Add CA1830 analyzer for StringBuilder.Append(new string(char, int))

[Copilot]

Summary

This PR extends the existing CA1830 analyzer to detect and optimize an inefficient StringBuilder pattern where new string(char, int) is used as an argument to StringBuilder.Append().

Changes Made

Analyzer

• Extended PreferTypedStringBuilderAppendOverloads.cs to detect new string(char, int) constructor calls in StringBuilder.Append() arguments
• Only applies to Append method (not Insert) since Insert(int, char, int) doesn't exist in StringBuilder

Code Fix

• Updated PreferTypedStringBuilderAppendOverloads.Fixer.cs to replace Append(new string(c, count)) with Append(c, count)

Resources

• Added PreferTypedStringBuilderAppendOverloadsReplaceStringConstructor resource string
• Updated CA1830 description to mention the new pattern

Tests

• Added comprehensive C# tests for the new analyzer functionality
• All tests pass

Performance Impact

The suggested change avoids allocating an intermediate string object.

Before: sb.Append(new string('*', 5)) - allocates string
After: sb.Append('*', 5) - no allocation

Fixes dotnet/runtime#117643

Original prompt

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This section details on the original issue you should resolve

<issue_title>[Analyzer Proposal]: Don't use stringBuilder.Append(new string(c, repeatCount))</issue_title>
<issue_description>### Background and motivation

In microsoft/testfx, we have had an occurrence of this pattern (see microsoft/testfx#6013).

This pattern can be replaced by stringBuilder.Append(c, repeatCount) to avoid the allocation of an intermediate string.

API Proposal

A CAxxx rule shipped in .NET SDK (Category=Performance) that suggests the simplified, and more performant overload of StringBuilder.Append

API Usage

Warning on:

builder.Append(new string(c, repeatCount))

A codefix is provided to change the above to:

builder.Append(c, repeatCount)

Alternative Designs

No response

Risks

No response</issue_description>

<agent_instructions>Implement an analyzer and fixer in the Microsoft.CodeAnalysis.NetAnalyzers project, following the same approach and style as is used in the rest of the project.</agent_instructions>

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

@bartonjs [Video](https://www.youtube.com/watch?v=MXg3EromQLM&t=1h14m55s)

Looks good as proposed

Category: Performance
Severity: Info</comment_new>

Fixes dotnet/runtime#117643

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[stephentoub]

@copilot, you changed hundreds of files that didn't need to be changed as part of this PR. Please revert all of those (did you maybe run dotnet format and include all those changes accidentally)?

[Copilot]

@copilot, you changed hundreds of files that didn't need to be changed as part of this PR. Please revert all of those (did you maybe run dotnet format and include all those changes accidentally)?

You're right, I accidentally included formatting changes from dotnet format that affected 228 unrelated files. I've reverted all of those changes and now only the files directly related to the CA1830 enhancement remain in the PR.

[Copilot]

Pull Request Overview

This PR extends the existing CA1830 analyzer to detect and optimize a new inefficient StringBuilder pattern where new string(char, int) constructor is used as an argument to StringBuilder.Append(). The optimization suggests using StringBuilder.Append(char, int) directly instead of creating an intermediate string object, improving performance by avoiding unnecessary allocations.

• Extends analyzer to detect Append(new string(char, int)) pattern
• Adds code fix to replace with direct Append(char, int) overload
• Updates documentation and localization resources across multiple languages

Reviewed Changes

Copilot reviewed 19 out of 19 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
PreferTypedStringBuilderAppendOverloadsTests.cs	Added comprehensive test cases for the new string constructor pattern detection
PreferTypedStringBuilderAppendOverloads.cs	Extended analyzer logic to detect new string(char, int) in Append calls
PreferTypedStringBuilderAppendOverloads.Fixer.cs	Added code fix to replace string constructor with direct Append overload
MicrosoftNetCoreAnalyzersResources.resx	Updated analyzer description and added new code fix message
Multiple .xlf files	Updated localization files with new resource strings
Microsoft.CodeAnalysis.NetAnalyzers.sarif	Updated analyzer description in SARIF metadata
Microsoft.CodeAnalysis.NetAnalyzers.md	Updated analyzer documentation