What difference does AsNoTracking make

Question

I have a question regarding the  AsNoTracking   extension  as this is all quite new and quite confusing   I m using a per-request context for a website   A lot of my entities don t change so don t need to be tracked  but I have the following scenario where I m unsure of what s going to the database  or even whether it makes a difference in this case   This example is what I m currently doing   context Set lt User gt    AsNoTracking      Step 1  Get user context Set lt User gt       Step 2  Update user   This is the same as above but removing the  AsNoTracking   from Step 1   context Set lt User gt        Step 1  Get user context Set lt User gt       Step 2  Update user   The Steps 1  amp  2 use the same context but occur at different times  What I can t work out is whether there is any difference  As Step 2 is an update I m guessing both will hit the database twice anyway   Can anyone tell me what the difference is

User · Answer

If you have something else altering the DB (say another process) and need to ensure you see these changes, use AsNoTracking(), otherwise EF may give you the last copy that your context had instead, hence it being good to usually use a new context every query:

http://codethug.com/2016/02/19/Entity-Framework-Cache-Busting/

User · Answer

AsNoTracking   allows the  unique key per record  requirement in EF to be bypassed  not mentioned explicitly by other answers    This is extremely helpful when  reading a View that does not support a unique key because perhaps some fields are nullable or the nature of the view is not logically indexable   For these cases the  key  can be set to any non-nullable column but then AsNoTracking   must be used with every query else records  duplicate by key  will be skipped

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

Disabling tracking will also cause your result sets to be streamed into memory  This is more efficient when you re working with large sets of data and don t need the entire set of data all at once   References    How to avoid memory overflow when querying large datasets with Entity Framework and LINQ Entity framework large data set  out of memory exception

[c#] What difference does .AsNoTracking() make?

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