PowerShell · Jordan Borean (jborean93) · Sep 29, 2026 · Ilya (iSazonov) · Sep 30, 2026 · Jordan Borean (jborean93)
@@ -487,10 +487,22 @@ internal static object DecodeObject(object obj, Type type)
             }
             else if (obj is PSObject && IsGenericIEnumerableOfInt(type))
             {
-                // we cannot create an instance of interface type like IEnumerable
-                // Since a Collection implements IEnumerable, falling back to use
-                // that.
-                return DecodeCollection((PSObject)obj, typeof(Collection<int>));
+                // We cannot create an instance of an interface type like IEnumerable<int>,
+                // so decode into a concrete type that implements it. EncodeObject picks
+                // the wire format from the runtime type so older servers that send an int[]
+                // using the array format while a Collection<int> uses the ArrayList format.
+                PSObject psObject = (PSObject)obj;
+                if (psObject.BaseObject is ArrayList)
+                {
+                    return DecodeCollection(psObject, typeof(Collection<int>));
+                }
+                else if (psObject.Properties[RemoteDataNameStrings.MethodArrayElements] is not null &&
+                    psObject.Properties[RemoteDataNameStrings.MethodArrayLengths] is not null)
+                {
+                    return DecodeArray(psObject, typeof(int[]));
+                }
+
+                throw RemoteHostExceptions.NewRemoteHostDataDecodingNotSupportedException(type);
             }
             else if (obj is PSObject && type == typeof(RemoteHostCall))
             {

@@ -76,8 +76,22 @@ public Collection<int> PromptForChoice(string caption,
             Collection<ChoiceDescription> choices,
             IEnumerable<int> defaultChoices)
         {
+            // The remote host encoder picks the wire format from the runtime
+            // type while the client decodes IEnumerable<int> as a
+            // Collection<int>. Normalize to Collection<int> so that arrays,
+            // lists, or any other IEnumerable<int> can round trip.
+            Collection<int> defaultChoicesCollection = null;
+            if (defaultChoices is not null)
+            {
+                defaultChoicesCollection = new Collection<int>();
+                foreach (int choice in defaultChoices)
+                {
+                    defaultChoicesCollection.Add(choice);
+                }
-                defaultChoicesCollection = new Collection<int>();
-                foreach (int choice in defaultChoices)
-                {
-                    defaultChoicesCollection.Add(choice);
-                }
+                defaultChoicesCollection = new Collection<int>(new List<int>(defaultChoices));
-                defaultChoicesCollection = new Collection<int>();
-                foreach (int choice in defaultChoices)
-                {
-                    defaultChoicesCollection.Add(choice);
-                }
+                defaultChoicesCollection = new Collection<int>(new List<int>(defaultChoices));
+            }
+
             return _serverMethodExecutor.ExecuteMethod<Collection<int>>(RemoteHostMethodId.PromptForChoiceMultipleSelection,
-                new object[] { caption, message, choices, defaultChoices });
+                new object[] { caption, message, choices, defaultChoicesCollection });
         }
 
         /// <summary>

@@ -0,0 +1,133 @@
+# Copyright (c) Microsoft Corporation.
+# Licensed under the MIT License.
