Many developers get confused between IEnumerable and IQueryable. When it comes to writing code, both look very similar. However, there are many differences between them which need to be taken care of while writing code. Both have some intended usability scenarios for which they are made.
Listed below are the differences between them based on their properties:
| | IEnumerable | IQueryable |
| Namespace | System.Collections Namespace | System.Linq Namespace |
| Derives from | No base interface | Derives from IEnumerable |
| Deferred Execution | Supported | Supported |
| Lazy Loading | Not Supported | Supported |
| How does it work | While querying data from database, IEnumerable executes select query on server side, load data in-memory on client side and then filter data. Hence does more work and becomes slow. | While querying data from database, IQueryable executes select query on server side with all filters. Hence does less work and becomes fast. |
| Suitable for | LINQ to Object and LINQ to XML queries | LINQ to SQL queries |
| Custom Query | Doesn’t support | Supports using CreateQuery and Execute methods |
Extension method
parameter | Extension methods supported in IEnumerable takes functional objects. | Extension methods supported in IEnumerable takes expression objects, i.e., expression tree. |
| When to use | When querying data from in-memory collections like List, Array, etc. | When querying data from out-memory (like remote database, service) collections. |
| Best Uses | In-memory traversal | Paging |
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