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Document EntityTypeConfigurationAttribute #4300

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Mar 28, 2023
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Document EntityTypeConfigurationAttribute #4300

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Document EntityTypeConfigurationAttribute
ajcvickers Mar 28, 2023
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Update entity-framework/core/modeling/index.md
ajcvickers Mar 28, 2023
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Expand Up @@ -33,13 +33,53 @@ Then just invoke the `Configure` method from `OnModelCreating`.

[!code-csharp[Main](../../../samples/core/Modeling/Misc/EntityTypeConfiguration.cs?Name=ApplyIEntityTypeConfiguration)]

#### Applying all configurations in an assembly

It is possible to apply all configuration specified in types implementing `IEntityTypeConfiguration` in a given assembly.

[!code-csharp[Main](../../../samples/core/Modeling/Misc/EntityTypeConfiguration.cs?Name=ApplyConfigurationsFromAssembly)]

> [!NOTE]
> The order in which the configurations will be applied is undefined, therefore this method should only be used when the order doesn't matter.

#### Using `EntityTypeConfigurationAttribute` on entity types

Rather than explicitly calling `Configure`, an <xref:Microsoft.EntityFrameworkCore.EntityTypeConfigurationAttribute > can instead be placed on the entity type such that EF Core can find and use appropriate configuration. For example:
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<!--
[EntityTypeConfiguration(typeof(BookConfiguration))]
public class Book
{
public int Id { get; set; }
public string Title { get; set; }
public string Isbn { get; set; }
}
-->
[!code-csharp[BookEntityType](../../../samples/core/Miscellaneous/NewInEFCore6/EntityTypeConfigurationAttributeSample.cs?name=BookEntityType)]

This attribute means that EF Core will use the specified `IEntityTypeConfiguration` implementation whenever the `Book` entity type is included in a model. The entity type is included in a model using one of the normal mechanisms. For example, by creating a <xref:Microsoft.EntityFrameworkCore.DbSet%601> property for the entity type:

<!--
public class BooksContext : DbContext
{
public DbSet<Book> Books { get; set; }

//...
-->
[!code-csharp[DbContext](../../../samples/core/Miscellaneous/NewInEFCore6/EntityTypeConfigurationAttributeSample.cs?name=DbContext)]

Or by registering it in <xref:Microsoft.EntityFrameworkCore.DbContext.OnModelCreating%2A>:

```csharp
protected override void OnModelCreating(ModelBuilder modelBuilder)
{
modelBuilder.Entity<Book>();
}
```

> [!NOTE]
> `EntityTypeConfigurationAttribute` types will not be automatically discovered in an assembly. Entity types must be added to the model before the attribute will be discovered on that entity type.

## Use data annotations to configure a model

You can also apply certain attributes (known as _Data Annotations_) to your classes and properties. Data annotations will override conventions, but will be overridden by Fluent API configuration.
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