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Theodore Karropoulos
Theodore Karropoulos

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Understanding IQueryable in C#


As developers, we often find ourselves tasked with fetching data from a database and storing it in our code for later use. The most common approach, or at least the one I've frequently encountered, is to store this data in IEnumerables.
In this article, we'll explore alternative methods for storing data using the IQueryable interface.

But first, let's understand what IQueryable is. To better understand this lets imagine we have a recipe book (which represents a data source) with many recipes (data items). Each recipe contains a list of ingredients and instructions. IQueryable is like having a set of tabs or bookmarks in our recipe book and instead of reading through the entire book we can open to a specific section (query) based on our needs. When compared, IEnumerable resembles reading a recipe book from beginning to end.

What is IQueryable

IQueryable is an interface in the .NET framework that represents a queryable collection of data. It inherits from the IEnumerable<T> interface and extends its functionality by providing support for composing and executing queries against data sources.
In simpler terms, IQueryable is like a smart container for data that allows you to ask questions about the data in a structured way. You can build complex questions by adding smaller questions together, and when you're ready, you can ask the container to find the answers for you from a specific data source.

When to use IQuearyable

Now that we know what IQueryable is, let's see in which cases it is beneficial to use it.
IQueryable is well-suited for situations where data retrieval from a source is required, such as when interacting with databases, particularly with ORM frameworks, or when querying data from remote sources like web services or APIs. Moreover, IQueryable is beneficial when dealing with large datasets, as it enables the utilization of query providers that optimize query execution and minimize memory usage by processing data in a streaming manner, rather than loading the entire dataset into memory.

Differences between IQueryable and IEnumerable

Okay, you might now ask, 'Well, why should I prefer IQueryable over IEnumerable?' Before we answer that, let's take a look at the fundamental differences between the two.

Deferred Execution

IQueryable: Supports deferred execution of queries, meaning that query operations are not executed immediately when defined but are instead executed when the results are actually needed. This allows for query optimization and composition.
IEnumerable: Does not support deferred execution. Query operations are executed immediately when invoked, resulting in all elements of the collection being enumerated at once.

Query Composition

IQueryable: Allows for dynamic composition of queries using LINQ operators. Query operators can be chained together to build complex queries, and the entire query is translated and executed at once.
IEnumerable: Supports only sequential enumeration of elements. Query operations cannot be dynamically composed, and each operation is applied separately to the entire collection.

Supported Query Operators

IQueryable: Supports a rich set of query operators provided by LINQ, including filtering (Where), sorting (OrderBy), projection (Select), grouping (GroupBy), and joining (Join).
IEnumerable: Supports a limited set of query operators, primarily those provided by the Enumerable class.

In general, we can say that IQueryable is suitable for scenarios where we need to query data from external data sources and perform dynamic, composable queries with deferred execution and query optimization. IEnumerable, on the other hand, is better suited for querying in-memory collections and performing simple, immediate operations on the data.

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