Comparison questionEasy~5 minASP.NET Core Framework Essentials / 22 of 26

Interview question

How do `Use`, `Run`, and `Map` differ when building an ASP.NET Core middleware pipeline?

Use, Run, and Map in the ASP.NET Core middleware pipeline

Explains middleware continuation, terminal delegates, path branches, short-circuiting, and two-way request and response flow.

1Say itAnswer out loud first

2CompareCompare with Core answer

3RefineRefine with Stronger and Advanced

TL;DR

Use adds middleware that can perform work before and after the next component. It receives a next delegate and continues the pipeline by awaiting it; if it does not call next, it short-circuits the request. Run adds a terminal delegate with no next component, so it produces or completes the response for requests that reach it. Map creates a branch selected by a request-path prefix and runs a separate pipeline for that branch. The request moves forward through middleware until an endpoint or terminal component handles it, then control unwinds back through middleware that awaited next. Order therefore affects both inbound work, such as authentication, and outbound work, such as logging duration or adding headers before the response starts.

Full answer below

Practice focus

Middleware composition, `RequestDelegate`, short-circuiting, path branching, response unwinding, and pipeline order.

Answer first

Answer path

Core answer first, then checkpoints toward a stronger answer.

Start
Core answer
Start here. This is the direct spoken answer to practice first.
Why this question matters
These three methods look similar in Program.cs, but they create different control flow. Understanding that flow makes middleware-order bugs much easier to predict.
Use adds middleware that can perform work before and after the next component. It receives a next delegate and continues the pipeline by awaiting it; if it does not call next, it short-circuits the request. Run adds a terminal delegate with no next component, so it produces or completes the response for requests that reach it. Map creates a branch selected by a request-path prefix and runs a separate pipeline for that branch. The request moves forward through middleware until an endpoint or terminal component handles it, then control unwinds back through middleware that awaited next. Order therefore affects both inbound work, such as authentication, and outbound work, such as logging duration or adding headers before the response starts.
Checkpoint
Stronger answer
Optional upgrade with examples, trade-offs, and sharper phrasing.
I use Use for cross-cutting behavior that normally wraps downstream execution, such as exception handling, correlation, authentication, or timing. I short-circuit deliberately for cases such as a rejected request or maintenance response, and I do not call next after writing a terminal body. I use Run when a branch truly ends there. Map is useful for an isolated path subtree; the matched path segment moves into PathBase, so code in the branch sees the remaining path. Endpoint methods such as MapGet and MapControllers register route endpoints and metadata, which is related composition but not the same API as path-based IApplicationBuilder.Map branching.
Finish
Advanced answer
Optional deeper angle for production judgment and nuance.
For production middleware I make continuation and response ownership explicit. Code after await next(context) can observe the final status and duration, but it may be too late to change headers if downstream code has started the response. A middleware that catches exceptions must sit before the components it protects. A branch can accidentally bypass authentication, authorization, rate limiting, or observability if it is composed outside their intended scope. I test representative matched, unmatched, rejected, and exceptional requests, including whether downstream middleware ran and whether the response was already committed. For complex conditional behavior I prefer clear endpoint metadata or a small, named branch over nested predicates scattered through startup. The goal is a pipeline whose control flow can be read top to bottom and whose short-circuits are intentional rather than side effects of forgetting to invoke next.

Review signals

Use these to catch weak phrasing, shallow reasoning, and risky claims.

Common mistakesCandidateCommon mistakesMistakes that weaken the answer or signal shallow understanding.Open details

Details

Using Use for terminal behavior and silently forgetting to call next.
Calling next after a complete response body has already been written.
Assuming code after await next runs before the endpoint.
Creating a branch that unintentionally bypasses security or telemetry middleware.

Red flagsCandidateSigns the approach is fragileSignals that an interviewer may treat as risky.Open details

Details

Middleware continuation depends on undocumented conditions.
A terminal delegate appears before endpoints that still need to run.
Path branching is confused with endpoint route mapping.
No tests prove which components execute for matched and rejected requests.

Follow-up prompts

Quick interviewer prompts inside this drill.

Follow-up questionsCandidateFollow-up questionsLikely next questions after the main answer.Open details

Details

Treat these as the next interviewer turn. Answer one out loud, then save the gap in your practice notes.

What does short-circuiting mean in middleware?
Why does code after await next run on the response path?
How is app.Map different from app.MapGet?

