Understanding AsNoTrackingWithIdentityResolution in Entity Framework Core

When working with Entity Framework Core, understanding change tracking behavior is crucial for both performance and data consistency. While I was ware of the AsNoTracking() method, I discovered a lesser-known but powerful alternative: AsNoTrackingWithIdentityResolution() during a code review. 

Let's explore what makes this method special and when you should use it.

Quick recap: What is AsNoTracking()?

Before diving into AsNoTrackingWithIdentityResolution, let's briefly review AsNoTracking(). By default, EF Core tracks all entities returned from queries in the change tracker. This tracking enables:

• Automatic detection of changes to entities
• Update operations without explicitly attaching entities
• Identity resolution (ensuring only one instance per entity exists in memory)

However, tracking comes with overhead. When you're performing read-only operations where you don't need to update data, AsNoTracking() improves performance by skipping the change tracker entirely.

The problem AsNoTracking() creates

While AsNoTracking() improves performance, it introduces a subtle but important issue: duplicate entity instances. Without the change tracker performing identity resolution, you can end up with multiple instances of the same entity in memory.

Consider this scenario:

If multiple orders belong to the same customer, you'll get separate Customer instances for each order, even though they represent the same database record. This means:

• Increased memory usage
• Potential confusion when comparing entities
• Loss of referential integrity in your object graph

Enter AsNoTrackingWithIdentityResolution()

AsNoTrackingWithIdentityResolution() solves this problem by providing the best of both worlds:

• No change tracking (performance benefit)
• Identity resolution (ensuring single instances of entities)

When you use AsNoTrackingWithIdentityResolution(), EF Core:

1. Executes the query without attaching entities to the change tracker
2. Maintains a temporary identity map during query materialization
3. Ensures that entities with the same key reference the same instance
4. Discards the identity map after the query completes

This means you get identity resolution during the query execution without the ongoing overhead of change tracking.

If you find yourself using AsNoTrackingWithIdentityResolution() frequently, you can set it as the default tracking behavior:

When to use each method

Use Default Tracking when:

• You need to update, delete, or track changes to entities
• Working with small result sets where tracking overhead is negligible
• You need automatic change detection

Use AsNoTracking when:

• Performing read-only operations
• You don't have navigation properties or related entities
• Maximum performance is critical
• Result sets don't contain duplicate entities

Use AsNoTrackingWithIdentityResolution when:

• Performing read-only operations with includes/joins
• You need referential integrity in your object graph
• Working with queries that return the same entity multiple times
• You want to compare entities by reference

More information

EntityFrameworkQueryableExtensions.AsNoTrackingWithIdentityResolution<TEntity> Method (Microsoft.EntityFrameworkCore) | Microsoft Learn

DbContextOptionsBuilder.UseQueryTrackingBehavior Method (Microsoft.EntityFrameworkCore) | Microsoft Learn