EF Core AsNoTracking(): When Does it Matter for Updates?

.AsNoTracking() in EF Core: Detailed explanation and impact of updates

The .AsNoTracking() method in EF Core is often confusing, especially for beginners. To illustrate its role, let's analyze a specific scenario and its impact on database interactions.

Assume that the website's entities are stored in a per-request context. Many entities remain unchanged and do not need to be tracked. However, the following scenario presents a difficult question:

<code class="language-csharp">context.Set<User>().AsNoTracking()
// 步骤 1) 获取用户
context.Set<User>()
// 步骤 2) 更新用户</code>

Alternatively, we can remove .AsNoTracking() from step 1:

<code class="language-csharp">context.Set<User>();
// 步骤 1) 获取用户
context.Set<User>()
// 步骤 2) 更新用户</code>

In both cases, context is used for retrieval (step 1) and update (step 2), but they happen at different times. The question is: does it make a difference?

The answer lies in the nature of tracking. In the first scenario, step 1 uses .AsNoTracking(), the retrieved user will not be tracked by the context. This means that when updating a user (step 2), you must manually attach the user and explicitly set its status to "Modified". This tells EF Core to update the existing user instead of creating a new record.

In contrast, in the second case, without .AsNoTracking(), you don't need to perform these manual steps if you load and save the user in the same context instance. The tracking mechanism handles this automatically, serving updated users without additional configuration.

To summarize, the difference between these two methods is that .AsNoTracking() disables tracking of retrieved entities, requiring manual management of object state and attachment during updates. Using the same context for retrieval and update without .AsNoTracking() simplifies this process, as the tracking mechanism handles these aspects transparently.

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