Migrating Ef6 Code First To Ef Core

EF6 Code-First to EF Core Migration

Overview

Migrates Entity Framework 6 Code-First projects to EF Core. This skill covers projects that already use DbContext with fluent or data-annotation-based models — no EDMX files involved.

> Scope: This skill targets EF6 Code-First projects (no .edmx files). For EDMX-based projects (Database-First/Model-First), use the migrating-edmx-to-code-first skill instead. For DbContext registration and DI setup during ASP.NET Core migration, also apply the migrating-ef-dbcontext skill — it is complementary to this one.

Workflow

Migration Progress:
- [ ] Step 1: Assess EF6 usage
- [ ] Step 2: Swap NuGet packages
- [ ] Step 3: Update namespaces
- [ ] Step 4: Migrate DbContext and configuration
- [ ] Step 5: Migrate entity configurations
- [ ] Step 6: Handle breaking API changes
- [ ] Step 7: Migrate migrations history
- [ ] Step 8: Validate

Step 1: Assess EF6 Usage

• Confirm the project uses EF6 Code-First (has a DbContext subclass, no .edmx files)
• List all DbContext subclasses and their DbSet<T> properties
• Find EntityTypeConfiguration<T> classes and OnModelCreating overrides
• Search for EF6-specific APIs: Database.SetInitializer, DbModelBuilder, HasDatabaseGeneratedOption, Map(), MapToStoredProcedures()
• Check for custom conventions (IConvention, Convention)
• Note any raw SQL usage: Database.SqlQuery<T>(), Database.ExecuteSqlCommand()
• Check for the EF6 Configuration class generated by Enable-Migrations (typically in Migrations/Configuration.cs) — this will be removed
• Confirm with the user before proceeding: if the project targets .NET Framework, advise upgrading to .NET 8 or later and updating EntityFramework to 6.5.1 first

Step 2: Swap NuGet Packages

Remove the EF6 package and add EF Core packages matching the target framework.

<!-- Remove -->
<PackageReference Include="EntityFramework" />

<!-- Add (choose the appropriate database provider) -->
<PackageReference Include="Microsoft.EntityFrameworkCore.SqlServer" />
<PackageReference Include="Microsoft.EntityFrameworkCore.Design" />
<PackageReference Include="Microsoft.EntityFrameworkCore.Tools" />

Use the EF Core major version that matches the project's target framework (e.g., EF Core 8.x for .NET 8, EF Core 10.x for .NET 10). Use the managing-package-references skill to add these packages — it handles version determination, CPM detection, and NuGet feed lookup.

Step 3: Update Namespaces

Replace all System.Data.Entity usings with Microsoft.EntityFrameworkCore. Note that System.Data.Entity.Validation and System.Data.Entity.Core.Objects are removed entirely in EF Core — replace with custom validation and DbContext APIs respectively.

Step 4: Migrate DbContext and Configuration

• Change constructor to accept DbContextOptions<T> instead of a connection string name
• Change OnModelCreating parameter from DbModelBuilder to ModelBuilder

Database initializers: EF Core does not support Database.SetInitializer or the MigrateDatabaseToLatestVersion initializer. Remove all initializer calls and convert seed logic to HasData() in OnModelCreating or a separate seed method called at startup. Also delete the EF6 Configuration class (typically Migrations/Configuration.cs) generated by Enable-Migrations — it has no equivalent in EF Core.

Step 5: Migrate Entity Configurations

• Convert EntityTypeConfiguration<T> to IEntityTypeConfiguration<T> (add Configure(EntityTypeBuilder<T>) method, prefix all calls with builder.)
• Replace HasRequired/HasOptional with HasOne + .IsRequired(), and WithRequired/WithOptional with WithOne + .IsRequired()
• Replace WillCascadeOnDelete(false) with OnDelete(DeleteBehavior.Restrict)
• Replace HasDatabaseGeneratedOption:
• DatabaseGeneratedOption.Identity → ValueGeneratedOnAdd() (database-generated IDs)
• DatabaseGeneratedOption.Computed → ValueGeneratedOnAddOrUpdate() (computed columns)
• DatabaseGeneratedOption.None → Check if you manually assign IDs before Add(). If yes, use ValueGeneratedNever(). If no (or using custom HiLo generator), use ValueGeneratedOnAdd() or EF Core's built-in UseHiLo() to avoid tracking conflicts from duplicate default IDs
• Replace Map(m => m.ToTable("Name")) with ToTable("Name") directly
• HasMany().WithMany() requires EF Core 5.0+. For join tables with payload columns, explicit join entity configuration is still required
• MapToStoredProcedures() is removed — use FromSql() or ExecuteSql()
• modelBuilder.Conventions.Remove<T>() is removed — override ConfigureConventions(ModelConfigurationBuilder) on the DbContext to add, remove, or replace conventions

