What difference does .AsNoTracking() make?
The difference is that in the first case the retrieved user is not tracked by the context so when you are going to save the user back to database you must attach it and set correctly state of the user so that EF knows that it should update existing user instead of inserting a new one. In the second case you don't need to do that if you load and save the user with the same context instance because the tracking mechanism handles that for you.
see this page Entity Framework and AsNoTracking
What AsNoTracking Does
Entity Framework exposes a number of performance tuning options to help you optimise the performance of your applications. One of these tuning options is .AsNoTracking(). This optimisation allows you to tell Entity Framework not to track the results of a query. This means that Entity Framework performs no additional processing or storage of the entities which are returned by the query. However, it also means that you can't update these entities without reattaching them to the tracking graph.
there are significant performance gains to be had by using AsNoTracking
No Tracking LINQ to Entities queries
Usage of AsNoTracking() is recommended when your query is meant for read operations. In these scenarios, you get back your entities but they are not tracked by your context.This ensures minimal memory usage and optimal performance
Pros
- Improved performance over regular LINQ queries.
- Fully materialized objects.
- Simplest to write with syntax built into the programming language.
Cons
- Not suitable for CUD operations.
- Certain technical restrictions, such as: Patterns using DefaultIfEmpty for OUTER JOIN queries result in more complex queries than simple OUTER JOIN statements in Entity SQL.
- You still can’t use LIKE with general pattern matching.
More info available here:
Performance considerations for Entity Framework
Entity Framework and NoTracking