Setting up EF Core 9.0 Cosmos Provider and .NET Aspire 9.4

Modern cloud apps need databases that work across the globe with lightning-fast response times and rock-solid reliability. Azure Cosmos DB ticks all these boxes. If you're building with .NET Aspire (Microsoft's framework for cloud-native applications) you can integrate Cosmos DB with Entity Framework Core (EF Core) without breaking a sweat.

Lets examine how to utilise the Cosmos DB provider for EF Core in a .NET Aspire project, explore some of Cosmos's unique features, and see how Aspire simplifies the setup process.

Why Use Aspire, EF Core, and Cosmos DB Together?

Aspire handles the heavy lifting – It manages configuration, environment variables, secrets, and service dependencies (like Cosmos DB) so you don't have to.

Stick with what you know – You can use familiar LINQ queries and DbContext patterns instead of learning a completely new SDK from scratch.

Built for the cloud – Aspire was designed for distributed workloads, and Cosmos DB is built for exactly that kind of environment.

Setting Up Cosmos DB in a .NET Aspire Project

In your Aspire AppHost Project install the Aspire.Hosting.Azure.CosmosDB nuget package

dotnet add package Aspire.Hosting.Azure.CosmosDB

Install the following packages into your API project

dotnet add package Aspire.Microsoft.Azure.Cosmos
dotnet add package Aspire.Microsoft.EntityFrameworkCore.Cosmos

Configure the aspire resources so your API project can talk to CosmosDB (This is set up for development using the emulator rather than connecting to a real database)

#pragma warning disable ASPIRECOSMOSDB001
var builder = DistributedApplication.CreateBuilder(args);

var cosmos = builder.AddAzureCosmosDB("cosmos")
    .RunAsPreviewEmulator(options =>
    {
        options.WithLifetime(ContainerLifetime.Persistent);
        options.WithDataVolume();
        options.WithDataExplorer();
    }).AddCosmosDatabase("MyCosmosDatabase");

#pragma warning restore ASPIRECOSMOSDB001

builder.AddProject<Projects.MyApi>("myapi")
    .WithReference(cosmos);

builder.Build().Run();

Finally in your project configure your DB Context to use the cosmo db connection

builder.AddCosmosDbContext<BloggingContext>("MyCosmosDatabase", "MyCosmosDatabase");

Key Considerations

• Partition Keys – Cosmos DB uses partitioning for scale. EF Core supports defining partition keys via Fluent API:

modelBuilder.Entity<Post>()
    .ToContainer("Posts")
    .HasPartitionKey(p => p.Title);

• No Joins – Unlike SQL databases, Cosmos doesn’t support cross-container joins. Consider embedding related data (e.g., Posts within Blog).
• Local Development – Aspire + the Cosmos emulator makes local dev frictionless. You won’t need to spin up real Azure resources until deployment.

Example Query with EF Core

using var context = new BloggingContext();

var recentPosts = await context.Posts
    .Where(p => p.Title.Contains("EF Core"))
    .OrderByDescending(p => p.Id)
    .ToListAsync();

When Should You Use Aspire + EF Core + Cosmos?

• If you’re building cloud-native apps with Aspire and want Cosmos DB as your backing store.
• If your team is comfortable with EF Core and LINQ but new to Cosmos DB.
• If you want local dev simplicity, Aspire + the Cosmos emulator makes setup nearly zero-config.

For advanced scenarios (change feed processing, multi-region writes, fine-grained indexing), you may still prefer using the Cosmos SDK directly. But Aspire + EF Core is a powerful combo for most application data needs.

Final Thoughts

Using Azure Cosmos DB with EF Core inside .NET Aspire gives you the best of all worlds:

• A familiar programming model via EF Core.
• Cloud-native orchestration and local dev support via Aspire.
• Scalability and global distribution from Cosmos DB.

If you’re starting a new distributed app on Azure, Aspire + EF Core + Cosmos DB is one of the fastest ways to be productive while still building on modern, cloud-ready foundations.