EF Core Data Seeding: Production-Ready Patterns for .NET

Practical code samples for Advanced Data Seeding Strategies in Entity Framework Core — covering both model-based HasData seeding and custom runtime seeders with full idempotency guards.

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🚀 What You'll Learn

This repository demonstrates two production-ready EF Core data seeding strategies:

Strategy	Best For	Pros	Cons
Model-based HasData	Static reference data (lookup tables, roles, categories)	Migrations-aware, zero startup overhead, deterministic	Hard-coded IDs required; large datasets inflate migration files
Custom Runtime Seeders	Dynamic/computed data, large datasets, external source data	Full C# expressiveness, no migration bloat, DB-generated IDs	Runs at startup, must guard against duplicates

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📁 Project Structure

efcore-data-seeding-seeddata/
├── src/
│   └── EFCore.DataSeeding.Api/
│       ├── Controllers/
│       │   └── ProductsController.cs        # REST endpoints to verify seeded data
│       ├── Data/
│       │   ├── AppDbContext.cs               # DbContext with HasData configuration
│       │   └── Seeding/
│       │       ├── IDataSeeder.cs            # Seeder marker interface
│       │       ├── DatabaseSeeder.cs         # Orchestrates all seeders in order
│       │       └── ProductSeeder.cs          # Custom runtime seeder (Strategy 2)
│       ├── Models/
│       │   ├── Category.cs
│       │   └── Product.cs
│       ├── Repositories/
│       │   ├── IProductRepository.cs
│       │   └── ProductRepository.cs         # Repository pattern with AsNoTracking
│       └── Program.cs                       # DI wiring + startup seeding
└── tests/
    └── EFCore.DataSeeding.Tests/
        ├── HasDataSeedingTests.cs            # Tests for Strategy 1
        └── ProductSeederTests.cs            # Tests for Strategy 2 (idempotency verified)

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⚡ Quick Start

Prerequisites

• .NET 10 SDK (or .NET 8/9 — update TargetFramework in .csproj)

Run Locally (In-Memory Database)

git clone https://github.com/codingdroplets/efcore-data-seeding-seeddata.git
cd efcore-data-seeding-seeddata

dotnet run --project src/EFCore.DataSeeding.Api

Open Swagger UI: https://localhost:{port}/swagger

Run Tests

dotnet test -c Release

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🗄️ Strategy 1: Model-Based HasData Seeding

Configured inside OnModelCreating in AppDbContext.cs. EF Core applies this data through migrations (or EnsureCreated for InMemory).

modelBuilder.Entity<Category>().HasData(
    new Category { Id = 1, Slug = "electronics", Name = "Electronics" },
    new Category { Id = 2, Slug = "books",        Name = "Books" },
    new Category { Id = 3, Slug = "clothing",     Name = "Clothing" }
);

⚠️ Important: HasData requires hard-coded primary keys. Shadow properties and navigation properties are not supported — use flat scalar values only.

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⚙️ Strategy 2: Custom Runtime Seeder

ProductSeeder implements IDataSeeder and is called at startup by DatabaseSeeder. It includes an idempotency guard so it is safe to run on every startup.

public async Task SeedAsync(CancellationToken cancellationToken = default)
{
    // Guard: skip if already seeded
    bool alreadySeeded = await _db.Products
        .AnyAsync(p => p.Stock > 500, cancellationToken);

    if (alreadySeeded) return;

    // Fetch FK by slug — no hard-coded IDs
    var electronics = await _db.Categories
        .FirstAsync(c => c.Slug == "electronics", cancellationToken);

    await _db.Products.AddRangeAsync(new List<Product>
    {
        new() { Name = "USB-C Hub (7-in-1)", Price = 49.99m, Stock = 750, CategoryId = electronics.Id },
        // ...
    }, cancellationToken);

    await _db.SaveChangesAsync(cancellationToken);
}

Registering Seeders

In Program.cs:

builder.Services.AddScoped<IDataSeeder, ProductSeeder>();
builder.Services.AddScoped<DatabaseSeeder>();

Add more seeders by implementing IDataSeeder and registering them. Control execution order via the Order property.

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🔄 Switching to SQL Server

The sample uses InMemory for zero-friction local runs. To use SQL Server:

1. Update Program.cs:

   builder.Services.AddDbContext<AppDbContext>(options =>
       options.UseSqlServer(builder.Configuration.GetConnectionString("DefaultConnection")));

2. Replace EnsureCreatedAsync() with:

   await db.Database.MigrateAsync();

3. Add your connection string to appsettings.json.

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🧪 Test Coverage

Test	Strategy
HasData_Seeds_ThreeCategories	Strategy 1
HasData_Seeds_ElectronicsCategory_WithCorrectSlug	Strategy 1
HasData_Seeds_FourBaseProducts	Strategy 1
HasData_Products_HaveCorrectCategoryAssignments	Strategy 1
ProductSeeder_InsertsHighStockProducts	Strategy 2
ProductSeeder_IsIdempotent_DoesNotDuplicate	Strategy 2
ProductSeeder_AssignsCorrectCategories	Strategy 2

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📚 Further Reading

• EF Core — Data Seeding (Microsoft Docs)
• EF Core — Migrations Overview

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📄 License

MIT © Coding Droplets