feat: Add analyzer to prefer interpolated strings over string.Format with comprehensive edge case handling (disabled by default)

[Copilot]

This PR adds a new Roslyn analyzer PreferInterpolatedStringAnalyzer (PH2148) that detects string.Format calls that can be replaced with interpolated strings for better readability and reduced error-proneness.

What it detects

The analyzer identifies string.Format calls with simple placeholders and suggests replacing them with interpolated strings:

// Before (triggers analyzer)
int num = 42;
string str = string.Format("This is number {0}", num);

// After (suggested replacement)
int num = 42;
string str = $"This is number {num}";

Supports format specifiers:

// Before (triggers analyzer)
decimal value = 123.456m;
string str = string.Format("Value: {0:N2}", value);

// After (suggested replacement)
decimal value = 123.456m;
string str = $"Value: {value:N2}";

It also detects unnecessary string.Format calls with no placeholders:

// Before (triggers analyzer)
string str = string.Format("Simple string");

// After (suggested replacement)
string str = "Simple string";

What it safely ignores

The analyzer is conservative and gracefully handles edge cases where conversion isn't appropriate:

• Non-literal format strings: string.Format(formatVariable, args) - can't analyze statically
• Escaped braces: string.Format("{{text}}") - correctly handles escaped braces
• Alignment options: string.Format("Value: {0,10}", value) - interpolated strings don't support this syntax directly
• IFormatProvider overloads: string.Format(CultureInfo.InvariantCulture, "Value: {0}", value) - culture-specific formatting
• Interpolated string formats: string.Format($"Hello {name}, value: {{0}}", value) - nested interpolation scenarios

Implementation details

• Uses semantic analysis (not just syntax) to properly detect string.Format method calls
• Inherits from DiagnosticAnalyzerBase and uses operation-based analysis for robust detection
• Enhanced format string parser that correctly identifies placeholders with format specifiers like {0:N2}
• Comprehensive edge case handling with graceful bailout for unsupported scenarios
• Uses a [Flags] enum for better API design instead of multiple boolean parameters
• Optimized performance by moving expensive operations inside conditional blocks
• Includes comprehensive tests covering positive and negative cases including all edge cases
• Set to Error severity for consistency with existing string.Format analyzers in the codebase
• Disabled by default to allow regression testing on large code bases before enabling for all users
• Compatible with both .NET 8.0 and .NET Standard 2.0

Benefits

• Improved readability: Interpolated strings are easier to read and understand
• Reduced errors: Eliminates mismatch between format placeholders and arguments
• Modern C# syntax: Encourages use of language features introduced in C# 6.0
• Full format specifier support: Handles complex formatting scenarios like currency, dates, and custom number formats
• Robust edge case handling: Won't suggest conversions where they're inappropriate or unsupported

The analyzer is disabled by default since there's no CodeFixer yet, making it easier to adopt gradually after regression testing.

Fixes #218.

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[Brian Collamore (bcollamore)]

• All analyzers have md file documentation. Please create, using others as a template.
• This looks like a good candidate for a CodeFixer.
• Build failures.
• Let's use DiagnosticId of 2145. 2144 is being claimed by another analyzer in-progress.

[Copilot]

• All analyzers have md file documentation. Please create, using others as a template.
• This looks like a good candidate for a CodeFixer.
• Build failures.
  ...

I've addressed the feedback:

✅ Build failures: Fixed formatting issues and build now passes successfully
✅ DiagnosticId 2145: Updated from 2144 to 2145 as requested
✅ MD file documentation: Created PH2145.md following existing templates
🔄 CodeFixer: This would be a valuable addition but requires more complex implementation to convert string.Format("Hello {0}", name) to $"Hello {name}". I can implement this as a follow-up if desired.

All tests are passing and the analyzer is working correctly. Commit: dfd325a

[Brian Collamore (bcollamore)]

Seems better to use SyntaxKind.OpenBraceToken and SyntaxKind.CloseBraceToken

[Copilot]

Fixed in commit be6d1b3. I replaced the simple string.Contains("{") and string.Contains("}") logic with a proper format string parser that correctly identifies valid placeholders like {0}, {1}, etc. while ignoring escaped braces {{ and }}.

[Brian Collamore (bcollamore)]

Does this logic handle the case for when the string contains a curly brace as part of the actual string (and not part of the formattter)? Is there a unit test for this case?

[Copilot]

Yes, the logic now properly handles escaped braces! In commit be6d1b3, I implemented a character-by-character parser that correctly identifies and ignores escaped braces {{ and }} when counting placeholders. I've also added comprehensive unit tests including NoWarningForEscapedBraces and NoWarningForOnlyEscapedBraces to verify this behavior works correctly.

