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Document HostingStartup incompatibility and migration to modern hosting patterns for .NET Aspire (#4187)
* Initial plan * Add troubleshooting documentation for HostingStartup incompatibility Co-authored-by: IEvangelist <[email protected]> * Fix duplicate link and complete documentation Co-authored-by: IEvangelist <[email protected]> * Fix style guide compliance: remove package-lock.json, add trailing newline, move code snippets to external files, fix ordered list format Co-authored-by: IEvangelist <[email protected]> * Remove Option 4 and dependency injection plugin example code Co-authored-by: IEvangelist <[email protected]> * Refactor troubleshooting doc to focus on migration concepts rather than multiple options Co-authored-by: IEvangelist <[email protected]> * Fix bullet points to be complete sentences with proper punctuation Co-authored-by: IEvangelist <[email protected]> * Update docs/troubleshooting/hosting-startup-not-supported.md * Fix build errors in hosting-startup snippet files Co-authored-by: IEvangelist <[email protected]> * Replace non-standard arrow characters with table format for better accessibility Co-authored-by: adegeo <[email protected]> * Fix punctuation in table benefit column - add periods to complete sentences Co-authored-by: adegeo <[email protected]> * Update ai-usage metadata from ai-assisted to ai-generated Co-authored-by: adegeo <[email protected]> * Update ai-usage metadata back to ai-assisted based on team decision Co-authored-by: adegeo <[email protected]> --------- Co-authored-by: copilot-swe-agent[bot] <[email protected]> Co-authored-by: IEvangelist <[email protected]> Co-authored-by: David Pine <[email protected]> Co-authored-by: adegeo <[email protected]>
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‎docs/fundamentals/integrations-overview.md

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These packages configure existing client libraries to connect to hosting integrations. They extend the <xref:Microsoft.Extensions.Hosting.IHostApplicationBuilder> interface allowing client-consuming projects, such as your web app or API, to use the connected resource. The official [client integration NuGet packages](https://www.nuget.org/packages?q=owner%3A+aspire+tags%3A+aspire+client+integration&includeComputedFrameworks=true&prerel=true&sortby=relevance) are tagged with `aspire`, `integration`, and `client`. In addition to the official client integrations, the [community has created client integrations](../community-toolkit/overview.md) for various services and platforms as part of the Community Toolkit.
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> [!IMPORTANT]
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> .NET Aspire integrations require <xref:Microsoft.Extensions.Hosting.IHostApplicationBuilder> and are **not compatible** with `HostingStartup` implementations, which only provide access to <xref:Microsoft.AspNetCore.Hosting.IWebHostBuilder>. If you're using `HostingStartup` for modular configuration, see [HostingStartup is not supported with .NET Aspire integrations](../troubleshooting/hosting-startup-not-supported.md) for migration guidance.
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For more information on creating a custom client integration, see [Create custom .NET Aspire client integrations](../extensibility/custom-client-integration.md).
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### Relationship between hosting and client integrations

‎docs/toc.yml

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- name: Allow unsecure transport
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displayName: unsecure transport,http,non-tls
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href: troubleshooting/allow-unsecure-transport.md
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- name: HostingStartup is not supported
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displayName: hosting startup,IWebHostBuilder,IHostApplicationBuilder,migration
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href: troubleshooting/hosting-startup-not-supported.md
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- name: Untrusted localhost certificate
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href: troubleshooting/untrusted-localhost-certificate.md
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- name: Unable to install .NET Aspire workload

