PowerShell · SufficientDaikon · Mar 28, 2026 · Mar 28, 2026 · Mar 28, 2026 · Mar 28, 2026
@@ -23,4 +23,10 @@
             <supportedOS Id="{35138b9a-5d96-4fbd-8e2d-a2440225f93a}"/>
         </application>
     </compatibility>
+    <!-- The asm.v3 xmlns is required for windowsSettings; it is distinct from the root asm.v1 namespace. -->
+    <application xmlns="urn:schemas-microsoft-com:asm.v3">
+        <windowsSettings>
+            <consoleAllocationPolicy xmlns="http://schemas.microsoft.com/SMI/2024/WindowsSettings">detached</consoleAllocationPolicy>
+        </windowsSettings>
+    </application>
 </assembly>
@@ -420,7 +420,14 @@ internal enum KeyboardFlag : uint
         internal static void SetConsoleMode(ProcessWindowStyle style)
         {
             IntPtr hwnd = GetConsoleWindow();
-            Dbg.Assert(hwnd != IntPtr.Zero, "Console handle should never be zero");
+            if (hwnd == IntPtr.Zero)
+            {
+                // No console window to modify. This can happen when running with
+                // consoleAllocationPolicy=detached before the console is allocated,
+                // or in a GUI-hosted scenario.
+                return;
+            }
+
             switch (style)
             {
                 case ProcessWindowStyle.Hidden:

@@ -51,6 +51,15 @@ public static int Start([MarshalAs(UnmanagedType.LPArray, ArraySubType = Unmanag
         {
             ArgumentNullException.ThrowIfNull(args);
 
+#if !UNIX
+            // On Windows with consoleAllocationPolicy=detached in the manifest,
+            // no console is auto-allocated by the OS. We must allocate one ourselves
+            // before anything touches CONOUT$/CONIN$ handles.
+            // On older Windows the manifest element is ignored and the OS auto-allocates
+            // a console, so GetConsoleWindow() != 0 and EarlyConsoleInit is a no-op.
+            EarlyConsoleInit(args);
+#endif
+
 #if DEBUG
             if (args.Length > 0 && !string.IsNullOrEmpty(args[0]) && args[0]!.Equals("-isswait", StringComparison.OrdinalIgnoreCase))
             {
@@ -120,5 +129,95 @@ public static int Start([MarshalAs(UnmanagedType.LPArray, ArraySubType = Unmanag
 
