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IEnumerable vs IQueryable in C#

In C#, IEnumerable and IQueryable are both interfaces that are commonly used in LINQ (Language Integrated Query) to work with collections of data, but they serve different and have distinct characteristics.

What is IEnumerable?

IEnumerable is a fundamental interface that represents a forward-only cursor of data. It is used to query and manipulate in-memory collections such as arrays, lists, and other data structures that implement this interface.

IEnumerable is an interface that is part of the System.Collection namespace, and is used to represent a collection of objects that can be enumerated (iterated) one at a time. It is a fundamental interface for working with collections and provides a standard way to iterate over the elements of a collection without exposing the underlying data structure.

Immediate Execution: Operations performed on IEnumerable sequences are executed immediately. When you perform a LINQ operation on an IEnumerable, the data is queried and processed in-memory, and the result is returned as a new sequence or collection.

Lack of Deferred Execution: The operations are not deferred; they are performed as soon as you call the LINQ method (e.g., Where, Select, ToList, ToArray, etc.).

Suitable for In-Memory Collections: It is ideal for working with in-memory data collections where the entire data set can fit into memory.

LINQ to Objects: It is mainly used for LINQ to Objects, which means it works with collections that are already loaded into memory.

Strongly Typed to a Degree:

  • IEnumerable is not as strongly typed as IQueryable. It works with collections of objects in a general way.
  • Type safety is somewhat present, but you may need to use explicit casting when working with objects.



It has several characteristics of the IEnumerable in C#. Some main characteristics of IEnumerable are as follows:

Collection Agnosticism:

IEnumerable is a way to interact with collections of objects in a generic, collection-agnostic manner. It doesn't matter whether you're dealing with an array, a list, or a custom collection; if it implements IEnumerable, you can work with it using a consistent set of methods.

Iteration Abstraction:

Instead of directly manipulating the underlying data structures of a collection, IEnumerable provides an abstraction for iteration. It allows you to ask the collection for an enumerator (an IEnumerator) who knows how to traverse through the elements.

Foreach Loop Compatibility:

One of the key advantages of IEnumerable is that it enables the use of the foreach loop to easily iterate through elements in a collection. This loop relies on the GetEnumerator method to get an enumerator and then uses it to loop through the elements without needing to know the specific collection type.

LINQ Integration:

IEnumerable is closely tied to LINQ, which is a powerful query language for working with collections in C#. Collections that implement IEnumerable can leverage LINQ's rich set of extension methods to perform operations like filtering, sorting, and projection.

Custom Iteration:

You can make your own classes iterable by implementing IEnumerable. It is helpful when you have a custom collection and want to enable standard iteration patterns. Implementing IEnumerable means you need to provide an enumerator that defines how to move through the elements within your collection.

Basic LINQ Support:

  • IEnumerable provides a basic set of LINQ operators, which are useful for working with in-memory collections.
  • Common LINQ operations like Where, Select, OrderBy, and GroupBy are supported.

Limited Query Optimization:

  • Query optimization is limited because operations are executed immediately and on the client side.
  • Operations are not translated into more efficient query languages, such as SQL, for external data sources.
  • In essence, IEnumerable provides a unified way to work with collections, whether they are part of the .NET Framework's standard libraries or your custom data structures. It abstracts the process of iterating over elements, making your code more versatile and adaptable to different types of collections. This flexibility is fundamental to C#'s approach to working with data.


Let's take an example to illustrate the iEnumerable in C#.


Name: Alice, Age: 30
Name: Bob, Age: 25
Name: Charlie, Age: 35


  • In this example, person is a class that represents individuals with two properties: Name and Age. It's a simple data structure to hold information about people.
  • PeopleCollection is a custom collection class that stores a list of Person objects.
  • It implements the IEnumerable interface, indicating that it contains a sequence of Person objects that can be enumerated.
  • The AddPerson method allows you to add new Person objects to the collection.
  • The GetEnumerator method is crucial for making the collection iterable. It returns an enumerator for the internal list of people. This enumerator is used to traverse the collection.
  • We create an instance of PeopleCollection called peopleCollection.
  • We use the AddPerson method to add three people to the collection: "Alice", "Bob", and "Charlie", each with their respective ages.
  • After that, we use a foreach loop to iterate through the peopleCollection object. It is possible because PeopleCollection implements IEnumerable.

