Instruction file imported from hebelmx/Veriqan (.cursor/rules/1033_EXXER200NullParameterRefactoring.mdc). Copyright stays with the author.

Rule: EXXER200 Null Parameter Validation Refactoring

Objective

Establish a systematic process for refactoring classes marked with // EXXER200: Validate null parameters at method entry comments to use the standardized Result<T> pattern with existing infrastructure (MultipleNullArgumentsError, NullArgumentValidation, and ResultExtensions).

Core Principles

1. Systematic Approach

• Use Serena MCP Server: Leverage mcp_serena_* tools for all file operations
• Use RefactorMCP Server: Utilize mcp_refactorMcp_* tools for automated refactoring
• Use Sequential Thinking: Apply mcp_sequential-thinking_sequentialthinking for complex scenarios
• Preserve Behavior: Maintain existing error messages and logging

2. Existing Infrastructure Usage

• MultipleNullArgumentsError: For handling multiple null parameters
• NullArgumentValidation: Static factory methods for Result creation
• ResultExtensions: Extension methods like ValidateNotNull, FailForNullArguments
• Result Pattern: Consistent error handling approach

Discovery Phase

1. Search for EXXER200 Instances

Use Serena MCP to Find All EXXER200 Comments

# Step 1: Activate the project
mcp_serena_activate_project("ExxerAI")

# Step 2: Search for EXXER200 pattern
mcp_serena_search_for_pattern(
    substring_pattern="// EXXER200: Validate null parameters at method entry",
    relative_path=".",
    restrict_search_to_code_files=true
)

# Step 3: Search for shortened EXXER200 pattern
mcp_serena_search_for_pattern(
    substring_pattern="// EXXER200",
    relative_path=".",
    restrict_search_to_code_files=true
)

Catalog and Classify Each Instance

# For each file found, read and analyze the methods
mcp_serena_read_file(relative_path="path/to/file.cs")

# Get symbols overview to understand method structure
mcp_serena_get_symbols_overview(relative_path="path/to/file.cs")

2. Classification Criteria

Group EXXER200 instances by:

• Return Type: ValidationDocumentResult, Result<T>, Result, primitives, void
• Parameter Count: Single vs multiple null checks
• Method Visibility: Public vs private methods
• Error Handling: Logging vs simple returns

Refactoring Templates

1. ValidationDocumentResult Methods

Before (Current Pattern)

public ValidationDocumentResult Validate(ParsedStatement statement, ArtifactSet templates)
{
    // EXXER200: Validate null parameters at method entry
    if (statement == null)
    {
        _logger.LogWarning(ImageConstants.LogNullStatementWarning);
        return ValidationDocumentResult.WithErrors([ImageConstants.NullStatementError]);
    }
    if (templates == null)
    {
        _logger.LogWarning(ImageConstants.LogNullTemplatesWarning);
        return ValidationDocumentResult.WithErrors([ImageConstants.NullTemplatesError]);
    }
    // ... rest of method
}

After (Using Result Pattern)

public ValidationDocumentResult Validate(ParsedStatement statement, ArtifactSet templates)
{
    // EXXER200: Validate null parameters at method entry using Result<T> pattern
    var validation = ResultExtensions.ValidateNotNull(
        (statement, nameof(statement)),
        (templates, nameof(templates))
    );
    if (validation.IsFailure)
    {
        _logger.LogWarning("Null parameter validation failed: {Errors}", string.Join(", ", validation.Errors));
        return ValidationDocumentResult.WithErrors(validation.Errors);
    }
    // ... rest of method
}

RefactorMCP Command for ValidationDocumentResult

mcp_refactorMcp_replace_regex(
    solutionPath="path/to/solution.sln",
    filePath="path/to/file.cs",
    regex="// EXXER200: Validate null parameters at method entry\\s*\\n(\\s*if \\([^)]+\\s*==\\s*null\\)\\s*\\{[^}]+\\}\\s*\\n?)+",
    repl="// EXXER200: Validate null parameters at method entry using Result<T> pattern\\nvar validation = ResultExtensions.ValidateNotNull(\\n    (statement, nameof(statement)),\\n    (templates, nameof(templates))\\n);\\nif (validation.IsFailure)\\n{\\n    _logger.LogWarning(\"Null parameter validation failed: {Errors}\", string.Join(\", \", validation.Errors));\\n    return ValidationDocumentResult.WithErrors(validation.Errors);\\n}"
)

