AZ-204 Develop for Azure storage Practice Question
You are developing a .NET application that stores JSON documents in Azure Blob Storage. The application must ensure that a client can only write a new blob if it does not already exist, preventing accidental overwrites. You need to configure the blob upload operation. Which condition should you use?
⚠ Common exam trap
Many exam-takers confuse If-None-Match: * with If-Match: *, assuming both enforce some form of existence check, when only the former checks for non-existence.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
If-None-Match: *
To prevent overwriting an existing blob, the application must use a conditional header that fails if the blob is present. The If-None-Match header with a wildcard value checks for the absence of the blob, making the upload succeed only when the blob does not exist. This is the correct approach for create-if-absent semantics in Azure Blob Storage.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
If-Modified-Since: <date>
Why it's wrong here
If-Modified-Since is used to perform the operation only if the blob has been modified since a specified date. It does not check for existence; it checks modification time. This condition would not prevent overwriting an existing blob that was modified after the given date, nor would it allow creation when the blob does not exist.
- ✗
If-Match: *
Why it's wrong here
If-Match: * requires that the blob already exists and its ETag matches any value. It is used for optimistic concurrency on existing blobs, ensuring the blob has not been modified. In this scenario, the requirement is to write only if the blob does not exist, so this condition would cause the operation to fail when the blob is absent.
- ✓
If-None-Match: *
Why this is correct
The If-None-Match header with a wildcard (*) instructs Azure Blob Storage to perform the operation only if the blob does not exist. This is the standard way to achieve create-if-absent semantics, ensuring that the upload fails with a 409 Conflict if the blob already exists, thus preventing overwrites.
- ✗
If-Unmodified-Since: <date>
Why it's wrong here
If-Unmodified-Since ensures the operation proceeds only if the blob has not been modified since a specified date. It is used to avoid conflicts when updating an existing blob. It does not enforce that the blob must be absent; if the blob does not exist, the condition is typically ignored, allowing the write, which does not meet the requirement.
Quick reference
Azure Blob Storage Tier Comparison
| Tier | Storage Cost | Retrieval Cost | Latency | Use Case |
|---|---|---|---|---|
| Hot | Highest | Lowest | Immediate | Active data, frequent reads |
| Cool | Lower | Higher | Immediate | Data accessed < once / month |
| Cold | Lower still | Higher | Immediate | Data accessed < once / quarter |
| Archive | Lowest | Highest + rehydration delay | Hours | Long-term compliance retention |
Go deeper
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Senior Network & Security Engineer · founder of Courseiva
Last reviewed September 2026 · checked against the official Microsoft exam blueprint
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