Complexity Analysis:

Time Complexity:

Adding a Person to the PeopleCollection: O(1)

When adding a Person to the PeopleCollection, it directly appends the person to the internal list. It is an O(1) operation because it doesn't depend on the size of the collection.

Iterating Through the PeopleCollection with foreach: O(n)

When you iterate through the PeopleCollection using a foreach loop, it essentially involves iterating over each element in the collection. The time complexity is O(n), where 'n' is the number of Person objects in the collection.

Space Complexity:

Space Complexity of PeopleCollection: O(n)

The PeopleCollection class internally uses a List to store the Person objects. The space complexity is O(n), where 'n' is the number of Person objects stored in the collection.

Space Complexity of 'Person' Objects: O(1) per Person object

Each Person object has a fixed amount of memory allocated for its Name and Age properties. Therefore, the space complexity for each Person object is O(1).

In summary, the time complexity for adding a person to the collection is O(1), and the time complexity for iterating through the collection is O(n). The space complexity of the PeopleCollection class is O(n), and the space complexity for each Person object is O(1).

What is IQueryable?

IQueryable is an interface that extends IEnumerable and is designed for querying data from a data source that supports querying, such as a database. It is part of LINQ to SQL, LINQ to Entities and other LINQ providers.

Deferred Execution:

IQueryable allows you to create queries that are not executed immediately when constructed. Instead, they are executed only when you explicitly request the results.

This deferred execution is a fundamental feature, as it allows query optimization and minimizes the data transferred from a data source.

Query Optimization: It allows for query optimization, meaning the query provider can convert LINQ queries into efficient SQL (or equivalent) queries when working with a database.

Suitable for Remote Data Sources: It is ideal for working with data sources that can handle query operations, like databases. The query is executed on the remote data source, reducing the amount of data transferred.

LINQ to SQL, LINQ to Entities: It is mainly used for LINQ to SQL, LINQ to Entities, or other LINQ providers where you work with data stored in databases or remote services.



It has several characteristics of the IQueryable in C#. Some main characteristics of the IQueryable are as follows:

Integration with Data Sources:

  • IQueryable is often used with data sources, such as databases (e.g., SQL Server) and Object-Relational Mapping (ORM) frameworks (e.g., Entity Framework).
  • It allows you to express queries in C# that can be translated into SQL or other query languages for efficient execution against data stores.

Type Safety:

  • Queries created with IQueryable are strongly typed. It means that you get compile-time type checking, reducing the chances of runtime errors.
  • You can work with strongly typed entities and properties, providing IntelliSense support and improved code readability.

Query Composition:

  • IQueryable enables you to build complex queries by chaining multiple query operators (e.g., Where, OrderBy, Select) together in a readable and modular way.
  • It promotes maintainability and expressiveness in query code.

Expression Trees:

  • IQueryable represents queries as expression trees, which are abstract syntax trees that represent code logic.
  • These expression trees can be analyzed and modified, allowing for advanced query optimization and transformation.

Custom Query Providers:

  • You can create custom query providers to extend IQueryable to work with various data sources beyond databases, such as in-memory collections, web services, or other data stores.
  • It makes IQueryable highly adaptable to different scenarios.

External Data Source Integration:

  • IQueryable is designed for querying external data sources, such as databases, web services, and other data stores.
  • It can translate LINQ queries into native query languages (e.g., SQL) for efficient data retrieval from these sources.

Asynchronous Support:

  • Supports asynchronous execution of queries, enabling parallel processing and improved performance when working with external data sources.
  • Asynchronous methods like ToListAsyncAsync are available to support async operations.
  • It involves query optimization, translating LINQ queries into native query languages that are efficient for data retrieval from external data sources.
  • Query optimization can significantly improve performance when working with large datasets and databases.


Let's take an example to illustrate the IQueryable in C#.


People over 30:
Charlie, 35 years old

Names of all people:

Sum of ages: 140 years


Person Class:

We define a simple Person class with two properties, Name and Age, to represent individuals. This class will be used to create objects that hold person-related data.

List of People:

We create a list called people and populate it with instances of the Person class. This list represents a collection of people, each with a name and an age.

IQueryable Creation:

We convert the people list into an IQueryable by using the AsQueryable method. This conversion allows us to use LINQ operators against the list, treating it as a queryable data source.

Query Operations:

We perform various query operations on the IQueryable object:

Filtering (Where): We create an IQueryable called over 30, which represents a query to find people whose age is greater than 30.