Continue the section

Next in ASP.NET Core Framework Essentials: ApiController Conventions

Step 23 of 26

ApiController conventions and automatic validation responses

Opening drill

Comparison questionEasy~5 minASP.NET Core Framework Essentials / 22 of 26

Interview question

How do `Use`, `Run`, and `Map` differ when building an ASP.NET Core middleware pipeline?

Use, Run, and Map in the ASP.NET Core middleware pipeline

Explains middleware continuation, terminal delegates, path branches, short-circuiting, and two-way request and response flow.

1Say itAnswer out loud first

2CompareCompare with Core answer

3RefineRefine with Stronger and Advanced

TL;DR

Full answer below

Practice focus

Middleware composition, `RequestDelegate`, short-circuiting, path branching, response unwinding, and pipeline order.

Answer first

Answer path

Core answer first, then checkpoints toward a stronger answer.

Start
Core answer
Start here. This is the direct spoken answer to practice first.
Why this question matters
These three methods look similar in Program.cs, but they create different control flow. Understanding that flow makes middleware-order bugs much easier to predict.
Use adds middleware that can perform work before and after the next component. It receives a next delegate and continues the pipeline by awaiting it; if it does not call next, it short-circuits the request. Run adds a terminal delegate with no next component, so it produces or completes the response for requests that reach it. Map creates a branch selected by a request-path prefix and runs a separate pipeline for that branch. The request moves forward through middleware until an endpoint or terminal component handles it, then control unwinds back through middleware that awaited next. Order therefore affects both inbound work, such as authentication, and outbound work, such as logging duration or adding headers before the response starts.
Checkpoint
Stronger answer
Optional upgrade with examples, trade-offs, and sharper phrasing.
I use Use for cross-cutting behavior that normally wraps downstream execution, such as exception handling, correlation, authentication, or timing. I short-circuit deliberately for cases such as a rejected request or maintenance response, and I do not call next after writing a terminal body. I use Run when a branch truly ends there. Map is useful for an isolated path subtree; the matched path segment moves into PathBase, so code in the branch sees the remaining path. Endpoint methods such as MapGet and MapControllers register route endpoints and metadata, which is related composition but not the same API as path-based IApplicationBuilder.Map branching.
Finish
Advanced answer
Optional deeper angle for production judgment and nuance.
For production middleware I make continuation and response ownership explicit. Code after await next(context) can observe the final status and duration, but it may be too late to change headers if downstream code has started the response. A middleware that catches exceptions must sit before the components it protects. A branch can accidentally bypass authentication, authorization, rate limiting, or observability if it is composed outside their intended scope. I test representative matched, unmatched, rejected, and exceptional requests, including whether downstream middleware ran and whether the response was already committed. For complex conditional behavior I prefer clear endpoint metadata or a small, named branch over nested predicates scattered through startup. The goal is a pipeline whose control flow can be read top to bottom and whose short-circuits are intentional rather than side effects of forgetting to invoke next.

Review signals

Use these to catch weak phrasing, shallow reasoning, and risky claims.

Common mistakesCandidateCommon mistakesMistakes that weaken the answer or signal shallow understanding.Open details

Details

Using Use for terminal behavior and silently forgetting to call next.
Calling next after a complete response body has already been written.
Assuming code after await next runs before the endpoint.
Creating a branch that unintentionally bypasses security or telemetry middleware.

Red flagsCandidateSigns the approach is fragileSignals that an interviewer may treat as risky.Open details

Details

Middleware continuation depends on undocumented conditions.
A terminal delegate appears before endpoints that still need to run.
Path branching is confused with endpoint route mapping.
No tests prove which components execute for matched and rejected requests.

Follow-up prompts

Quick interviewer prompts inside this drill.

Follow-up questionsCandidateFollow-up questionsLikely next questions after the main answer.Open details

Details

Treat these as the next interviewer turn. Answer one out loud, then save the gap in your practice notes.

What does short-circuiting mean in middleware?
Why does code after await next run on the response path?
How is app.Map different from app.MapGet?

Continue the section

Next in ASP.NET Core Framework Essentials: ApiController Conventions

Step 23 of 26

ApiController conventions and automatic validation responses

How do `Use`, `Run`, and `Map` differ when building an ASP.NET Core middleware pipeline?

Practice focus

Answer path

Why this question matters

Review signals

Follow-up prompts

Continue the section

How do `Use`, `Run`, and `Map` differ when building an ASP.NET Core middleware pipeline?

Practice focus

Answer path

Why this question matters

Review signals

Follow-up prompts

Continue the section