IEnumerable and IQueryable
Tags: dotnet, collection, csharp
Published: 2023-03-22 07:30:23

#IEnumerable and IQueryable

In C#, both `IEnumerable` and `IQueryable` are interfaces that allow you to work with collections of objects, but they differ in terms of how they retrieve data.

`IEnumerable` is defined in the `System.Collections` namespace and is the base interface for all non-generic collections that can be enumerated. When you work with an `IEnumerable`, you are working with an in-memory collection of objects that are already loaded into memory. You can use the `foreach` loop to iterate over the collection and access each element in turn.

Here is an example of using `IEnumerable` in C#:

```csharp
IEnumerable<int> numbers = new List<int> { 1, 2, 3, 4, 5 };

foreach (int num in numbers)
{
    Console.WriteLine(num);
}

```

In the above example, we create an `IEnumerable` collection of integers and use the `foreach` loop to iterate over each element and print it to the console.

`IQueryable`, on the other hand, is defined in the `System.Linq` namespace and is used to query data from a data source, such as a database, rather than working with an in-memory collection. When you work with an `IQueryable`, you are creating a query that is executed when you actually enumerate the results.

Here is an example of using `IQueryable` in C#:
```csharp
IQueryable<Customer> customers = db.Customers.Where(c => c.City == "London");

foreach (Customer customer in customers)
{
    Console.WriteLine(customer.Name);
}
```
In the above example, we create an `IQueryable` collection of Customer objects and use the Where method to filter the results to only those customers who live in London. The query is not executed until we enumerate the results using the `foreach` loop.

Note that `IQueryable` also supports deferred execution, which means that the query is not actually executed until you try to access the results. This can be useful when working with large datasets that may take a long time to retrieve from a data source.


`IEnumerable` and `IQueryable` are both interfaces in C# that allow you to work with collections of objects, but they differ in terms of how they retrieve data. Let's take a closer look at the internals of these interfaces.

`IEnumerable` is defined in the `System.Collections` namespace and is the base interface for all non-generic collections that can be enumerated. It has a single method, `GetEnumerator()`, which returns an `IEnumerator` object that can be used to iterate over the collection. When you use `foreach` to iterate over an `IEnumerable`, the compiler generates code that calls `GetEnumerator()` and uses the `MoveNext()` and Current properties of the `IEnumerator` to iterate over the collection.

Here is an example of how `IEnumerable` works internally:

```csharp
IEnumerable<int> numbers = new List<int> { 1, 2, 3, 4, 5 };

IEnumerator<int> enumerator = numbers.GetEnumerator();

while (enumerator.MoveNext())
{
    int num = enumerator.Current;
    Console.WriteLine(num);
}
```

In the above example, we create an `IEnumerable` collection of integers and use the `GetEnumerator()` method to get an `IEnumerator` object. We then use the `MoveNext()` and Current properties of the `IEnumerator` to iterate over the collection and print each element to the console.

`IQueryable`, on the other hand, is defined in the `System.Linq` namespace and is used to query data from a data source, such as a database. It extends `IEnumerable` and adds methods for filtering, sorting, and projecting data before it is retrieved from the data source. When you use `IQueryable`, you are creating a query that is executed when you actually enumerate the results.

Here is an example of how `IQueryable` works internally:

```csharp
IQueryable<Customer> customers = db.Customers.Where(c => c.City == "London");

IEnumerator<Customer> enumerator = customers.GetEnumerator();

while (enumerator.MoveNext())
{
    Customer customer = enumerator.Current;
    Console.WriteLine(customer.Name);
}
```

In the above example, we create an `IQueryable` collection of Customer objects and use the Where method to filter the results to only those customers who live in London. The query is not executed until we enumerate the results using the `GetEnumerator()` method. When we call `MoveNext()`, the query is executed and the results are returned one at a time.


In summary, `IEnumerable` is used to work with in-memory collections, whereas `IQueryable` is used to query data from a data source. `IEnumerable` is simpler and provides basic functionality for working with collections, whereas `IQueryable` provides additional functionality for querying data from a data source in a more efficient manner by performing filtering and sorting operations in the data source itself before retrieving the data.


Disclaimer : This article has been written using AI and MSDN resources