From 8948c417c0ceb0a4f954b51599294ac06881eb5b Mon Sep 17 00:00:00 2001
From: Yoshifumi <ytsuda@gmail.com>
Date: Sat, 22 Nov 2025 16:38:35 +0900
Subject: [PATCH] Make Export-Csv -Append and -NoHeader mutually exclusive
 (#26472)

---
 .../commands/utility/CsvCommands.cs                       | 8 ++++++++
 .../resources/CsvCommandStrings.resx                      | 3 +++
 .../Microsoft.PowerShell.Utility/Export-Csv.Tests.ps1     | 4 ++++
 3 files changed, 15 insertions(+)

diff --git a/src/Microsoft.PowerShell.Commands.Utility/commands/utility/CsvCommands.cs b/src/Microsoft.PowerShell.Commands.Utility/commands/utility/CsvCommands.cs
index fa49c33d2a..e84a79b99b 100644
--- a/src/Microsoft.PowerShell.Commands.Utility/commands/utility/CsvCommands.cs
+++ b/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)
             {
diff --git a/src/Microsoft.PowerShell.Commands.Utility/resources/CsvCommandStrings.resx b/src/Microsoft.PowerShell.Commands.Utility/resources/CsvCommandStrings.resx
index 0eff0d6f84..8c5ded1346 100644
--- a/src/Microsoft.PowerShell.Commands.Utility/resources/CsvCommandStrings.resx
+++ b/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>
diff --git a/test/powershell/Modules/Microsoft.PowerShell.Utility/Export-Csv.Tests.ps1 b/test/powershell/Modules/Microsoft.PowerShell.Utility/Export-Csv.Tests.ps1
index db5c6aff9e..ea05d66d39 100644
--- a/test/powershell/Modules/Microsoft.PowerShell.Utility/Export-Csv.Tests.ps1
+++ b/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 = $object.pstypenames | ForEach-Object -Process {$_}