Replace string concatenation with StringBuilder and invariant interpolation

[kamilla11]

Summary

Closes issue #648.

• Replace string concatenation with StringBuilder or string interpolation.
• Introduce StringExtensions.AppendInvariant(StringBuilder, …) as a single cross-TFM API.
• Add polyfill attributes (InterpolatedStringHandlerAttribute, etc.) for targets below .NET 6 so the compiler can emit correct interpolation code.

Extension method sb.AppendInvariant("...") on .NET 6+ is not possible (CS8944), so a static call is used StringExtensions.AppendInvariant(sb, "...")

Summary by CodeRabbit

• Refactor

  • Optimized string building operations across payload generators for improved memory efficiency
  • Enhanced invariant culture string handling in payment and QR code formats
  • Streamlined PDF generation string construction logic

• Chores

  • Added internal compiler attribute support for .NET 6+ compatibility

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro

Run ID: aaa1ad1f-5a33-4e9d-8c85-1adec053cf97

📥 Commits

Reviewing files that changed from the base of the PR and between 4b97eb1 and f39e154.

📒 Files selected for processing (15)

• QRCoder/Attributes/InterpolatedStringHandlerAttributes.cs
• QRCoder/Extensions/StringExtensions.cs
• QRCoder/PayloadGenerator.cs
• QRCoder/PayloadGenerator/BezahlCode.cs
• QRCoder/PayloadGenerator/BitcoinLikeCryptoCurrencyAddress.cs
• QRCoder/PayloadGenerator/ContactData.cs
• QRCoder/PayloadGenerator/Girocode.cs
• QRCoder/PayloadGenerator/Mail.cs
• QRCoder/PayloadGenerator/OneTimePassword.cs
• QRCoder/PayloadGenerator/RussiaPaymentOrder.cs
• QRCoder/PayloadGenerator/SwissQrCode.cs
• QRCoder/PayloadGenerator/Url.cs
• QRCoder/PayloadGenerator/WiFi.cs
• QRCoder/PdfByteQRCode.cs
• QRCoder/QRCodeGenerator/Polynom.cs

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📝 Walkthrough

Walkthrough

This PR systematically refactors QRCoder's payload generators to use invariant-culture interpolated strings instead of string concatenation. It adds compiler attributes for pre-.NET 6 compatibility, introduces an AppendInvariant extension helper, and applies the pattern across payment, contact, authentication, and PDF generation classes.

Changes

Interpolated String Handler Refactoring

Layer / File(s)	Summary
Compiler Attributes & StringExtensions Infrastructure QRCoder/Attributes/InterpolatedStringHandlerAttributes.cs, QRCoder/Extensions/StringExtensions.cs	Adds InterpolatedStringHandlerAttribute, InterpolatedStringHandlerArgumentAttribute, and CompilerFeatureRequiredAttribute (conditional for !NET6_0_OR_GREATER). Introduces AppendInvariant helper that accepts interpolated-string handler refs for invariant-culture appends, with existing numeric overloads refactored to delegate through it.
Core PayloadGenerator Updates QRCoder/PayloadGenerator.cs	IsValidIban checksum computation replaces LINQ Aggregate with StringBuilder loop; EscapeInput switches per-character replacement to interpolation; NET5+ conditional code uses interpolated substring construction.
Simple String Format Changes QRCoder/PayloadGenerator/BitcoinLikeCryptoCurrencyAddress.cs, QRCoder/PayloadGenerator/Mail.cs, QRCoder/PayloadGenerator/Url.cs, QRCoder/PayloadGenerator/WiFi.cs	Straightforward conversions from string concatenation to string interpolation for URL prefixes, query parameters, and HEX-string wrapping—no behavioral changes.
OneTimePassword OTP Query Building QRCoder/PayloadGenerator/OneTimePassword.cs	HOTP counter, TOTP period, issuer:label combination, and common query fields refactored from sb.Append concatenation to StringExtensions.AppendInvariant with interpolation.
Payment & Financial Payload Builders QRCoder/PayloadGenerator/BezahlCode.cs, QRCoder/PayloadGenerator/Girocode.cs, QRCoder/PayloadGenerator/RussiaPaymentOrder.cs	BezahlCode non-contact/contact query construction, Girocode BCD payload assembly, and RussiaPaymentOrder mandatory fields refactored to use StringBuilder + AppendInvariant, preserving conditional field inclusion and encoding normalization.
Contact & Address Data Builders QRCoder/PayloadGenerator/ContactData.cs, QRCoder/PayloadGenerator/SwissQrCode.cs	ContactData (MeCard and vCard output) and SwissQrCode (contact block and full payload) refactored from concatenation to StringBuilder + AppendInvariant, maintaining address ordering rules and conditional formatting by output type/version. Formatting comments updated.
PDF Byte Generation QRCoder/PdfByteQRCode.cs	PDF object construction (catalog, pages, content stream, xref table, trailer) switched from concatenation to StringBuilder + AppendInvariant; path rectangle commands and color RGB strings use interpolation. xref offset tracking and stream length computation preserved.
Polynomial String Representation QRCoder/QRCodeGenerator/Polynom.cs	Polynomial term formatting updated to use AppendInvariant with interpolated algebraic notation instead of direct Append concatenation.

Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~25 minutes

Possibly related PRs

• Shane32/QRCoder#635: Modifies the same PDF generation code paths in PdfByteQRCode.cs (GetGraphic, CreatePathFromModules, ColorToPdfRgb).

Suggested labels

refactoring, performance

Suggested reviewers

• gfoidl

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title accurately summarizes the main change: replacing string concatenation with StringBuilder and invariant interpolation throughout the codebase.
Docstring Coverage	✅ Passed	Docstring coverage is 93.10% which is sufficient. The required threshold is 80.00%.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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[kamilla11]

I also want to warn you that IDE0055 errors occurred during the build, which are not present in <NoWarn>$(NoWarn);IDE0005;CA1510;</NoWarn>, which triggers configuration errors <TreatWarningsAsErrors>. These errors are not related to the code I changed.