[bitsandfoxes]

Description

The SDK already documents that attachment streams must be seekable:

sentry-dotnet/src/Sentry/Scope.cs

Line 784 in 9130077

/// Note: the stream must be seekable.

and explicitly logs when they aren't:

sentry-dotnet/src/Sentry/Scope.cs

Lines 792 to 800 in 9130077

	var length = stream.TryGetLength();
	if (length is null)
	{
	Options.LogWarning(
	"Cannot evaluate the size of attachment '{0}' because the stream is not seekable.",
	fileName);
	return;
	}

So the assumption is already there. Instead of relying on Stream.Length throwing and the try-catch swallowing it, TryGetLength now checks CanSeek up front. This avoids a first-chance exception for every non-seekable stream, which helps on platforms like Unity's IL2CPP where exceptions are prohibitively expensive.

For well-behaved streams the result is unchanged: per the Stream.Length docs, Length throws NotSupportedException when the stream doesn't support seeking.

The one difference is for non-standard streams that report CanSeek == false but still return a Length. These now return null, which is consistent with the expectations and the contract.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 75.31%. Comparing base (9130077) to head (45a56c0).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5736      +/-   ##
==========================================
- Coverage   75.31%   75.31%   -0.01%     
==========================================
  Files         517      517              
  Lines       19093    19093              
  Branches     3704     3705       +1     
==========================================
- Hits        14380    14379       -1     
  Misses       3859     3859              
- Partials      854      855       +1     

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