[Bug]: Duration encoded as integer (e.g. milliseconds) can serialize as floating point due to use of TimeSpan.TotalMilliseconds

[joseharriaga]

Describe the bug

In TypeSpec, it is possible to define a property of type duration with different encodings as described here: https://typespec.io/docs/libraries/http/encoding/#duration

Example definition:

@encode(DurationKnownEncoding.milliseconds, integer)
audio_end_ms: duration;

This indicates that the audio_end_ms property is an integer (milliseconds). However, the generated C# code represents this as a TimeSpan and serializes it as follows:

if (!Patch.Contains("$.audio_end_ms"u8))
{
    writer.WritePropertyName("audio_end_ms"u8);
    writer.WriteNumberValue(AudioEndMs.TotalMilliseconds);
}

The issue: TimeSpan.TotalMilliseconds is a double, so this code can serialize values with fractional milliseconds (e.g., 123.45), violating the contract where audio_end_ms must be an integer. This can result in serialization output not matching the expected API contract.

Expected: The code should round TotalMilliseconds up or down (or something like AudioEndMs.Ticks / TimeSpan.TicksPerMillisecond) to ensure only integers are written for the property.

Please consider updating the serialization logic to guarantee integer values when the original TypeSpec contract expects an integer.

Reproduction

Define a property in TypeSpec using:

@encode(DurationKnownEncoding.milliseconds, integer)
audio_end_ms: duration;

Generate the C# client and observe the serialization code for this member: AudioEndMs will use TotalMilliseconds resulting in double output values. Test with a TimeSpan representing fractional milliseconds and see if output is a floating point value. The value should be serialized as an integer.

Checklist

• Follow our Code of Conduct
• Check that there isn't already an issue that request the same bug to avoid creating a duplicate.
• Check that this is a concrete bug. For Q&A open a GitHub Discussion.
• The provided reproduction is a minimal reproducible example of the bug.

[github-actions]

🎯 Issue Triage

The C# client emitter incorrectly uses TimeSpan.TotalMilliseconds (which returns a double) when serializing a duration property encoded as integer milliseconds, potentially producing fractional values that violate the API contract.

Category: Bug
Area: emitter:client:csharp

📋 Reproduction code

`@encode`(DurationKnownEncoding.milliseconds, integer)
audio_end_ms: duration;

🔍 Analysis

• The @encode(DurationKnownEncoding.milliseconds, integer) decorator signals that the wire format should be an integer number of milliseconds.
• The generated C# serialization calls writer.WriteNumberValue(AudioEndMs.TotalMilliseconds), where TotalMilliseconds is double, allowing fractional values like 123.45.
• A correct implementation should cast to long or use AudioEndMs.Ticks / TimeSpan.TicksPerMillisecond to ensure integer-only output.
• This affects any duration property with an integer encoding (e.g., seconds, milliseconds) in the C# emitter's serialization code generation.

📝 Suggested next steps

• In the C# emitter, identify the code path that emits serialization for duration with integer encoding.
• Replace TotalMilliseconds (and equivalent double-returning properties) with an integer cast, e.g., (long)AudioEndMs.TotalMilliseconds or AudioEndMs.Ticks / TimeSpan.TicksPerMillisecond.
• Add/update a Spector test case covering integer-encoded durations to catch regressions.

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