+
+Describe "Remote host method call tests" -Tag Feature {
+    BeforeAll {
+        if (-not ('RemoteHostCallsTest.TestHost' -as [type])) {
+            Add-Type -TypeDefinition @'
+using System;
+using System.Collections.Generic;
+using System.Collections.ObjectModel;
+using System.Globalization;
+using System.Management.Automation;
+using System.Management.Automation.Host;
+using System.Security;
+
+namespace RemoteHostCallsTest
+{
+    public class TestHostUserInterface : PSHostUserInterface, IHostUISupportsMultipleChoiceSelection
+    {
+        public int[] ReceivedDefaultChoices;
+        public Collection<int> ChoicesToReturn = new Collection<int>();
+
+        public override PSHostRawUserInterface RawUI => null;
+        public override string ReadLine() => throw new NotImplementedException();
+        public override SecureString ReadLineAsSecureString() => throw new NotImplementedException();
+        public override void Write(string value) { }
+        public override void Write(ConsoleColor foregroundColor, ConsoleColor backgroundColor, string value) { }
+        public override void WriteLine(string value) { }
+        public override void WriteErrorLine(string value) { }
+        public override void WriteDebugLine(string message) { }
+        public override void WriteProgress(long sourceId, ProgressRecord record) { }
+        public override void WriteVerboseLine(string message) { }
+        public override void WriteWarningLine(string message) { }
+
+        public override Dictionary<string, PSObject> Prompt(string caption, string message, Collection<FieldDescription> descriptions)
+            => throw new NotImplementedException();
+
+        public override PSCredential PromptForCredential(string caption, string message, string userName, string targetName)
+            => throw new NotImplementedException();
+
+        public override PSCredential PromptForCredential(string caption, string message, string userName, string targetName, PSCredentialTypes allowedCredentialTypes, PSCredentialUIOptions options)
+            => throw new NotImplementedException();
+
+        public override int PromptForChoice(string caption, string message, Collection<ChoiceDescription> choices, int defaultChoice)
+            => throw new NotImplementedException();
+
+        public Collection<int> PromptForChoice(string caption, string message, Collection<ChoiceDescription> choices, IEnumerable<int> defaultChoices)
+        {
+            ReceivedDefaultChoices = defaultChoices == null ? null : new List<int>(defaultChoices).ToArray();
+            return ChoicesToReturn;
+        }
+    }
+
+    public class TestHost : PSHost
+    {
+        private readonly TestHostUserInterface _ui = new TestHostUserInterface();
+        private readonly Guid _instanceId = Guid.NewGuid();
+
+        public override CultureInfo CurrentCulture => CultureInfo.CurrentCulture;
+        public override CultureInfo CurrentUICulture => CultureInfo.CurrentUICulture;
+        public override Guid InstanceId => _instanceId;
+        public override string Name => "RemoteHostCallsTest";
+        public override PSHostUserInterface UI => _ui;
+        public override Version Version => new Version(1, 0);
+        public override void EnterNestedPrompt() => throw new NotImplementedException();
+        public override void ExitNestedPrompt() => throw new NotImplementedException();
+        public override void NotifyBeginApplication() { }
+        public override void NotifyEndApplication() { }
+        public override void SetShouldExit(int exitCode) { }
+    }
+}
+'@
+        }
+
+        $testHost = [RemoteHostCallsTest.TestHost]::new()
+
+        # Connecting to the current process over the named pipe gives a remote
+        # runspace whose host calls are marshalled back to our custom host.
+        $connInfo = [System.Management.Automation.Runspaces.NamedPipeConnectionInfo]::new($PID)
+        $runspace = [runspacefactory]::CreateRunspace($testHost, $connInfo)
+        $runspace.Open()
+    }
+
+    AfterAll {
+        if ($runspace) {
+            $runspace.Dispose()
+        }
+    }
+
+    Context "IHostUISupportsMultipleChoiceSelection.PromptForChoice" {
+        BeforeEach {
+            $testHost.UI.ReceivedDefaultChoices = $null
+            $testHost.UI.ChoicesToReturn.Clear()
+        }
+
+        It "Passes default choices as <Name>" -TestCases @(
+            @{ Name = 'int[]'; DefaultChoices = '[int[]]@(0, 2)'; Expected = @(0, 2) }
+            @{ Name = 'List[int]'; DefaultChoices = '[System.Collections.Generic.List[int]]@(0, 2)'; Expected = @(0, 2) }
+            @{ Name = 'Collection[int]'; DefaultChoices = '[System.Collections.ObjectModel.Collection[int]]@(0, 2)'; Expected = @(0, 2) }
+            @{ Name = 'empty int[]'; DefaultChoices = '[int[]]@()'; Expected = @() }
+            @{ Name = 'null'; DefaultChoices = '$null'; Expected = $null }
+        ) {
+            param ($DefaultChoices, $Expected)
+
+            $testHost.UI.ChoicesToReturn.Add(1)
+            $testHost.UI.ChoicesToReturn.Add(2)
+
+            $ps = [PowerShell]::Create()
+            try {
+                $ps.Runspace = $runspace
+                $null = $ps.AddScript(@"
+`$choices = [System.Collections.ObjectModel.Collection[System.Management.Automation.Host.ChoiceDescription]]@('&a', '&b', '&c')
+`$Host.UI.PromptForChoice('caption', 'message', `$choices, $DefaultChoices)
+"@)
+                $actual = $ps.Invoke()
+
+                $ps.Streams.Error | Should -BeNullOrEmpty
+                $actual | Should -Be @(1, 2)
+
+                if ($null -eq $Expected) {
+                    $null -eq $testHost.UI.ReceivedDefaultChoices | Should -BeTrue
+                }
+                else {
+                    $null -eq $testHost.UI.ReceivedDefaultChoices | Should -BeFalse
+                    $testHost.UI.ReceivedDefaultChoices -join ',' | Should -BeExactly ($Expected -join ',')
+                }
+            }
+            finally {
+                $ps.Dispose()
+            }
+        }
+    }
+}
@@ -0,0 +1,74 @@
+// Copyright (c) Microsoft Corporation.
