When to Use IEnumerable vs IQueryable in C#
IEnumerable<T> is for in-memory collections and executes queries in the application. IQueryable<T> is for out-of-memory data sources like databases and translates queries to the data source's native language, such as SQL.
If you are asking "ienumerable vs iqueryable c#", the short answer is: use IEnumerable<T> for local collections and IQueryable<T> for database-backed queries where the provider can translate the expression.
IEnumerable uses deferred execution with LINQ to Objects, while IQueryable uses expression trees that can be translated to query languages like SQL.
Sample Data
// IEnumerable - in-memory collection
List<Product> products = new List<Product>
{
new Product { Id = 1, Name = "Laptop", Price = 1200, Category = "Electronics" },
new Product { Id = 2, Name = "Mouse", Price = 25, Category = "Electronics" },
new Product { Id = 3, Name = "Desk", Price = 350, Category = "Furniture" },
new Product { Id = 4, Name = "Chair", Price = 200, Category = "Furniture" },
new Product { Id = 5, Name = "Monitor", Price = 400, Category = "Electronics" }
};Using IEnumerable for In-Memory Filtering
// IEnumerable - filtering happens in memory
IEnumerable<Product> enumerable = products;
var expensiveProducts = enumerable.Where(p => p.Price > 100);
Console.WriteLine("IEnumerable (in-memory filtering):");
foreach (var product in expensiveProducts)
{
Console.WriteLine($"{product.Name}: ${product.Price}");
}Using IQueryable for Expression-Based Queries
// IQueryable - can be translated to SQL or other query language
IQueryable<Product> queryable = products.AsQueryable();
var electronicsQuery = queryable.Where(p => p.Category == "Electronics");
Console.WriteLine("\nIQueryable (expression-based):");
foreach (var product in electronicsQuery)
{
Console.WriteLine($"{product.Name}: ${product.Price}");
}Query Composition and Chaining
// Demonstrating query composition
Console.WriteLine("\nQuery Composition:");
var complexQuery = queryable
.Where(p => p.Price > 100)
.OrderBy(p => p.Price)
.Select(p => new { p.Name, p.Price });
foreach (var item in complexQuery)
{
Console.WriteLine($"{item.Name}: ${item.Price}");
}FAQ
Which one should I use with Entity Framework Core?
Use IQueryable<T> when you want EF Core to translate the filter and projection to SQL.
Can I convert between them?
Yes. AsEnumerable() moves from IQueryable<T> to IEnumerable<T>, while AsQueryable() wraps an in-memory source as IQueryable<T>.
Performance Patterns and Query Translation
// IEnumerable: All filtering after ToList()
Console.WriteLine("\nIEnumerable Performance Pattern:");
var inMemory = products
.Where(p => p.Category == "Electronics") // Happens in memory
.ToList(); // Already in memory
Console.WriteLine($"Found {inMemory.Count} electronics");
// IQueryable: Filtering would be done at data source
// (In real scenarios with Entity Framework or LINQ to SQL)
Console.WriteLine("\nIQueryable Pattern (simulated DB query):");
var dbQuery = products.AsQueryable()
.Where(p => p.Category == "Electronics") // Would translate to SQL WHERE
.Select(p => p.Name); // Would translate to SQL SELECT
Console.WriteLine($"Query created (would execute on database)");
// Conversion between them
IEnumerable<Product> fromQueryable = queryable.AsEnumerable();
IQueryable<Product> fromEnumerable = enumerable.AsQueryable();Product Class
public class Product
{
public int Id { get; set; }
public string Name { get; set; }
public decimal Price { get; set; }
public string Category { get; set; }
}Take It Further
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