How do you implement custom extensions or operators for IEnumerable and IQueryable?

IEnumerable vs IQueryable in C#

If you have used Language Integrated Query (LINQ) in C#, you have probably encountered the interfaces IEnumerable and IQueryable. They both allow you to perform queries on collections of data, but they have some important differences. In this article, you will learn what they are, how they work, and how to implement custom extensions or operators for them.

1 What is IEnumerable?

IEnumerable is the base interface for all collections that can be iterated over using the foreach loop. It defines a single method, GetEnumerator, that returns an IEnumerator object. An IEnumerator can move through the collection one element at a time, and expose the current element through the Current property. IEnumerable does not support any query operations by itself, but it can be used with LINQ extension methods that are defined in the System.Linq namespace. These methods return a new IEnumerable object that represents the result of the query.

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2 What is IQueryable?

IQueryable is an interface that inherits from IEnumerable, and represents a queryable data source. It defines a property, Expression, that holds an expression tree that describes the query. It also defines a property, Provider, that is responsible for executing the query against the data source. IQueryable can be used with LINQ extension methods as well, but they return a new IQueryable object that preserves the expression tree. This allows the query to be deferred and optimized by the provider.

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Paulo Zëmek

Software Engineer - I used talk to computers and they understood me even without AI, we just spoke the same language... now that they got smarter we are so in-sync that sometimes it feels we talk to each-other mentally!
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That's too much implementation detail without explaining the most obvious thing: An IEnumerable will just go from one record to the next. If there's any filtering, you first read all the data, then you filter it; An IQueryable can potentially pre-filter data using indices and avoid reading unnecessary data. To try to make this make sense, imagine that you just want records from WA state. In an IEnumerable, you would enumerate all records, check if they have state as WA, and that would be it. In an indexed (and queryable) object, you tell in advance that you just want records from WA... and it can use any indexes it has to just go to WA, avoiding all other States.

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3 When to use IEnumerable or IQueryable?

The choice between IEnumerable and IQueryable depends on the type and location of your data source, and the complexity and performance of your queries. In general, you should use IEnumerable when your data source is in-memory or local, and your queries are simple or need to be executed immediately. You should use IQueryable when your data source is remote or large, and your queries are complex or need to be executed lazily. For example, if you are querying a database using Entity Framework, you should use IQueryable to avoid loading all the data into memory and to take advantage of the SQL translation and optimization.

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4 How to convert between IEnumerable and IQueryable?

Sometimes, you may need to convert between IEnumerable and IQueryable, depending on your scenario. You can use the AsEnumerable and AsQueryable extension methods to do so. AsEnumerable converts an IQueryable to an IEnumerable, and forces the query execution at that point. AsQueryable converts an IEnumerable to an IQueryable, and wraps the collection in a queryable object. However, this does not mean that the query will be executed by the provider, as the expression tree will still contain the original IEnumerable source. Therefore, you should be careful when using AsQueryable, as it may not improve the performance or functionality of your query.

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Paulo Zëmek

Software Engineer - I used talk to computers and they understood me even without AI, we just spoke the same language... now that they got smarter we are so in-sync that sometimes it feels we talk to each-other mentally!
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Definitely needs more context on these 2. In my opinion these methods shouldn't exist as they cause more harm than good (and this is somewhat said, so maybe an explanation why avoiding those AsQueryable and AsEnumerable would be better than just explaining an obvious conversion).

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5 How to implement custom extensions or operators for IEnumerable and IQueryable?

Creating your own extension methods or operators for IEnumerable and IQueryable is one of the benefits of LINQ. To do so, you need to define a static class that contains the extension method or operator as a public static method, and use the this keyword to mark the first parameter as the source of the query. For IQueryable, use the Expression class to build an expression tree representing your logic, and for IEnumerable use the yield return statement. Additionally, use the standard query operator attributes such as ExtensionAttribute to mark your method or operator as a LINQ operator. An example of a custom extension method for IEnumerable is one that filters out null values, and an example of a custom operator for IQueryable is one that performs a full-text search.

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Paulo Zëmek

Software Engineer - I used talk to computers and they understood me even without AI, we just spoke the same language... now that they got smarter we are so in-sync that sometimes it feels we talk to each-other mentally!
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Extension methods are a C# feature, not a LINQ feature (although they came together) and so, saying it is one of the benefits of LINQ is false. All classes in C# got that benefit (extension methods) since it was introduced in the language. LINQ was just the first official user of such a feature.

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How do you implement custom extensions or operators for IEnumerable and IQueryable?

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1 What is IEnumerable?

2 What is IQueryable?

3 When to use IEnumerable or IQueryable?

4 How to convert between IEnumerable and IQueryable?

5 How to implement custom extensions or operators for IEnumerable and IQueryable?

Language Integrated Query (LINQ)

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