Projection (Select): We create an IQueryable called names, which represents a query to extract the names of all people.

Aggregation (Sum): We calculate the sum of ages using the Sum operator, which results in an int called ageSum.

Result Execution:

We explicitly execute the queries by iterating through the results. It is when the queries are executed, and the data is retrieved:

  • We print the names and ages of people over 30 to the console.
  • We print the names of all people to the console.
  • We print the sum of ages to the console.

Complexity Analysis:

Time Complexity:

Creating and Populating the people list:

Time Complexity: O(n)

Populating a list with n items takes linear time, as each person is added one by one to the list.

Creating the IQueryable from people:

Time Complexity: O(1)

Converting the people list into an IQueryable using AsQueryable is a constant-time operation and doesn't depend on the size of the list.

Query Operations (Where, Select, Sum):

Time Complexity: O(n)

Executing query operations using LINQ, such as Where, Select, and Sum, typically involves iterating through the entire data source once. These operations have a linear time complexity in relation to the size of the data source.

Iterating Through Results:

Time Complexity: O(n)

Iterating through the results of the query operations also takes linear time because it requires visiting each item in the result set.

Space Complexity:

people List: O(n)

The space complexity of the people list is linear in relation to the number of people stored in the list. Each person object consumes a fixed amount of memory.

IQueryable and Query Operators: O(1)

Creating the IQueryable and defining query operators does not significantly increase the memory usage. The space used is constant and does not depend on the size of the data source.

Query Results (over30, names, ageSum): O(m)

The space complexity of the query results (over30, names, and ageSum) depends on the size of the result set. If the result set contains m items, the space complexity is O(m).

The space complexity depends on the size of the result sets and the initial list of people, with the space complexity being O(n) for the list and O(m) for the query results where "n" is the number of people in the list and "m" is the number of results in each query.

Key differences between IEnumerable and IQueryable:

There are several differences between the IEnumerable and IQueryable. Some main differences between the IEnumerable and IQueryable are as follows:



  • It is primarily used for in-memory collections, such as arrays, lists, and other collections.
  • IEnumerable is suitable for working with data that is already in memory and can be used for general-purpose iteration and manipulation of collections.


  • It is designed for querying data from various data sources, such as databases, web services, or other external data stores.
  • IQueryable is used for constructing and executing complex queries against external data sources and supports deferred execution.

Immediate vs. Deferred Execution:


  • Operations on IEnumerable collections are executed immediately when the method is called. There is no query optimization or deferred execution.
  • It is suitable for in-memory collections where all data is readily available.


  • IQueryable allows for deferred execution. Query operations are not executed until the results are explicitly requested.
  • This deferred execution enables query optimization and reduces the amount of data transferred from external data sources.

Data Sources:


  • It is typically used for in-memory collections like arrays, lists, and dictionaries.
  • It is not optimized for querying external data sources, such as databases.


  • It is designed for querying external data sources, including databases, web services, and custom data stores.
  • It supports translating LINQ queries into native query languages (e.g., SQL) for efficient data retrieval.

Strongly Typed:


  • It works with collections of objects in a general way.
  • It's less type-safe because it operates with objects and may require casting.


  • It provides strong typing and is used with entities and properties in a strongly-typed manner.
  • The type safety makes it easier to catch errors at compile time.



  • It does not involve query optimization, as operations are executed immediately.
  • It may require loading all data into memory before filtering or projecting.


  • It involves query optimization and deferred execution. The query provider optimizes and translates the query to the most efficient form for the data source.
  • It minimizes data transfer and improves query efficiency, especially when querying databases.

Integration with LINQ:


  • It provides a basic set of LINQ operators for working with in-memory collections.
  • LINQ operations are executed on the client side.


  • It offers an extended set of LINQ operators for complex queries.
  • LINQ queries using IQueryable are translated into native query languages (e.g., SQL) and executed on the data source side.

Custom Data Sources:


  • It is typically used for in-memory collections, making it less suitable for custom data sources.
  • Querying non-collection data sources requires custom logic.


  • It can be extended to work with custom data sources by creating custom query providers.
  • It enables querying a wide range of data sources beyond collections and databases.


In summary, IEnumerable is best suited for in-memory collections and immediate execution of operations, while IQueryable is designed for querying external data sources with deferred execution and query optimization. The choice between them depends on the nature of your data and the specific requirements of your application.

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