‎docs/troubleshooting/hosting-startup-not-supported.md

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---
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title: HostingStartup is not supported with .NET Aspire integrations
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description: Learn how to migrate from HostingStartup to the IHostApplicationBuilder pattern for use with .NET Aspire integrations.
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ms.date: 08/04/2025
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ai-usage: ai-assisted
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---
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# HostingStartup is not supported with .NET Aspire integrations
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.NET Aspire integrations require the use of <xref:Microsoft.Extensions.Hosting.IHostApplicationBuilder>, but `HostingStartup` only provides access to <xref:Microsoft.AspNetCore.Hosting.IWebHostBuilder>. This fundamental incompatibility means that you can't configure .NET Aspire integrations from within a `HostingStartup` implementation.
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## Symptoms
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When attempting to use .NET Aspire integrations within a HostingStartup implementation, you might encounter:
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- **Compilation errors**: Aspire integration extension methods like `AddNpgsqlDbContext` or `AddRedis` are not available on `IWebHostBuilder`.
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- **Runtime configuration issues**: Even if you access the underlying services, the proper configuration and service registration won't occur.
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- **Missing telemetry and resilience**: Aspire's built-in observability, health checks, and resilience patterns won't be applied.
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## Why HostingStartup doesn't work with .NET Aspire
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.NET Aspire integrations extend <xref:Microsoft.Extensions.Hosting.IHostApplicationBuilder> to provide:
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- Standardized configuration patterns.
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- Built-in health checks.
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- Telemetry and observability.
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- Resilience patterns.
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- Service discovery integration.
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The `HostingStartup` feature was designed for the older ASP.NET Core hosting model and only provides access to <xref:Microsoft.AspNetCore.Hosting.IWebHostBuilder>, which doesn't include these modern hosting capabilities.
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## Migrating from HostingStartup
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The `HostingStartup` feature represents an older ASP.NET Core hosting model that predates the modern <xref:Microsoft.Extensions.Hosting.IHostApplicationBuilder> pattern that .NET Aspire requires. Migration is necessary to leverage .NET Aspire's integrations and modern hosting capabilities.
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### Understanding the API changes
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The fundamental difference lies in the hosting abstractions:
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**Before (HostingStartup pattern):**
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:::code language="csharp" source="snippets/hosting-startup-not-supported/hosting-startup-before.cs":::
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**After (Modern hosting pattern):**
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:::code language="csharp" source="snippets/hosting-startup-not-supported/host-application-builder-after.cs":::
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### Key conceptual changes
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When migrating from `HostingStartup` to the modern hosting model, you're moving between these approaches:
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| Legacy pattern | Modern pattern | Benefit |
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|---|---|---|
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| `IWebHostBuilder` | `IHostApplicationBuilder` | Access to modern hosting features and .NET Aspire integrations. |
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| Separate startup classes | Program.cs configuration | Service configuration moves directly into the application's entry point for better clarity and debugging. |
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| Manual service registration | Integration packages | .NET Aspire integrations handle service registration, configuration, health checks, and telemetry automatically. |
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### Migration resources
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For detailed migration guidance, see:
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- [Migrate from ASP.NET Core 5.0 to 6.0](/aspnet/core/migration/50-to-60?view=aspnetcore-9.0) - Covers the transition to the minimal hosting model
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- [David Fowl's ASP.NET Core 6.0 migration guide](https://gist.github.com/davidfowl/0e0372c3c1d895c3ce195ba983b1e03d) - Provides practical migration patterns and examples
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## Additional considerations
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- **Service discovery**: .NET Aspire integrations automatically configure service discovery. If you were using HostingStartup for service-to-service communication, consider using Aspire's [service discovery features](../service-discovery/overview.md).
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- **Configuration management**: Instead of hard-coding connection strings in HostingStartup, use .NET Aspire's configuration patterns with connection string names that map to resources in your app host.
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- **Testing**: .NET Aspire provides [testing capabilities](../testing/overview.md) that work with the new hosting model.
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For more information about .NET Aspire integrations and the hosting model, see [.NET Aspire integrations overview](../fundamentals/integrations-overview.md).

‎docs/troubleshooting/snippets/hosting-startup-not-supported/MyDbContext.cs

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using Microsoft.EntityFrameworkCore;
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public class MyDbContext : DbContext
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{
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public MyDbContext(DbContextOptions<MyDbContext> options) : base(options) { }
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}

‎docs/troubleshooting/snippets/hosting-startup-not-supported/host-application-builder-after.cs

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using Microsoft.AspNetCore.Builder;
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using Microsoft.Extensions.Hosting;
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// Example of modern hosting pattern that works with .NET Aspire
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public class ModernHostingExample
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{
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public static void ConfigureServices()
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{
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// Program.cs equivalent
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var builder = WebApplication.CreateBuilder();
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// Add service defaults first
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// builder.AddServiceDefaults();
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// Now you can use Aspire integrations
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// builder.AddNpgsqlDbContext<MyDbContext>("postgres");
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var app = builder.Build();
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// app.MapDefaultEndpoints();
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// app.Run();
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}
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}

‎docs/troubleshooting/snippets/hosting-startup-not-supported/hosting-startup-before.cs

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using Microsoft.AspNetCore.Hosting;
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using Microsoft.EntityFrameworkCore;
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using Microsoft.Extensions.DependencyInjection;
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// MyDatabaseStartup.cs
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public class MyDatabaseStartup : IHostingStartup
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{
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public void Configure(IWebHostBuilder builder)
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{
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builder.ConfigureServices(services =>
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{
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// This won't work with Aspire integrations
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services.AddDbContext<MyDbContext>(options =>
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options.UseNpgsql("Host=localhost;Database=mydb;Username=user;Password=password"));
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});
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}
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}

‎docs/troubleshooting/snippets/hosting-startup-not-supported/hosting-startup-not-supported.csproj

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<Project Sdk="Microsoft.NET.Sdk">
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<PropertyGroup>
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<TargetFramework>net8.0</TargetFramework>
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<ImplicitUsings>enable</ImplicitUsings>
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<Nullable>enable</Nullable>
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<!-- This is just a snippet project, not meant to be executed -->
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<OutputType>Library</OutputType>
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</PropertyGroup>
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<ItemGroup>
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<FrameworkReference Include="Microsoft.AspNetCore.App" />
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<PackageReference Include="Microsoft.AspNetCore.Hosting.Abstractions" Version="2.2.0" />
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<PackageReference Include="Microsoft.EntityFrameworkCore" Version="8.0.11" />
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<PackageReference Include="Npgsql.EntityFrameworkCore.PostgreSQL" Version="8.0.11" />
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</ItemGroup>
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</Project>

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