diff --git a/src/BuiltInTools/DotNetWatchTasks/Utilities/IsExternalInit.cs b/src/BuiltInTools/DotNetWatchTasks/Utilities/IsExternalInit.cs
new file mode 100644
index 000000000000..7d1e00360964
--- /dev/null
+++ b/src/BuiltInTools/DotNetWatchTasks/Utilities/IsExternalInit.cs
@@ -0,0 +1,20 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+// Copied from:
+// https://github.com/dotnet/runtime/blob/218ef0f7776c2c20f6c594e3475b80f1fe2d7d08/src/libraries/System.Private.CoreLib/src/System/Runtime/CompilerServices/IsExternalInit.cs
+
+using System.ComponentModel;
+
+namespace System.Runtime.CompilerServices
+{
+    /// <summary>
+    /// Reserved to be used by the compiler for tracking metadata.
+    /// This class should not be used by developers in source code.
+    /// </summary>
+    [EditorBrowsable(EditorBrowsableState.Never)]
+    internal static class IsExternalInit
+    {
+    }
+}
diff --git a/src/BuiltInTools/dotnet-watch/Internal/MSBuildFileSetResult.cs b/src/BuiltInTools/dotnet-watch/Internal/MSBuildFileSetResult.cs
index 16c91b4c5c23..295afb4d72b6 100644
--- a/src/BuiltInTools/dotnet-watch/Internal/MSBuildFileSetResult.cs
+++ b/src/BuiltInTools/dotnet-watch/Internal/MSBuildFileSetResult.cs
@@ -3,39 +3,55 @@
 
 using System;
 using System.Collections.Generic;
+using System.Runtime.Serialization;
 
 namespace Microsoft.DotNet.Watcher.Internal
 {
+    [DataContract]
     internal sealed class MSBuildFileSetResult
     {
-        public string RunCommand { get; set; }
+        [DataMember]
+        public string RunCommand { get; init; }
 
-        public string RunArguments { get; set; }
+        [DataMember]
+        public string RunArguments { get; init; }
 
-        public string RunWorkingDirectory { get; set; }
+        [DataMember]
+        public string RunWorkingDirectory { get; init; }
 
-        public bool IsNetCoreApp { get; set; }
+        [DataMember]
+        public bool IsNetCoreApp { get; init; }
 
-        public string TargetFrameworkVersion { get; set; }
+        [DataMember]
+        public string TargetFrameworkVersion { get; init; }
 
-        public string RuntimeIdentifier { get; set; }
+        [DataMember]
+        public string RuntimeIdentifier { get; init; }
 
-        public string DefaultAppHostRuntimeIdentifier { get; set; }
+        [DataMember]
+        public string DefaultAppHostRuntimeIdentifier { get; init; }
 
-        public Dictionary<string, ProjectItems> Projects { get; set; }
+        [DataMember]
+        public Dictionary<string, ProjectItems> Projects { get; init; }
     }
 
+    [DataContract]
     internal sealed class ProjectItems
     {
-        public List<string> Files { get; set; } = new();
+        [DataMember]
+        public List<string> Files { get; init; } = new();
 
-        public List<StaticFileItem> StaticFiles { get; set; } = new();
+        [DataMember]
+        public List<StaticFileItem> StaticFiles { get; init; } = new();
     }
 
+    [DataContract]
     internal sealed class StaticFileItem
     {
-        public string FilePath { get; set; }
+        [DataMember]
+        public string FilePath { get; init; }
 
-        public string StaticWebAssetPath { get; set; }
+        [DataMember]
+        public string StaticWebAssetPath { get; init; }
     }
 }
diff --git a/src/Tests/dotnet-watch.Tests/FileSetSerializerTests.cs b/src/Tests/dotnet-watch.Tests/FileSetSerializerTests.cs
new file mode 100644
index 000000000000..495142f9d6b1
--- /dev/null
+++ b/src/Tests/dotnet-watch.Tests/FileSetSerializerTests.cs
@@ -0,0 +1,49 @@
+// Copyright (c) .NET Foundation and contributors. All rights reserved.
+// Licensed under the MIT license. See LICENSE file in the project root for full license information.
+//
+
+using System.IO;
+using System.Runtime.Serialization.Json;
+using System.Text;
+using System.Text.Json;
+using System.Threading;
+using System.Threading.Tasks;
+using Microsoft.DotNet.Watcher.Internal;
+using Xunit;
+
+namespace Microsoft.DotNet.Watcher.Tools;
+
+public class FileSetSerializerTests
+{
+    [Fact]
+    public async Task Serialize()
+    {
+        var fileSetResult = new MSBuildFileSetResult()
+        {
+            IsNetCoreApp = true,
+            TargetFrameworkVersion = "net5.0",
+            RuntimeIdentifier = "win-arm64",
+            DefaultAppHostRuntimeIdentifier = "",
+            RunCommand = "run",
+            RunArguments = "args",
+            RunWorkingDirectory = "dir",
+            Projects = new() { { "proj", new() { Files = new() { "a.cs", "b.cs" }, StaticFiles = new() { new() { FilePath = "path1", StaticWebAssetPath = "path2" } } } } }
+        };
+
+        using var stream = new MemoryStream();
+
+        var serializer = new DataContractJsonSerializer(fileSetResult.GetType(), new DataContractJsonSerializerSettings
+        {
+            UseSimpleDictionaryFormat = true,
+        });
+
+        using (var writer = JsonReaderWriterFactory.CreateJsonWriter(stream, Encoding.UTF8, ownsStream: false, indent: true))
+        {
+            serializer.WriteObject(writer, fileSetResult);
+        }
+
+        stream.Position = 0;
+
+        await JsonSerializer.DeserializeAsync<MSBuildFileSetResult>(stream, cancellationToken: CancellationToken.None);
+    }
+}
diff --git a/src/Tests/dotnet-watch.Tests/dotnet-watch.Tests.csproj b/src/Tests/dotnet-watch.Tests/dotnet-watch.Tests.csproj
index c126799ec11f..7cac2bac18ac 100644
--- a/src/Tests/dotnet-watch.Tests/dotnet-watch.Tests.csproj
+++ b/src/Tests/dotnet-watch.Tests/dotnet-watch.Tests.csproj
@@ -3,6 +3,7 @@
     <OutputType>Exe</OutputType>
     <TargetFramework>$(ToolsetTargetFramework)</TargetFramework>
     <StrongNameKeyId>MicrosoftAspNetCore</StrongNameKeyId>
+    <RootNamespace>Microsoft.DotNet.Watcher.Tools</RootNamespace>
   </PropertyGroup>
 
   <ItemGroup>