             return exitCode;
         }
+
+#if !UNIX
+        /// <summary>
+        /// Allocates a console early in startup to support consoleAllocationPolicy=detached.
+        /// On newer Windows (with the detached policy active), the OS does not auto-allocate
+        /// a console for CUI apps. On older Windows the manifest is ignored, so the OS
+        /// auto-allocates a console and GetConsoleWindow() returns non-zero (early return).
+        /// </summary>
+        private static void EarlyConsoleInit(string[] args)
+        {
+            nint existingConsole = Interop.Windows.GetConsoleWindow();
+            if (existingConsole != nint.Zero)
+            {
+                // Console already exists (inherited from parent or auto-allocated on older Windows).
+                // Leave -WindowStyle handling to the existing SetConsoleMode code path.
+                return;
+            }
+
+            // No console exists (GetConsoleWindow() == 0). This means either:
+            //   (a) The detached manifest policy is active (newer Windows), or
+            //   (b) DETACHED_PROCESS — no console at all, or
+            //   (c) CREATE_NO_WINDOW — console session exists but no window.
+            //
+            // For (c), AllocConsoleWithOptions returns ExistingConsole (no-op).
+            // For (a) and (b), behavior depends on the mode:
+            //   Default mode: allocates if the parent would have given us a console
+            //     on prior Windows versions, returns NoConsole for DETACHED_PROCESS.
+            //   NoWindow mode: always creates a console session (overrides DETACHED).
+            //
+            // When -WindowStyle Hidden is specified, we intentionally use NoWindow
+            // even though it overrides DETACHED_PROCESS — the user explicitly asked
+            // for invisible PowerShell with working I/O, and the alternative (no
+            // console, crashing on stdin/stdout access) is strictly worse.
+            //
+            // If the API is not available (older Windows), TryAlloc* returns false.
+            // On older Windows the manifest is ignored and the OS auto-allocates
+            // a console, so GetConsoleWindow() would have returned non-zero above.
+            // The only older-Windows path here is DETACHED_PROCESS, where the
+            // existing behavior is no console I/O; we preserve that by not
+            // falling back to plain AllocConsole() (per DHowett's guidance).
+            if (EarlyCheckForHiddenWindowStyle(args))
+            {
+                Interop.Windows.TryAllocConsoleNoWindow();
+            }
+            else
+            {
+                Interop.Windows.TryAllocConsoleDefault();
+            }
+        }
+
+        /// <summary>
+        /// Minimal early scan for -WindowStyle Hidden in command line args.
+        /// Matches any unambiguous prefix of "windowstyle" starting from "w"
+        /// (e.g. -w, -wi, -win, ..., -windowstyle, --windowstyle) followed by "hidden".
+        /// This is a best-effort check that runs before the full parser. False positives
+        /// (e.g. a hypothetical future -w parameter) are acceptable because the worst case
+        /// is allocating a hidden console that the full parser would later show.
+        /// </summary>
+        private static bool EarlyCheckForHiddenWindowStyle(string[] args)
+        {
+            for (int i = 0; i < args.Length - 1; i++)
+            {
+                string arg = args[i];
+                if (arg.Length >= 2 && (arg[0] == '-' || arg[0] == '/'))
+                {
+                    int start = 1;
+
+                    // Strip second dash for --windowstyle (matches full parser behavior).
+                    if (arg.Length >= 3 && arg[0] == '-' && arg[1] == '-')
+                    {
+                        start = 2;
+                    }
+
+                    ReadOnlySpan<char> key = arg.AsSpan(start);
+                    if (key.Length >= 1
+                        && key.Length <= "windowstyle".Length
+                        && "windowstyle".AsSpan().StartsWith(key, StringComparison.OrdinalIgnoreCase))
+                    {
+                        if (args[i + 1].Equals("hidden", StringComparison.OrdinalIgnoreCase))
+                        {
+                            return true;
+                        }
+                    }
+                }
+            }
+
+            return false;
+        }
+
+#endif
     }
 }
@@ -0,0 +1,101 @@
+// Copyright (c) Microsoft Corporation.
+// Licensed under the MIT License.
+
+#nullable enable
+
+using System;
+using System.Runtime.InteropServices;
+
+internal static partial class Interop
+{
+    internal static unsafe partial class Windows
+    {
+        /// <summary>Console allocation mode for AllocConsoleWithOptions.</summary>
+        internal enum AllocConsoleMode : int
+        {
+            /// <summary>Allocate only if the parent process requested it.</summary>
+            Default = 0,
+
+            /// <summary>Force allocation of a console with a visible window.</summary>
+            NewWindow = 1,
+
+            /// <summary>Allocate console I/O handles without creating a visible window.</summary>
+            NoWindow = 2,
+        }
+
+        /// <summary>Result of an AllocConsoleWithOptions call.</summary>
+        internal enum AllocConsoleResult : int
+        {
+            /// <summary>No console was allocated.</summary>
+            NoConsole = 0,
+
+            /// <summary>A new console session was created.</summary>
+            NewConsole = 1,
+
+            /// <summary>An existing console session was attached.</summary>
+            ExistingConsole = 2,
+        }
+
+        /// <summary>Options struct passed to AllocConsoleWithOptions.</summary>
+        [StructLayout(LayoutKind.Sequential)]
+        internal struct AllocConsoleOptions
+        {
+            /// <summary>The allocation mode (Default, NewWindow, or NoWindow).</summary>
+            public AllocConsoleMode Mode;
+
+            /// <summary>If non-zero, the ShowWindow field is used as the initial show state.</summary>
+            public int UseShowWindow;
+
+            /// <summary>The initial show state (e.g. SW_HIDE) when UseShowWindow is set.</summary>
+            public ushort ShowWindow;
+        }
+
+        [LibraryImport("kernel32.dll")]
+        internal static partial int AllocConsoleWithOptions(
+            ref AllocConsoleOptions allocOptions,
+            out AllocConsoleResult result);
+
+        /// <summary>
+        /// Attempts to allocate a console without a visible window using AllocConsoleWithOptions.
+        /// Returns false if the API is not available (older Windows), the call fails,
+        /// or no console was allocated (e.g. process was started with DETACHED_PROCESS).
+        /// </summary>
+        internal static bool TryAllocConsoleNoWindow()
+        {
+            return TryAllocConsoleWithMode(AllocConsoleMode.NoWindow);
+        }
+
+        /// <summary>
+        /// Attempts to allocate a console using AllocConsoleWithOptions with Default mode.
+        /// Default mode respects DETACHED_PROCESS from the parent's CreateProcess call:
+        /// it returns NoConsole if the parent intended this process to run without a console,
+        /// whereas plain AllocConsole() would override that and force-create a console.
+        /// Returns false if the API is not available (older Windows), the call fails,
+        /// or no console was allocated.
+        /// </summary>
+        internal static bool TryAllocConsoleDefault()
+        {
+            return TryAllocConsoleWithMode(AllocConsoleMode.Default);
+        }
+
+        private static bool TryAllocConsoleWithMode(AllocConsoleMode mode)
+        {
+            try
+            {
+                var options = new AllocConsoleOptions
+                {
+                    Mode = mode,
+                    UseShowWindow = 0,
+                    ShowWindow = 0,
+                };
+
+                int hr = AllocConsoleWithOptions(ref options, out AllocConsoleResult result);
+                return hr >= 0 && result != AllocConsoleResult.NoConsole;
+            }
+            catch (EntryPointNotFoundException)
+            {
+                return false;
+            }
+        }
+    }
+}
@@ -2489,30 +2489,32 @@ internal static bool AllocateHiddenConsole()
             // save the foreground window since allocating a console window might remove focus from it
             IntPtr savedForeground = Interop.Windows.GetForegroundWindow();
 
