Microsoft·Software Engineer·Technical Phone Screen·Senior

SeniorPrefer not to say

Jun 2026

Summary

Microsoft technical phone screen, C# focused. One question, pretty deep, and they wanted working async code not just pseudocode. Left feeling like I probably over-engineered part of it but the core logic held up.

Questions Asked (1)

You have three functions: an async ReadStream() that returns bytes, DecodeBytesToObject(bytes) that turns bytes into an object, and DecodeBytesToInt(bytes) that parses bytes as an integer. Using these, write a method in C# that reads a length-prefixed stream and returns a list of decoded objects. The interviewer wanted async/await and IAsyncEnumerable used appropriately.

System DesignAPI & IntegrationsTechnical Trade-offs

Author's notes

The length-prefix framing took me a second to internalize.

Create a free account to read the full note

AI HintsAI Generated

Suggested Approach

Start by clarifying the length-prefix format (e.g., fixed-size integer) and error handling expectations. Then design an async method that reads the length, reads exactly that many bytes, decodes them into an object, and repeats until the stream ends. Use IAsyncEnumerable to yield objects as they are decoded, and ensure proper disposal of resources.

Pro tip: Mention that you would use a helper method to read exactly N bytes from the stream, as ReadStream might return fewer bytes than requested. Also, discuss cancellation support and how to handle partial reads gracefully.

1. Clarify requirements and assumptions

Ask about the length-prefix format (e.g., 4-byte little-endian integer), error handling, and whether the stream can be empty. Confirm that the method should return IAsyncEnumerable<object>.

2. Design the reading loop

Outline a loop that reads the length prefix, then reads exactly that many bytes, decodes them into an object, and yields it. Continue until the stream ends or cancellation is requested.

3. Implement helper for exact reads

Write an async helper method that reads exactly N bytes from the stream, handling partial reads by looping until all bytes are received or the stream ends.

4. Use IAsyncEnumerable and async/await

Implement the method as an async iterator (using yield return) that awaits ReadStream and DecodeBytesToObject. Ensure proper disposal of resources and support cancellation.

5. Discuss trade-offs and edge cases

Explain how you handle errors (e.g., malformed length, incomplete data), performance considerations (buffering, memory), and why IAsyncEnumerable is appropriate for streaming.

Key Points to Mention

Length-prefix format: typically a fixed-size integer (e.g., 4 bytes) indicating the number of bytes in the payload.
Reading exactly N bytes: must loop because ReadStream may return fewer bytes than requested.
Using IAsyncEnumerable to stream results as they are decoded, avoiding buffering the entire list in memory.
Proper async/await usage: avoid blocking calls, use ConfigureAwait(false) where appropriate, and support cancellation tokens.
Error handling: decide how to handle malformed data (e.g., throw exception, skip, or return partial results).
Resource management: ensure the stream is disposed properly, possibly using await using.

AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.