2. Result Methods

Before (Current Pattern)

public Result<ProcessedData> ProcessData(InputData data, Configuration config)
{
    // EXXER200: Validate null parameters at method entry
    if (data == null)
        return Result<ProcessedData>.WithFailure("Data parameter cannot be null");
    if (config == null)
        return Result<ProcessedData>.WithFailure("Config parameter cannot be null");
    // ... rest of method
}

After (Using NullArgumentValidation)

public Result<ProcessedData> ProcessData(InputData data, Configuration config)
{
    // EXXER200: Validate null parameters at method entry using Result<T> pattern
    var validation = ResultExtensions.ValidateNotNull(
        (data, nameof(data)),
        (config, nameof(config))
    );
    if (validation.IsFailure)
    {
        return Result<ProcessedData>.WithFailure(validation.Errors);
    }
    // ... rest of method
}

Alternative Using NullArgumentValidation Factory

public Result<ProcessedData> ProcessData(InputData data, Configuration config)
{
    // EXXER200: Validate null parameters at method entry using Result<T> pattern
    if (data == null || config == null)
    {
        var nullParams = new List<string>();
        if (data == null) nullParams.Add(nameof(data));
        if (config == null) nullParams.Add(nameof(config));
        return NullArgumentValidation.Failure<ProcessedData>(nullParams.ToArray());
    }
    // ... rest of method
}

3. Non-Generic Result Methods

Before (Current Pattern)

public Result UpdateConfiguration(Configuration config)
{
    // EXXER200: Validate null parameters at method entry
    if (config == null)
        return Result.WithFailure("Configuration cannot be null");
    // ... rest of method
}

After (Using NullArgumentValidation)

public Result UpdateConfiguration(Configuration config)
{
    // EXXER200: Validate null parameters at method entry using Result pattern
    if (config == null)
        return NullArgumentValidation.Failure(nameof(config));
    // ... rest of method
}

4. Private Methods with Early Returns

Before (Current Pattern)

private void ValidatePromotionImage(PromotionImage promotionImage, List<PromotionTemplate> templates, List<string> errors, List<string> warnings)
{
    // EXXER200: Validate null parameters at method entry
    if (promotionImage == null)
        return;
    if (templates == null)
        return;
    if (errors == null)
        return;
    if (warnings == null)
        return;
    // ... rest of method
}

After (Consistent Pattern)

private void ValidatePromotionImage(PromotionImage promotionImage, List<PromotionTemplate> templates, List<string> errors, List<string> warnings)
{
    // EXXER200: Validate null parameters at method entry - early returns for private helper method
    var validation = ResultExtensions.ValidateNotNull(
        (promotionImage, nameof(promotionImage)),
        (templates, nameof(templates)),
        (errors, nameof(errors)),
        (warnings, nameof(warnings))
    );
    if (validation.IsFailure)
        return; // Early return for private helper method
    // ... rest of method
}

5. Methods Returning Primitives

Before (Current Pattern)

public bool ValidateImageQuality(PromotionImage promotionImage)
{
    // EXXER200: Validate null parameters at method entry
    if (promotionImage == null)
        return false;
    // ... rest of method
}

Option 1: Keep Primitive Return (Simple)

public bool ValidateImageQuality(PromotionImage promotionImage)
{
    // EXXER200: Validate null parameters at method entry - primitive return
    if (promotionImage == null)
        return false; // Consistent with existing behavior
    // ... rest of method
}

Option 2: Change to Result (Recommended)

public Result<bool> ValidateImageQuality(PromotionImage promotionImage)
{
    // EXXER200: Validate null parameters at method entry using Result<T> pattern
    if (promotionImage == null)
        return NullArgumentValidation.Failure<bool>(nameof(promotionImage));
    // ... rest of method implementation
    return Result<bool>.Success(qualityCheckResult);
}

RefactorMCP Implementation Guide

1. Systematic Refactoring Process

Step 1: Load Solution and Find Symbols

# Load the solution
mcp_refactorMcp_load_solution(solutionPath="/absolute/path/to/solution.sln")

# Find all methods with EXXER200 comments
mcp_serena_find_symbol(
    name_path="EXXER200",
    relative_path=".",
    substring_matching=true
)

Step 2: Group by Refactoring Pattern

Use Sequential Thinking to plan refactoring:

mcp_sequential-thinking_sequentialthinking(
    thought="Analyze the EXXER200 instances found and group them by return type and complexity",
    thoughtNumber=1,
    totalThoughts=5,
    nextThoughtNeeded=true
)

