Minor refactoring

[michelebastione]

Taken care of a some warnings, removed unused imports, added pattern matching where deemed appropriate, general code and comment cleanup

[gemini-code-assist]

Summary of Changes

Hello @michelebastione, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

I've undertaken a minor refactoring effort to enhance the overall quality and maintainability of the codebase. My primary focus was on addressing compiler warnings, removing redundant code, and modernizing existing logic. This involved a widespread cleanup of unused imports, the strategic application of C# pattern matching for more concise expressions, and improvements to nullability handling. These changes contribute to a cleaner, more robust, and easier-to-understand codebase without altering existing functionality.

Highlights

• Removed Unused Imports: I've performed a comprehensive cleanup by removing numerous unused 'using' directives across various core and test files. This reduces compilation warnings and improves code readability by eliminating unnecessary dependencies.
• Enhanced Code with Pattern Matching: I've modernized the codebase by extensively applying C# pattern matching, including 'is { Count: > 0 }', 'is IEnumerable value and not string', and switch expressions. This makes the code more concise, readable, and expressive, particularly in data handling and XML processing logic.
• Improved Nullability and Type Safety: I've improved nullability handling throughout the project, including initializing dictionaries, using null-forgiving operators (!), and null-coalescing operators (??). This helps prevent potential NullReferenceExceptions and aligns with modern C# best practices.
• Refactored Dimension Reference Logic: I've refactored the 'GetDimensionRef' method in 'OpenXmlWriter.DefaultOpenXml.cs' to use a C# 8.0 switch expression, providing a more elegant and readable way to determine Excel sheet dimensions based on row and column counts.
• Simplified Cell Value Conversion: I've simplified the 'ConvertCellValue' method in 'OpenXmlReader.cs' by streamlining its logic with 'break' statements and ternary operators, making the value conversion process more efficient and easier to follow.
• Streamlined XML Style Generation: I've updated the XML writing logic in style builders ('DefaultSheetStyleBuilder.cs', 'MinimalSheetStyleBuilder.cs', 'SheetStyleBuilderBase.cs') by introducing local 'OldReader' and 'NewWriter' properties. This significantly cleans up the XML generation code, making it more maintainable and less verbose.
• Updated Test Cases for Consistency: I've made minor adjustments to several test files to align with the refactoring changes, including explicit type declarations in foreach loops, updating constant names, and ensuring assertions remain valid with the new nullable types and code structures.

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[gemini-code-assist]

Code Review

This pull request contains a lot of good refactoring, including removing unused using statements and adopting modern C# features like pattern matching and expression-bodied members, which improves code clarity and maintainability. The changes are generally solid. I've found one potential issue regarding a NullReferenceException that could occur when reading a malformed OpenXML file, and I've left a comment with details on how to address it.