AZ-400 Implement an instrumentation strategy Practice Question
This AZ-400 practice question tests your understanding of implement an instrumentation strategy. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
Exhibit
Refer to the exhibit.
```
Application Insights Snapshot Debugger
Snapshot 1:
Thread: 1234
Exception: NullReferenceException
Stack:
Contoso.Web.Pages.Index.OnGet() line 42
Microsoft.AspNetCore.Mvc.RazorPages.Infrastructure.PageActionInvoker.InvokeHandlerMethod()
...
Local variables:
_dbContext: null (Contoso.Data.AppDbContext)
```
You are debugging a production issue using Application Insights Snapshot Debugger. The exhibit shows a snapshot from a NullReferenceException. The variable _dbContext is null. What is the most likely root cause?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "most likely"
Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The Index page model is not registered in the dependency injection container.
The Snapshot Debugger shows _dbContext is null, which indicates the dependency was never injected. In ASP.NET Core, page models like IndexModel must be registered in the dependency injection (DI) container (e.g., via AddTransient<IndexModel> or AddScoped<IndexModel>) for DI to resolve and inject the DbContext. Without registration, the runtime creates the page model manually, leaving _dbContext uninitialized and causing a NullReferenceException when OnGet accesses it.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✗
The call to the database is not awaited, causing a race condition.
Why it's wrong here
NullReferenceException is due to null reference, not async.
✓
The Index page model is not registered in the dependency injection container.
Why this is correct
If the page model isn't registered, DI can't inject _dbContext.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
The database connection string is invalid in appsettings.json.
Why it's wrong here
A bad connection string would cause a different exception at runtime.
✗
The OnGet method is missing a null check for _dbContext before usage.
Why it's wrong here
Null check is a workaround, not root cause.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often assume a NullReferenceException always means a missing null check (Option D), but the snapshot's context—showing a dependency that is null—points to a DI registration failure, not a defensive coding oversight.
Detailed technical explanation
How to think about this question
In ASP.NET Core, page models are resolved by the framework's DI container only if they are explicitly registered (e.g., via services.AddTransient<IndexModel>() in Startup.ConfigureServices). Without registration, the framework falls back to the default parameterless constructor, leaving any injected dependencies (like _dbContext) as null. This is a common pitfall when using constructor injection in Razor Pages, as the framework does not automatically register page models—unlike controllers in MVC, which are registered by default via AddControllers().
KKey Concepts to Remember
Read the scenario before looking for a memorised answer.
Find the constraint that changes the correct option.
Eliminate answers that are true in general but not in this case.
TExam Day Tips
→Watch for words such as best, first, most likely and least administrative effort.
→Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
A company's IT admin needs to give a contractor read-only access to production logs without sharing account credentials. Using role-based access control (RBAC) and temporary scoped permissions — not a permanent shared password — is the correct pattern. Questions like this test whether you can apply least-privilege access across cloud identity services.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this AZ-400 question in full detail.
Implement an instrumentation strategy — This question tests Implement an instrumentation strategy — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The Index page model is not registered in the dependency injection container. — The Snapshot Debugger shows _dbContext is null, which indicates the dependency was never injected. In ASP.NET Core, page models like IndexModel must be registered in the dependency injection (DI) container (e.g., via AddTransient<IndexModel> or AddScoped<IndexModel>) for DI to resolve and inject the DbContext. Without registration, the runtime creates the page model manually, leaving _dbContext uninitialized and causing a NullReferenceException when OnGet accesses it.
What should I do if I get this AZ-400 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Are there clue words in this question I should notice?
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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