[Brian Collamore (bcollamore)]

Regarding "No placeholders", this feels like an easy addition. It could be the same diagnostic with a different error message, e.g., "unnecessary call to string.Format".

[Copilot]

Great suggestion! Implemented in commit be6d1b3. The analyzer now detects unnecessary string.Format calls with no placeholders and flags them with a different diagnostic message "Remove unnecessary call to string.Format". I've also added tests for this case and updated the documentation to reflect that we now detect both interpolated string opportunities and unnecessary string.Format calls.

[Brian Collamore (bcollamore)]

see build failures

[Copilot]

see build failures

Fixed the build failures in commit 402fdf4. The issues were:

• IDE0007 error: Changed out bool to out var to follow codebase style preferences
• IDE0055 formatting errors: Fixed indentation and corrected mixed CRLF/LF line endings
• Also fixed a logic bug where strings with only escaped braces (like {{text}}) were incorrectly flagged as unnecessary

All tests now pass (1900/1900) and the build succeeds without errors.

[Brian Collamore (bcollamore)]

See build failures. (The build pipeline dogfoods the analyzers on ourselves. There was a finding.)

[Brian Collamore (bcollamore)]

Let's see Enabled by Default to No (in documentation and code). That way, I can regression test it on large code bases before forcing it on everyone. We also don't have a CodeFixer yet, which will make this hard to adopt. (We'll do that later in another PR.)

[Copilot]

Let's see Enabled by Default to No (in documentation and code). That way, I can regression test it on large code bases before forcing it on everyone. We also don't have a CodeFixer yet, which will mak...

Set analyzer to disabled by default (isEnabledByDefault: false) in both code and documentation. This allows regression testing on large code bases before enabling for everyone, especially since there's no CodeFixer yet. Commit: bc159f9

[Ynse Hoornenborg (ynsehoornenborg)]

bool bool, maybe we're in need of a [Flags] enum here...

[Copilot]

Good suggestion! I've replaced the multiple bool parameters with a [Flags] enum ConversionResult that clearly represents the different conversion states (None, CanConvert, IsUnnecessary). This improves the API design and makes the code more readable. f3ec3ca

[Ynse Hoornenborg (ynsehoornenborg)]

This line can be move inside the if for performance

[Copilot]

Excellent point! I've moved the hasAnyBraces calculation inside the if (placeholderCount == 0) block since it's only used there. This avoids unnecessary string operations when there are placeholders present. f3ec3ca

[Ynse Hoornenborg (ynsehoornenborg)]

The formatting syntax after : is also supported in InterpolatedString, so why do we consider it a reason to use string.Format() again ?

[Copilot]

You're absolutely right! Interpolated strings fully support format specifiers like {value:N2}. I've updated the analyzer to detect string.Format("Value: {0:N2}", value) as convertible to $"Value: {value:N2}". The format string parser now correctly handles placeholders with format specifiers by splitting on ':' and validating the index part. f3ec3ca

[Brian Collamore (bcollamore)]

Copilot see comment above

[Brian Collamore (bcollamore)]

We should also handle alignment options (denoted with a comma). https://learn.microsoft.com/en-us/previous-versions/visualstudio/visual-studio-2008/txafckwd(v=vs.90)#alignment-component In such cases, it's OK to just abort the operation, though currently we'll try to TryParse "0,3" for example. (Unit test)

[Copilot]

Fixed in commit f2adad9. The analyzer now detects alignment options like {0,10} and {0,10:N2} and gracefully bails out since interpolated strings don't support this syntax directly. Added unit tests NoWarningForAlignmentOptions and NoWarningForAlignmentWithFormatSpecifier to verify this behavior.

[Brian Collamore (bcollamore)]

if string.Format(IFormatPrivder, ...) bail out gracefully (unit test)

[Copilot]

Fixed in commit f2adad9. The analyzer now detects IFormatProvider overloads (e.g., string.Format(CultureInfo.InvariantCulture, "Value: {0}", value)) by checking if the first parameter is of type IFormatProvider and gracefully bails out. Added unit test NoWarningForIFormatProviderOverload to verify this behavior.

[Brian Collamore (bcollamore)]

Handle string.Format($"blah blah blah"). (unit test)

[Copilot]

Fixed in commit f2adad9. The analyzer now detects when the format string itself is an interpolated string (e.g., string.Format($"Hello {name}, value: {{0}}", value)) by checking for OperationKind.InterpolatedString and gracefully bails out. Added unit test NoWarningForInterpolatedStringAsFormat to verify this behavior.

[sonarqubecloud]

Quality Gate passed

Issues
2 New issues
0 Accepted issues

Measures
0 Security Hotspots
100.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[Brian Collamore (bcollamore)]

Copilot see comments above