fix: Don't fail metadata updates on missing assemblies

src/BuiltInTools/DotNetDeltaApplier/HotReloadAgent.cs

@@ -82,24 +82,36 @@ private UpdateHandlerActions GetMetadataUpdateHandlerActions()
             var handlerActions = new UpdateHandlerActions();
             foreach (var assembly in sortedAssemblies)
             {
-                foreach (var attr in assembly.GetCustomAttributesData())
+                try
                 {
-                    // Look up the attribute by name rather than by type. This would allow netstandard targeting libraries to
-                    // define their own copy without having to cross-compile.
-                    if (attr.AttributeType.FullName != "System.Reflection.Metadata.MetadataUpdateHandlerAttribute")
+                    foreach (var attr in assembly.GetCustomAttributesData())
                     {
-                        continue;
+                        // Look up the attribute by name rather than by type. This would allow netstandard targeting libraries to
+                        // define their own copy without having to cross-compile.
+                        if (attr.AttributeType.FullName != "System.Reflection.Metadata.MetadataUpdateHandlerAttribute")
+                        {
+                            continue;
+                        }
+
+                        IList<CustomAttributeTypedArgument> ctorArgs = attr.ConstructorArguments;
+                        if (ctorArgs.Count != 1 ||
+                            ctorArgs[0].Value is not Type handlerType)
+                        {
+                            _log($"'{attr}' found with invalid arguments.");
+                            continue;
+                        }
+
+                        GetHandlerActions(handlerActions, handlerType);
                     }
-
-                    IList<CustomAttributeTypedArgument> ctorArgs = attr.ConstructorArguments;
-                    if (ctorArgs.Count != 1 ||
-                        ctorArgs[0].Value is not Type handlerType)
-                    {
-                        _log($"'{attr}' found with invalid arguments.");
-                        continue;
-                    }
-
-                    GetHandlerActions(handlerActions, handlerType);
+                }
+                catch (Exception e)
+                {
+                    // In cross-platform scenarios, such as debugging in VS through WSL, Roslyn
+                    // runs on Windows, and the agent runs on Linux. Assemblies accessible to Windows
+                    // may not be available or loaded on linux (such as WPF's assemblies).
+                    // In such case, we can ignore the assemblies and continue enumerating handlers for
+                    // the rest of the assemblies of current domain.
+                    _log($"'{assembly.FullName}' is not loaded ({e.Message})");
                 }
             }
 

test/TestAssets/TestProjects/WatchAppMissingAssemblyFailure/App/App.csproj

@@ -0,0 +1,13 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <OutputType>Exe</OutputType>
+    <TargetFramework>$(CurrentTargetFramework)</TargetFramework>
+    <ImplicitUsings>enable</ImplicitUsings>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <ProjectReference Include="..\Dep\Dep.csproj" />
+  </ItemGroup>
+
+</Project>

test/TestAssets/TestProjects/WatchAppMissingAssemblyFailure/App/Program.cs

@@ -0,0 +1,34 @@
+using System.Diagnostics;
+using System.Reflection.Metadata;
+
+[assembly: MetadataUpdateHandler(typeof(UpdateHandler))]
+[assembly: MetadataUpdateHandler(typeof(Dep.UpdateHandler))]
+
+// delete the dependency dll to cause load failure of DepSubType
+var depPath = Path.Combine(Path.GetDirectoryName(typeof(Program).Assembly.Location!)!, "Dep.dll");
+File.Delete(depPath);
+Console.WriteLine($"File deleted: {depPath}");
+
+while (true)
+{
+    lock (UpdateHandler.Guard)
+    {
+        Printer.Print();
+    }
+
+    Thread.Sleep(100);
+}
+
+static class UpdateHandler
+{
+    // Lock to avoid the updated Print method executing concurrently with the update handler.
+    public static object Guard = new object();
+
+    public static void UpdateApplication(Type[] types)
+    {
+        lock (Guard)
+        {
+            Console.WriteLine($"Updated types: {(types == null ? "<null>" : types.Length == 0 ? "<empty>" : string.Join(",", types.Select(t => t.Name)))}");
+        }
+    }
+}

test/TestAssets/TestProjects/WatchAppMissingAssemblyFailure/App/Update.cs

@@ -0,0 +1,13 @@
+using System;
+
+class DepSubType : Dep.DepType
+{
+    int F() => 1;
+}
+
+class Printer
+{
+    public static void Print()
+        => Console.WriteLine("Hello!");
+}
+

test/TestAssets/TestProjects/WatchAppMissingAssemblyFailure/Dep/Dep.cs

@@ -0,0 +1,23 @@
+namespace Dep;
+
+public class DepType
+{
+    void F()
+    {
+        Console.WriteLine(1);
+    }
+}
+
+public static class UpdateHandler
+{
+    // Lock to avoid the updated Print method executing concurrently with the update handler.
+    public static object Guard = new object();
+
+    public static void UpdateApplication(Type[] types)
+    {
+        lock (Guard)
+        {
+            Console.WriteLine($"Dep Updated types: {(types == null ? "<null>" : types.Length == 0 ? "<empty>" : string.Join(",", types.Select(t => t.Name)))}");
+        }
+    }
+}

test/TestAssets/TestProjects/WatchAppMissingAssemblyFailure/Dep/Dep.csproj

@@ -0,0 +1,8 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <TargetFramework>$(CurrentTargetFramework)</TargetFramework>
+    <ImplicitUsings>enable</ImplicitUsings>
+  </PropertyGroup>
+
+</Project>

test/dotnet-watch.Tests/HotReload/ApplyDeltaTests.cs

@@ -120,5 +120,56 @@ public async Task BlazorWasm()
             //UpdateSourceFile(Path.Combine(testAsset.Path, "Pages", "Index.razor"), newSource);
             //await App.AssertOutputLineStartsWith(MessageDescriptor.HotReloadSucceeded);
         }
+
+        // Test is timing out on .NET Framework: https://github.com/dotnet/sdk/issues/41669
+        [CoreMSBuildOnlyFact(Skip = "https://github.com/dotnet/sdk/issues/42850")]
+        public async Task HandleMissingAssemblyFailure()
+        {
+            var testAsset = TestAssets.CopyTestAsset("WatchAppMissingAssemblyFailure")
+                .WithSource();
+
+            App.Start(testAsset, [], "App");
+
+            await App.AssertWaitingForChanges();
+
+            var newSrc = /* lang=c#-test */"""
+                class DepSubType : Dep.DepType
+                {
+                    int F() => 2;
+                }
+
+                class Printer
+                {
+                    public static void Print()
+                    {
+                        Console.WriteLine("Changed!");
+                    }
+                }
+
+                public static class UpdateHandler
+                {
+                    // Lock to avoid the updated Print method executing concurrently with the update handler.
+                    public static object Guard = new object();
+
+                    public static void UpdateApplication(Type[] types)
+                    {
+                        lock (Guard)
+                        {
+                            Console.WriteLine($"Dep Updated types: {(types == null ? "<null>" : types.Length == 0 ? "<empty>" : string.Join(",", types.Select(t => t.Name)))}");
+                        }
+                    }
+                }
+                """;
+
+            // Delete all files in testAsset.Path named Dep.dll
+            foreach (var depDll in Directory.GetFiles(testAsset.Path, "Dep.dll", SearchOption.AllDirectories))
+            {
+                File.Delete(depDll);
+            }
+
+            File.WriteAllText(Path.Combine(testAsset.Path, "App", "Update.cs"), newSrc);
+
+            await App.AssertOutputLineStartsWith("Updated types: Printer");
+        }
     }
 }