Decimal property precision: EF6 automatically mapped decimal properties to decimal(18,2). EF Core requires explicit precision configuration to avoid silent data truncation:

// EF6 - automatic precision
public decimal Price { get; set; }  // Became decimal(18,2) in database

// EF Core - must specify precision
protected override void OnModelCreating(ModelBuilder modelBuilder)
{
    modelBuilder.Entity<Product>()
        .Property(p => p.Price)
        .HasPrecision(18, 2);  // Or use [Column(TypeName = "decimal(18,2)")]
}

Register configurations: Replace modelBuilder.Configurations.Add(...) with modelBuilder.ApplyConfiguration(...) or use modelBuilder.ApplyConfigurationsFromAssembly(typeof(AppDbContext).Assembly) to apply all at once.

Step 6: Handle Breaking API Changes

Raw SQL: Replace Database.SqlQuery<T>() with Set<T>().FromSql() and Database.ExecuteSqlCommand() with Database.ExecuteSql(). Use interpolated strings ($"...") for automatic parameterization — do not use string concatenation, which bypasses EF Core's parameterization and creates SQL injection vulnerabilities.

Lazy loading: EF Core disables lazy loading by default.

• Option 1: Install Microsoft.EntityFrameworkCore.Proxies, call UseLazyLoadingProxies()
• Option 2 (preferred): Use explicit .Include() for eager loading

Complex types: In EF Core 7 and earlier, [ComplexType] → OwnsOne(). EF Core 8+ reintroduces native support — use ComplexProperty() instead.

Validation: EF Core does not call IValidatableObject.Validate() on SaveChanges(). Add validation in the application layer or override SaveChanges() to call it explicitly.

Step 7: Migrate Migrations History

Advise the user to apply all pending EF6 migrations to their database before starting the EF Core migration — this is a manual prerequisite. Once confirmed:

• Delete the existing Migrations/ folder (contains EF6 DbMigration classes and the Configuration class generated by Enable-Migrations)
• Create a baseline EF Core migration. Present the command and offer to run it with user confirmation:

dotnet ef migrations add InitialCreate

• Before marking the migration as applied, the user must verify that the existing database schema matches the EF Core model. The user should review the generated migration file (InitialCreate) and confirm it reflects their current schema — not new changes. If the migration contains unexpected schema alterations, applying it could cause data loss. Only proceed once the user confirms the migration is a clean baseline.

• Once verified, mark the migration as applied without running it. Always pause and confirm with the user before executing database-modifying commands, even in auto-execute mode — this connects to a live database:

dotnet ef database update

Alternatively, to avoid connecting to the database directly, generate an idempotent SQL script:

dotnet ef migrations script --idempotent --output mark-migration.sql

Advise the user to review the generated script and apply only the INSERT INTO __EFMigrationsHistory statement to their database. This registers the migration as applied without modifying the schema.

Step 8: Validate

Compilation alone is not sufficient. Many EF6→EF Core differences only surface at runtime.

LLM responsibilities (automated):

• Build the project and fix compilation errors
• Confirm no EF6 artifacts remain (System.Data.Entity namespaces, EntityFramework package reference, Migrations/Configuration.cs)
• Run existing unit tests

User responsibilities (require database access and manual verification):

• Test CRUD operations against the database
• Verify navigation property loading works as expected (eager vs lazy)
• Validate change tracking behavior matches expectations (Added, Modified, Deleted states)
• If stored procedures were migrated, test FromSql/ExecuteSql results
• Compare query results with the EF6 version for any complex LINQ queries

Present items 4–8 as a checklist for the user — the LLM cannot validate runtime database behavior.

Success Criteria

• EntityFramework NuGet package removed, EF Core packages added
• All System.Data.Entity namespaces replaced with Microsoft.EntityFrameworkCore
• DbContext constructor accepts DbContextOptions
• All EntityTypeConfiguration<T> classes converted to IEntityTypeConfiguration<T>
• Fluent API calls updated (HasRequired → HasOne, etc.)
• Raw SQL calls migrated to FromSql/ExecuteSql
• Lazy loading strategy decided and configured
• Complex types converted to owned entities or complex properties
• Database initializers and EF6 Configuration class removed, seed data migrated
• EF Core migrations baseline created
• Project builds, tests pass, queries return correct results
• Adding a new EF Core migration produces one without any operations (model matches database)
• No secrets are stored in plain text