All 17 ASP.NET Core Middleware & Request Pipeline questions
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How do you conditionally apply middleware only to certain requests using UseWhen or MapWhen? How does the HttpContext object serve as the shared state carrier through the middleware pipeline? What is middleware in ASP.NET Core, and how does the order in which it's registered affect request handling? What are Minimal APIs in ASP.NET Core, and when would you choose them over traditional MVC controllers? How does UseRouting and endpoint routing (via MapControllers, MapGet, etc.) fit into the middleware pipeline conceptually? How do minimal API endpoint filters (IEndpointFilter) compare to traditional middleware for cross-cutting logic? How does the middleware pipeline interact with asynchronous streaming responses, and what precautions are needed? What is the recommended standard order for common middleware in a typical ASP.NET Core Web API pipeline, and why? Why is it dangerous to capture and store an HttpContext reference in a singleton-scoped service, and what should you use instead?
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How do you inject scoped (per-request) services into middleware, given that middleware classes are constructed once at app startup?

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Answer

Since class-based middleware is instantiated once as a singleton for the app's lifetime, you can't inject scoped services (like a DbContext) directly into the constructor -- instead, inject them as parameters on the InvokeAsync method itself, which ASP.NET Core's DI container resolves per-request from the current request's scope, correctly respecting scoped service lifetimes.
public class AuditMiddleware {
    private readonly RequestDelegate _next;
    public AuditMiddleware(RequestDelegate next) => _next = next;

    // Scoped service injected per-request via method parameter, not constructor
    public async Task InvokeAsync(HttpContext context, AppDbContext db) {
        db.AuditLogs.Add(new AuditLog { Path = context.Request.Path });
        await db.SaveChangesAsync();
        await _next(context);
    }
}
Real-world example A middleware logging audit trails to a scoped DbContext initially crashes with a lifetime mismatch error until the team moves the DbContext injection from the constructor to the InvokeAsync method parameter list.

Common follow-ups: What error occurs if you inject a scoped service into a middleware constructor directly?;How does this differ for minimal API endpoint filters?

Dependency Injection;Entity Framework Core & Data Access
All 17 ASP.NET Core Middleware & Request Pipeline questions