Step 3: Apply Refactoring Templates

For each group, apply appropriate RefactorMCP commands:

ValidationDocumentResult Methods

mcp_refactorMcp_replace_regex(
    solutionPath="/path/to/solution.sln",
    filePath="relative/path/to/file.cs",
    regex="// EXXER200: Validate null parameters at method entry\\s*\\n(\\s*if \\([^)]+\\s*==\\s*null\\)\\s*\\{[^}]+return ValidationDocumentResult\\.WithErrors\\([^}]+\\}\\s*\\n?)+",
    repl="// EXXER200: Validate null parameters at method entry using Result<T> pattern\\nvar validation = ResultExtensions.ValidateNotNull(PARAMETERS_HERE);\\nif (validation.IsFailure)\\n{\\n    _logger.LogWarning(\"Null parameter validation failed: {Errors}\", string.Join(\", \", validation.Errors));\\n    return ValidationDocumentResult.WithErrors(validation.Errors);\\n}"
)

Result Methods

mcp_refactorMcp_replace_regex(
    solutionPath="/path/to/solution.sln",
    filePath="relative/path/to/file.cs",
    regex="// EXXER200: Validate null parameters at method entry\\s*\\n(\\s*if \\([^)]+\\s*==\\s*null\\)\\s*\\n?\\s*return Result<[^>]+>\\.WithFailure\\([^;]+;\\s*\\n?)+",
    repl="// EXXER200: Validate null parameters at method entry using Result<T> pattern\\nvar validation = ResultExtensions.ValidateNotNull(PARAMETERS_HERE);\\nif (validation.IsFailure)\\n{\\n    return Result<T>.WithFailure(validation.Errors);\\n}"
)

2. Manual Parameter Replacement

For each refactored method, manually replace PARAMETERS_HERE with:

(parameter1, nameof(parameter1)),
(parameter2, nameof(parameter2)),
// ... for each parameter that was null-checked

3. Method Signature Changes (When Needed)

For methods that should return Result<T> instead of primitives:

# Change method signature
mcp_refactorMcp_rename_symbol(
    solutionPath="/path/to/solution.sln",
    filePath="relative/path/to/file.cs",
    oldName="public bool MethodName",
    newName="public Result<bool> MethodName"
)

# Update method body to return Result<T>
mcp_refactorMcp_replace_regex(
    solutionPath="/path/to/solution.sln",
    filePath="relative/path/to/file.cs",
    regex="return (true|false);",
    repl="return Result<bool>.Success($1);"
)

Quality Assurance Checklist

1. Validation After Refactoring

Verify All EXXER200 Comments Are Addressed

# Search for any remaining unrefactored EXXER200 comments
mcp_serena_search_for_pattern(
    substring_pattern="// EXXER200:",
    relative_path=".",
    restrict_search_to_code_files=true
)

Check for Compilation Errors

# Build the solution to check for errors
mcp_serena_execute_shell_command(
    command="dotnet build",
    cwd="path/to/solution/directory"
)

2. Behavioral Validation

Ensure Error Messages Are Preserved

• Before: Custom error messages in individual null checks
• After: Error messages from MultipleNullArgumentsError.ToString()
• Action: Verify that the new error messages provide equivalent information

Verify Logging Is Maintained

• ValidationDocumentResult methods: Should include logging of validation failures
• Other methods: Logging behavior should be preserved or improved

Check Return Type Consistency

• ValidationDocumentResult: Should use ValidationDocumentResult.WithErrors(validation.Errors)
• Result: Should use Result<T>.WithFailure(validation.Errors)
• Result: Should use Result.WithFailure(validation.Errors)

3. Integration Testing

Test Null Parameter Scenarios

[Test]
public void Method_Should_ReturnFailure_When_ParameterIsNull()
{
    // Arrange
    var service = new ServiceUnderTest();

    // Act
    var result = service.MethodWithNullValidation(null);

    // Assert
    result.IsFailure.ShouldBeTrue();
    result.Errors.ShouldContain(error => error.Contains("parameter"));
}

Advanced Scenarios

1. Complex Methods with Mixed Validation

For methods that have both null validation and business logic validation:

Before

public Result<ProcessedData> ComplexMethod(InputData data, Configuration config)
{
    // EXXER200: Validate null parameters at method entry
    if (data == null)
        return Result<ProcessedData>.WithFailure("Data cannot be null");
    if (config == null)
        return Result<ProcessedData>.WithFailure("Config cannot be null");

