Add runtime migration creation and application

[thromel]

Summary

Implements #37342: Allow creating and applying migrations at runtime without recompiling.

This adds support for creating and applying migrations at runtime using Roslyn compilation, enabling scenarios like .NET Aspire and containerized applications where recompilation isn't possible.

CLI Usage

# Standard update (existing behavior)
dotnet ef database update [migration]

# Create and apply a new migration in one step
dotnet ef database update MigrationName --add [--output-dir <DIR>] [--namespace <NS>] [--json]

The -o/--output-dir, -n/--namespace, and --json options require --add to be specified.

PowerShell Usage

# Standard update (existing behavior)
Update-Database [-Migration <migration>]

# Create and apply a new migration in one step
Update-Database -Migration MigrationName -Add [-OutputDir <DIR>] [-Namespace <NS>]

Architecture

Component	Purpose
IMigrationCompiler / CSharpMigrationCompiler	Internal: Roslyn-based compilation of scaffolded migrations
IMigrationsAssembly.AddMigrations(Assembly)	Registers dynamically compiled migrations
MigrationsOperations.AddAndApplyMigration()	Orchestrates scaffold → compile → register → apply workflow

Design Decisions

• Extends existing services: Uses IMigrationsScaffolder for scaffolding and IMigrator for applying, adding only the new IMigrationCompiler service
• AddMigrations(Assembly): Extended IMigrationsAssembly interface to accept additional assemblies containing runtime-compiled migrations
• Always persists to disk: Like AddMigration, files are always saved to enable source control and future recompilation
• No pending changes behavior: If no model changes are detected, applies any existing pending migrations without creating a new one
• Internal compiler API: IMigrationCompiler and CSharpMigrationCompiler are in the .Internal namespace as they require design work for public API
• Error handling with cleanup: If compilation or migration application fails, saved migration files are cleaned up to prevent orphans
• Thread safety: MigrationsAssembly uses locking to protect against race conditions when adding migrations concurrently

Workflow

User runs: dotnet ef database update InitialCreate --add
    │
    ▼
MigrationsOperations.AddAndApplyMigration()
    │
    ├─► Check for pending model changes
    │       └─► If none: apply existing migrations, return
    │
    ├─► IMigrationsScaffolder.ScaffoldMigration() - Generate code
    │
    ├─► try {
    │       ├─► IMigrationsScaffolder.Save() - Write files to disk
    │       ├─► IMigrationCompiler.CompileMigration() - Roslyn compile
    │       ├─► IMigrationsAssembly.AddMigrations() - Register migration
    │       └─► IMigrator.Migrate() - Apply to database
    │   } catch {
    │       └─► Clean up saved files on failure
    │   }
    │
    └─► Return migration files

Robustness Features

1. Exception handling with cleanup: AddAndApplyMigration wraps the save-compile-register-apply chain in try-catch, deleting saved files on failure to prevent orphans
2. Context disposal on validation failure: PrepareForMigration ensures the DbContext is disposed if validation or service building fails
3. Thread-safe migration registration: MigrationsAssembly uses locking to protect shared state (migrations dictionary, model snapshot, additional assemblies list)

Limitations

• Requires dynamic code generation (incompatible with NativeAOT) - marked with [RequiresDynamicCode]
• C# only (no VB.NET/F# support)

Test plan

• Unit tests for CSharpMigrationCompiler
• Unit tests for MigrationsOperations.AddAndApplyMigration
• Integration tests in RuntimeMigrationTestBase (SQLite and SQL Server implementations)
• Tests for validation (empty name, invalid characters)
• Tests for RemoveMigration with dynamically created migrations
• All existing EFCore.Design.Tests pass
• All existing EFCore.Relational.Tests pass

Fixes #37342

[AndriySvyryd]

Thanks for your contribution!