+// Licensed under the MIT License.
+
+using System;
+using System.Collections.Generic;
+using System.Collections.ObjectModel;
+using System.IO;
+using System.Linq;
+using System.Management.Automation;
+using System.Management.Automation.Host;
+using System.Management.Automation.Remoting;
+using Xunit;
+
+namespace PSTests.Parallel
+{
+    public static class RemoteHostEncoderTests
+    {
+        public static TheoryData<IEnumerable<int>> MultipleChoiceDefaultChoices => new()
+        {
+            new int[] { 0, 2 },
+            Array.Empty<int>(),
+            new Collection<int> { 0, 2 },
+            new Collection<int>(),
+        };
+
+        [Theory]
+        [MemberData(nameof(MultipleChoiceDefaultChoices))]
+        public static void PromptForChoiceMultipleSelectionDefaultChoicesRoundTrip(IEnumerable<int> defaultChoices)
+        {
+            // Older servers pass the defaultChoices value through as is so the
+            // client needs to be able to decode both the array and collection
+            // wire formats.
+            Collection<ChoiceDescription> choices = new()
+            {
+                new ChoiceDescription("&a"),
+                new ChoiceDescription("&b"),
+                new ChoiceDescription("&c"),
+            };
+            RemoteHostCall call = new(
+                1,
+                RemoteHostMethodId.PromptForChoiceMultipleSelection,
+                new object[] { "caption", "message", choices, defaultChoices });
+
+            RemoteHostCall decoded = RemoteHostCall.Decode(SerializeForRemoting(call.Encode()));
+
+            Assert.Equal(RemoteHostMethodId.PromptForChoiceMultipleSelection, decoded.MethodId);
+            IEnumerable<int> actual = Assert.IsAssignableFrom<IEnumerable<int>>(decoded.Parameters[3]);
+            Assert.Equal(defaultChoices.ToArray(), actual.ToArray());
+        }
+
+        [Fact]
+        public static void DecodeEnumerableOfIntWithUnknownFormatFails()
+        {
+            PSObject data = new();
+            data.Properties.Add(new PSNoteProperty("Foo", "Bar"));
+
+            PSRemotingDataStructureException exc = Assert.Throws<PSRemotingDataStructureException>(
+                () => RemoteHostEncoder.DecodeObject(data, typeof(IEnumerable<int>)));
+            Assert.Equal(
+                string.Format(RemotingErrorIdStrings.RemoteHostDataDecodingNotSupported, typeof(IEnumerable<int>)),
+                exc.Message);
+        }
+
+        private static PSObject SerializeForRemoting(PSObject data)
+        {
+            Fragmentor fragmentor = new(32 * 1024, null);
+            using MemoryStream stream = new();
+            fragmentor.SerializeToBytes(data, stream);
+            stream.Position = 0;
+
+            return fragmentor.DeserializeToPSObject(stream);
+        }
+    }
+}