    // Business validation
    if (!data.IsValid)
        return Result<ProcessedData>.WithFailure("Invalid data format");

    // ... processing logic
}

After

public Result<ProcessedData> ComplexMethod(InputData data, Configuration config)
{
    // EXXER200: Validate null parameters at method entry using Result<T> pattern
    var nullValidation = ResultExtensions.ValidateNotNull(
        (data, nameof(data)),
        (config, nameof(config))
    );
    if (nullValidation.IsFailure)
        return Result<ProcessedData>.WithFailure(nullValidation.Errors);

    // Business validation
    if (!data.IsValid)
        return Result<ProcessedData>.WithFailure("Invalid data format");

    // ... processing logic
}

2. Async Methods

For async methods, the pattern remains the same:

public async Task<Result<ProcessedData>> ProcessDataAsync(InputData data, CancellationToken cancellationToken = default)
{
    // EXXER200: Validate null parameters at method entry using Result<T> pattern
    var validation = ResultExtensions.ValidateNotNull(
        (data, nameof(data))
    );
    if (validation.IsFailure)
        return Result<ProcessedData>.WithFailure(validation.Errors);

    // ... async processing logic
}

3. Methods with Custom Error Messages

When preserving specific error messages:

public Result<ProcessedData> ProcessData(InputData data, Configuration config)
{
    // EXXER200: Validate null parameters at method entry using Result<T> pattern
    if (data == null)
        return Result<ProcessedData>.WithFailure("InputData is required for processing");
    if (config == null)
        return Result<ProcessedData>.WithFailure("Configuration must be provided");

    // ... rest of method
}

Sequential Thinking Integration

For Complex Refactoring Decisions

Use Sequential Thinking when encountering complex scenarios:

mcp_sequential-thinking_sequentialthinking(
    thought="I found a method with EXXER200 that returns a complex custom type and has intricate error handling. Should I change it to Result<T> or keep the existing pattern?",
    thoughtNumber=1,
    totalThoughts=3,
    nextThoughtNeeded=true
)

Questions to Consider in Sequential Thinking

1. Return Type Impact: Will changing the return type break existing callers?
2. Error Message Preservation: Are the current error messages more specific than what MultipleNullArgumentsError provides?
3. Logging Requirements: Does the method need to maintain specific logging behavior?
4. Performance Considerations: Is this a high-frequency method where the validation overhead matters?
5. API Consistency: Should this method match the pattern of similar methods in the same class?

Enforcement and Maintenance

1. Code Review Checklist

• All EXXER200 comments have been addressed
• Appropriate Result<T> pattern is used for the return type
• MultipleNullArgumentsError and related infrastructure is used
• Error messages provide adequate information
• Logging behavior is preserved where appropriate
• No compilation errors introduced
• Unit tests pass and cover null validation scenarios

2. Static Analysis Rules

Consider adding analyzer rules to detect:

• Remaining EXXER200 comments
• Manual null checks that could use the standardized pattern
• Methods that should return Result<T> but don't

3. Documentation Updates

Update method documentation to reflect:

• New return types (if changed)
• Error conditions using Result<T> pattern
• Parameter validation behavior

Future Improvements

1. Automated Code Generation

Consider creating code generators for:

• Automatic null validation injection
• Result<T> wrapper generation
• Test case generation for null scenarios

2. Analyzer Development

Develop custom analyzers to:

• Detect EXXER200 patterns automatically
• Suggest refactoring to Result<T> pattern
• Validate proper usage of null validation infrastructure

3. Refactoring Tool Enhancement

Enhance RefactorMCP with:

• EXXER200-specific refactoring commands
• Automated parameter extraction for validation calls
• Return type conversion assistance

Conclusion

This rule provides a comprehensive framework for systematically refactoring EXXER200 null parameter validation patterns using the existing Result<T> infrastructure. By following this approach, teams can:

• Maintain Consistency: All null validation follows the same pattern
• Leverage Infrastructure: Use existing MultipleNullArgumentsError and NullArgumentValidation
• Preserve Behavior: Keep existing error handling and logging
• Enable Automation: Use RefactorMCP for systematic refactoring
• Ensure Quality: Follow validation and testing guidelines

The systematic approach ensures that all EXXER200 instances are properly addressed while maintaining code quality and consistency across the codebase.