Fix merge conflict checker for empty file lists and filter *.cs files (#26365)

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: TravisEz13 <10873629+TravisEz13@users.noreply.github.com>

.github/actions/infrastructure/merge-conflict-checker/README.md

@@ -56,8 +56,10 @@ jobs:
 - **File Handling**:
   - Checks only files that were added, modified, or renamed
   - Skips deleted files
+  - **Filters out `*.cs` files** (C# files are excluded from merge conflict checking)
   - Skips binary/unreadable files
   - Skips directories
+- **Empty File List**: Gracefully handles cases where no files need checking (e.g., PRs that only delete files)
 
 ## Example Output
 

.github/actions/infrastructure/merge-conflict-checker/action.yml

@@ -25,7 +25,8 @@ runs:
       run: |
         # Get changed files from environment variable (secure against injection)
         $changedFilesJson = $env:CHANGED_FILES_JSON
-        $changedFiles = $changedFilesJson | ConvertFrom-Json
+        # Ensure we always have an array (ConvertFrom-Json returns null for empty JSON arrays)
+        $changedFiles = @($changedFilesJson | ConvertFrom-Json)
 
         # Import ci.psm1 and run the check
         Import-Module "$env:GITHUB_WORKSPACE/tools/ci.psm1" -Force

test/infrastructure/ciModule.Tests.ps1

@@ -30,11 +30,17 @@ Describe "Test-MergeConflictMarker" {
     }
 
     Context "When no files are provided" {
-        It "Should handle empty file array" {
-            # The function parameter has Mandatory validation which rejects empty arrays by design
-            # This test verifies that behavior
+        It "Should handle empty file array gracefully" {
+            # The function now accepts empty arrays to handle cases like delete-only PRs
             $emptyArray = @()
-            { Test-MergeConflictMarker -File $emptyArray -WorkspacePath $script:testWorkspace -OutputPath $script:testOutputPath -SummaryPath $script:testSummaryPath } | Should -Throw -ExpectedMessage "*empty array*"
+            Test-MergeConflictMarker -File $emptyArray -WorkspacePath $script:testWorkspace -OutputPath $script:testOutputPath -SummaryPath $script:testSummaryPath
+            
+            $outputs = Get-Content $script:testOutputPath
+            $outputs | Should -Contain "files-checked=0"
+            $outputs | Should -Contain "conflicts-found=0"
+            
+            $summary = Get-Content $script:testSummaryPath -Raw
+            $summary | Should -Match "No Files to Check"
         }
     }
 

tools/ci.psm1

@@ -1039,8 +1039,9 @@ Function Test-MergeConflictMarker
     #>
     [CmdletBinding()]
     param(
-        [Parameter(Mandatory)]
-        [string[]] $File,
+        [Parameter()]
+        [AllowEmptyCollection()]
+        [string[]] $File = @(),
 
         [Parameter()]
         [string] $WorkspacePath = $PWD,
@@ -1054,10 +1055,63 @@ Function Test-MergeConflictMarker
 
     Write-Host "Starting merge conflict marker check..." -ForegroundColor Cyan
 
-    Write-Host "Checking $($File.Count) changed files for merge conflict markers" -ForegroundColor Cyan
+    # Helper function to write outputs when no files to check
+    function Write-NoFilesOutput {
+        param(
+            [string]$Message,
+            [string]$OutputPath,
+            [string]$SummaryPath
+        )
+        
+        # Output results to GitHub Actions
+        if ($OutputPath) {
+            "files-checked=0" | Out-File -FilePath $OutputPath -Append -Encoding utf8
+            "conflicts-found=0" | Out-File -FilePath $OutputPath -Append -Encoding utf8
+        }
+        
+        # Create GitHub Actions job summary
+        if ($SummaryPath) {
+            $summaryContent = @"
+# Merge Conflict Marker Check Results
+
+## Summary
+- **Files Checked:** 0
+- **Files with Conflicts:** 0
+
+## ℹ️ No Files to Check
+
+$Message
+
+"@
+            $summaryContent | Out-File -FilePath $SummaryPath -Encoding utf8
+        }
+    }
+
+    # Handle empty file list (e.g., when PR only deletes files)
+    if ($File.Count -eq 0) {
+        Write-Host "No files to check (empty file list)" -ForegroundColor Yellow
+        Write-NoFilesOutput -Message "No files were provided for checking (this can happen when a PR only deletes files)." -OutputPath $OutputPath -SummaryPath $SummaryPath
+        return
+    }
+
+    # Filter out *.cs files from merge conflict checking
+    $filesToCheck = @($File | Where-Object { $_ -notlike "*.cs" })
+    $filteredCount = $File.Count - $filesToCheck.Count
+    
+    if ($filteredCount -gt 0) {
+        Write-Host "Filtered out $filteredCount *.cs file(s) from merge conflict checking" -ForegroundColor Yellow
+    }
+    
+    if ($filesToCheck.Count -eq 0) {
+        Write-Host "No files to check after filtering (all files were *.cs)" -ForegroundColor Yellow
+        Write-NoFilesOutput -Message "All $filteredCount file(s) were filtered out (*.cs files are excluded from merge conflict checking)." -OutputPath $OutputPath -SummaryPath $SummaryPath
+        return
+    }
+
+    Write-Host "Checking $($filesToCheck.Count) changed files for merge conflict markers" -ForegroundColor Cyan
 
     # Convert relative paths to absolute paths for processing
-    $absolutePaths = $File | ForEach-Object {
+    $absolutePaths = $filesToCheck | ForEach-Object {
         if ([System.IO.Path]::IsPathRooted($_)) {
             $_
         } else {