What is the difference between IEnumerable and IQueryable in C#?

In C#, IEnumerable and IQueryable are both interfaces that represent collections of data, but they serve different purposes and have different capabilities:

1. IEnumerable:

   • IEnumerable is defined in the System.Collections namespace.
   • It represents a forward-only cursor of data, which means it can only move forward and read data one item at a time.
   • It is suitable for in-memory collections and supports LINQ (Language Integrated Query) to Objects.
   • LINQ queries executed on IEnumerable collections are done in-memory, meaning the entire collection is loaded into memory before querying.
   • Examples : 👉Examples for IEnumerable and IQueryable interface in C#

2. IQueryable:

   • IQueryable is defined in the System.Linq namespace.
   • It inherits from IEnumerable but extends it with additional functionality for querying data from data sources such as databases, web services, or any data provider that implements IQueryable.
   • It represents a query that can be executed against a data source, allowing for deferred execution.
   • LINQ queries executed on IQueryable collections are translated into a query expression (e.g., SQL for a database) and executed on the data source, minimizing the amount of data retrieved from the source.
   • Examples : 👉Examples for IEnumerable and IQueryable interface in C#

Key differences:

• Deferred Execution: IQueryable supports deferred execution, meaning the query is not executed until the result is enumerated (e.g., using foreach, ToList(), ToArray(), etc.). This allows for building complex queries dynamically and executing them efficiently.

• Expression Trees: IQueryable allows for expression trees to be built, which represent the query in a form that can be translated into the native query language of the underlying data source (e.g., SQL).

• In-Memory vs. Out-of-Memory Querying: IEnumerable queries are executed in-memory, while IQueryable queries can be executed both in-memory and out-of-memory (e.g., against a database).

  Feature	IEnumerable	IQueryable
  Namespace	System.Collections	System.Linq
  Cursor Movement	Forward-only	Forward-only
  Query Execution	In-memory	In-memory or out-of-memory (e.g., database)
  Query Capabilities	LINQ to Objects	LINQ to Objects and Providers
  Deferred Execution	No	Yes
  Expression Trees	No	Yes
  Suitable for	In-memory collections	External data sources

In summary, IEnumerable is suitable for working with in-memory collections and supports LINQ to Objects, while IQueryable is more powerful for querying external data sources and supports deferred execution and expression trees.