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IEnumerable vs IQueryable a High Level Comparison

In this short article, we will take a look at IEnumerable vs IQueryable to understand the essential key differences.

IEnumerable and IQueryable are both interface types in the C# programming language used to work with data collections. While they may seem similar at first glance, there are important differences between the two that can affect the performance and functionality of your code.

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The IEnumerable interface represents a collection of objects that can be enumerated or iterated through, one at a time. This interface is implemented by many of the built-in collections in the .NET Framework, such as lists, arrays, and dictionaries.

The IEnumerable interface defines a single method, GetEnumerator(), which is used to obtain an enumerator object that can be used to iterate through the collection. The foreach loop in C# is implemented in terms of the IEnumerable interface, and you can use it to iterate through any collection that implements this interface.

Here is an example of using the IEnumerable interface in C# to iterate over a collection of integers:

In this example, we create a List<int> and assign it to an IEnumerable<int> variable. We can then use a foreach loop to iterate over the elements in the IEnumerable just as we would with a List.

Notice that the List<T> class itself implements the IEnumerable<T> interface; that’s why the assignment is possible. The IEnumerable<T> interface gives a way to traverse a collection of a certain type, and it’s usually used when you don’t need to modify the collection, but just access its elements.

It’s important to note that the IEnumerable<T> interface only provides a way to read the elements of a collection. It doesn’t provide methods for adding or removing elements, and this is why it’s considered a “read-only” interface.



The IQueryable interface, on the other hand, is used to represent a collection of objects that can be queried using the LINQ (Language Integrated Query) technology in C#.

This interface is implemented by types that can translate LINQ queries into a specific query language, such as SQL, and execute those queries against a data source. IQueryable is built on top of IEnumerable and has the same method (GetEnumerator) as IEnumerable but also include expression trees, which can be used to generate dynamic queries.

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Here is an example of how to use IQueryable to query a list of integers:

This program will output:


The Difference Between IEnumerable and IQueryable

The main difference between the two is that when you use LINQ on an IEnumerable<T>, the query is executed locally in memory on the client side.

When you use LINQ on an IQueryable<T>, the query is executed on the server side and can be translated into a specific query language (like SQL) to be executed on a database. This is called ‘Deferred Execution’, where the query is executed only when the final result is retrieved.



When using Entity Framework, for example, a major advantage of IQueryable over IEnumerable is that when you use IQueryable, the query is executed in the database rather than in memory on the client. This can lead to significant performance improvements when working with large data sets.

Additionally, because IQueryable can translate LINQ queries into a specific query language like SQL it can also support features such as pagination, ordering, and filtering that are not available when using IEnumerable.

Another vital point to notice is that the IEnumerable is a read-only interface, and the IQueryable is not. This means that IQueryable can be used for CRUD (Create-Read-Update-Delete) operations on data.


However, there are also some downsides to using IQueryable. For example, because the query is executed on the server side, it can lead to more complex and harder to debug code, and also will introduce network overhead as well. Additionally, because the IQueryable interface is based on expression trees, which are a more complex and powerful feature in C#, the learning curve can be steeper than when using IEnumerable.

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In summary, IEnumerable and IQueryable are both powerful tools for working with collections of data in C#, but they are designed for different use cases. IEnumerable is best used when you want to iterate over a collection of data locally on the client side, while IQueryable is best used when you want to query a data source on the server side.

The IQueryable interface can lead to better performance and more advanced features when working with large data sets, but it can also lead to more complex and harder-to-debug code. Therefore, it’s essential to choose the proper interface for the job based on your specific requirements.


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