-            // Since there is no console window, allocate and then hide it...
-            // Suppress the PreFAST warning about not using Marshal.GetLastWin32Error() to
-            // get the error code.
-            Interop.Windows.AllocConsole();
-            hwnd = Interop.Windows.GetConsoleWindow();
+            // Try AllocConsoleWithOptions with NoWindow mode first to avoid the flash
+            // that the AllocConsole() + ShowWindow(SW_HIDE) pattern causes.
+            bool allocated = Interop.Windows.TryAllocConsoleNoWindow();
 
-            bool returnValue;
-            if (hwnd == nint.Zero)
+            if (!allocated)
             {
-                returnValue = false;
-            }
-            else
-            {
-                returnValue = true;
+                // Fallback for older Windows: allocate and then hide.
+                Interop.Windows.AllocConsole();
+                hwnd = Interop.Windows.GetConsoleWindow();
+                if (hwnd == nint.Zero)
+                {
+                    return false;
+                }
+
                 Interop.Windows.ShowWindow(hwnd, Interop.Windows.SW_HIDE);
-                AlwaysCaptureApplicationIO = true;
             }
 
+            AlwaysCaptureApplicationIO = true;
+
+            // Restore foreground window if focus changed during console allocation.
             if (savedForeground != nint.Zero && Interop.Windows.GetForegroundWindow() != savedForeground)
             {
                 Interop.Windows.SetForegroundWindow(savedForeground);
             }
 
-            return returnValue;
+            return true;
 #endif
         }
     }