A Brief Discussion on Default Value Behavior in Entity Framework

TLDR

• When a SQL Server column has a default value, if the Entity property value matches the C# default value for that type, EF Core may ignore the column during INSERT operations.
• It is recommended to avoid relying on SQL defaults whenever possible. If they must be used, ensure the SQL default value matches the C# default value.
• For string types, it is recommended to set them as NOT NULL Default '' to handle nulls and empty strings consistently.
• When updating an Entity property but the value remains unchanged, SaveChanges() will return 0. The logic should be adjusted to context.Entry(entity).State == EntityState.Unchanged || context.SaveChanges() > 0.
• Avoid using AsNoTracking() in scenarios where updates are required. If used, manually setting the state to EntityState.Modified will cause all fields of the Entity to be updated.

Entity Framework Default Value Write Behavior

When does this issue occur: When a database column has a DEFAULT constraint, and the developer has not explicitly assigned a value in the code (or has assigned a value identical to the C# default).

Testing reveals that Entity Framework does not determine changes based on whether a property was "assigned a value," but rather by comparing "new and old values" to decide whether to include the column in the INSERT statement. When the property value matches the type's default (such as null or 0), EF Core may ignore the column entirely, leaving the database to handle the default value.

WARNING

If using EF Core Power Tools for reverse engineering, specific types like bit may result in generated Entity property types (e.g., bool?) that differ from the Required setting. This can cause the INSERT statement to ignore the column entirely, thereby triggering the database's default value mechanism.

Logic for Updating Entities with Unchanged Values

When does this issue occur: During an update operation, if the Entity property value is the same as the existing value in the database, but the developer still executes SaveChanges().

When performing an update, if the property value has not changed, EF Core keeps the EntityState as Unchanged. In this case, SaveChanges() will not execute any UPDATE command and will return 0.

Recommended Approach: In the update logic of a Business Service, consider the Unchanged state to avoid misinterpreting it as an update failure:

int changedCount = context.SaveChanges();
bool isSuccess = context.Entry(entity).State == EntityState.Unchanged || changedCount > 0;

Side Effects of AsNoTracking and EntityState.Modified

When does this issue occur: In scenarios where data needs to be updated, AsNoTracking() is mistakenly used for queries, and subsequently, the state is manually set to EntityState.Modified to force an update.

An Entity queried using AsNoTracking() is not managed by the Change Tracker. If you manually set its state to EntityState.Modified, EF Core will be unable to track which fields have actually changed, causing the generated UPDATE statement to include "all fields" in the table rather than just the changed ones.

Recommended Approach:

• Do not use AsNoTracking() if you need to perform update operations.
• Use AsNoTracking() only in pure read-only query scenarios to improve performance.

Change Log

• 2025-07-12 Initial document creation.