data, the choice between IQueryable<T> and IEnumerable<T> significantly impacts performance and efficiency. Both represent sequences of data, but their execution differs dramatically. This guide clarifies when to use each.

Understanding the Core Differences

IQueryable<T> excels in scenarios involving remote data sources like databases (LINQ to SQL, LINQ to Entities). Its key feature is deferred execution: the query isn't executed until its results are actually needed. This optimizes resource usage and speeds up data retrieval.

Conversely, IEnumerable<T> operates on in-memory collections. Queries using IEnumerable<T> execute immediately, loading all matching data into memory. This is suitable for smaller datasets but can be inefficient for large databases.

Making the Right Choice

The optimal choice depends on your application's needs:

Use IQueryable<T> when:

• You're accessing a remote database and want to minimize data transfer.
• The query needs further refinement (filtering, sorting) before execution.
• You require server-side operations like aggregation, pagination, or projection for optimal performance.

Use IEnumerable<T> when:

• Your data resides in memory (e.g., a list, array).
• Immediate query execution is essential.
• You need to access individual elements repeatedly; IQueryable<T> can only be enumerated once.

Illustrative Examples

Consider these code snippets:

<code class="language-csharp">// IQueryable<Customer> - Deferred Execution
IQueryable<Customer> customers = from c in db.Customers
                                 where c.City == "New York"
                                 select c;</code>

<code class="language-csharp">// IEnumerable<Customer> - Immediate Execution
IEnumerable<Customer> customers = from c in db.Customers.ToList() // Loads all customers into memory
                                 where c.City == "New York"
                                 select c;</code>

In the IQueryable<T> example, the database query executes only when you iterate through customers. The IEnumerable<T> example, however, loads all customers from the database into memory before filtering, potentially causing performance bottlenecks with large datasets. The .ToList() call forces immediate execution.

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