Make Export-Csv -Append and -NoHeader mutually exclusive (#26472)

src/Microsoft.PowerShell.Commands.Utility/commands/utility/CsvCommands.cs

@@ -270,6 +270,14 @@ namespace Microsoft.PowerShell.Commands
                 this.ThrowTerminatingError(errorRecord);
             }
 
+            // Validate that Append and NoHeader are not specified together.
+            if (Append && NoHeader)
+            {
+                InvalidOperationException exception = new(CsvCommandStrings.CannotSpecifyAppendAndNoHeader);
+                ErrorRecord errorRecord = new(exception, "CannotSpecifyBothAppendAndNoHeader", ErrorCategory.InvalidData, null);
+                this.ThrowTerminatingError(errorRecord);
+            }
+
             _shouldProcess = ShouldProcess(Path);
             if (!_shouldProcess)
             {

src/Microsoft.PowerShell.Commands.Utility/resources/CsvCommandStrings.resx

@@ -157,4 +157,7 @@
   <data name="EOFIsReached" xml:space="preserve">
     <value>EOF is reached.</value>
   </data>
+  <data name="CannotSpecifyAppendAndNoHeader" xml:space="preserve">
+    <value>You must specify either the -Append or -NoHeader parameters, but not both.</value>
+  </data>
 </root>

test/powershell/Modules/Microsoft.PowerShell.Utility/Export-Csv.Tests.ps1

@@ -187,6 +187,10 @@ Describe "Export-Csv" -Tags "CI" {
         $results[1].PSObject.properties.Name | Should -Not -Contain 'third'
     }
 
+    It "Should throw when -Append and -NoHeader are specified together" {
+        { $P1 | Export-Csv -Path $testCsv -Append -NoHeader -ErrorAction Stop } | Should -Throw -ErrorId "CannotSpecifyBothAppendAndNoHeader,Microsoft.PowerShell.Commands.ExportCsvCommand"
+    }
+
     It "First line should be #TYPE if -IncludeTypeInformation used and pstypenames object property is empty" {
         $object = [PSCustomObject]@{first = 1}
         $pstypenames = $object.pstypenames | ForEach-Object -Process {$_}