Avoid NotSupportedException overhead in SharpCompressStream for non-seekable streams

[Copilot]

SharpCompressStream was attempting to access stream.Position without checking CanSeek, causing NotSupportedException to be thrown and caught for every non-seekable stream (e.g., XZ's ReadOnlyStream). This exception overhead occurred in the constructor and buffer allocation, impacting performance for formats using non-seekable streams.

Changes

• Check stream.CanSeek before accessing Position in two locations:
  • Constructor initialization of _baseInitialPos
  • BufferSize setter initialization of _internalPosition
• Use ternary operators for cleaner, simpler code

Impact

Eliminates exception overhead for non-seekable streams while maintaining identical behavior for seekable streams. No API changes.

// Before: Always tried Position, caught exception for non-seekable streams
try {
    _baseInitialPos = stream.Position;
} catch {
    _baseInitialPos = 0;
}

// After: Check CanSeek before accessing Position
_baseInitialPos = stream.CanSeek ? stream.Position : 0;

<issue_title>Handled System.NotSupportedException but can be avoided?</issue_title>
><issue_description>Open xz file like this:
>
> using var fileStream = new FileStream(updateFilename, FileMode.Open, FileAccess.Read);
> using var reader = ReaderFactory.Open(fileStream, new SharpCompress.Readers.ReaderOptions()
> {
> LeaveStreamOpen = true,
> });
>
>
> SharpCompressStream is wrapped around ReadOnlyStream and ReadOnlyStream doesnt' support Position get. In SharpCompressStream exception is handled:
>
> try
> {
> _baseInitialPos = stream.Position;
> }
> catch
> {
> _baseInitialPos = 0;
> }
>
> But this is still exception thrown for each entry I think which causes performance issues. Can it be avoided? Check for CanSeek ?</issue_description>
>
> ## Comments on the Issue (you are @copilot in this section)
>
>
>
>

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Handled System.NotSupportedException but can be avoided?</issue_title>
<issue_description>Open xz file like this:

            using var fileStream = new FileStream(updateFilename, FileMode.Open, FileAccess.Read);
            using var reader = ReaderFactory.Open(fileStream, new SharpCompress.Readers.ReaderOptions()
            {
                LeaveStreamOpen = true,
            });

SharpCompressStream is wrapped around ReadOnlyStream and ReadOnlyStream doesnt' support Position get. In SharpCompressStream exception is handled:

    try
    {
        _baseInitialPos = stream.Position;
    }
    catch
    {
        _baseInitialPos = 0;
    }

But this is still exception thrown for each entry I think which causes performance issues. Can it be avoided? Check for CanSeek ?</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Handled System.NotSupportedException but can be avoided? #1083

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[adamhathcock]

@copilot remove the try/catch...just check CanSeek

[Copilot]

@copilot remove the try/catch...just check CanSeek

Done in commit 6991900. Removed the try/catch blocks and simplified to just check CanSeek using ternary operators:

_baseInitialPos = stream.CanSeek ? stream.Position : 0;
_internalPosition = Stream.CanSeek ? Stream.Position : 0;

All tests pass (258 tests including XZ-specific tests).

[Copilot]

Pull request overview

This PR optimizes SharpCompressStream performance by replacing exception-based handling with proactive CanSeek checks when accessing stream positions. The change eliminates the overhead of throwing and catching NotSupportedException for non-seekable streams like ReadOnlyStream used in XZ file operations.

Key Changes:

• Replaced try/catch blocks with CanSeek ternary checks in two locations
• Added clarifying comments explaining the rationale for the check

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