Fix trim/AOT safety for custom event argument deserialization in Blazor

src/Components/Web/src/WebEventData/WebEventData.cs

@@ -58,8 +58,6 @@ private WebEventData(ulong eventHandlerId, EventFieldInfo? eventFieldInfo, Event
 
     public EventArgs EventArgs { get; }
 
-    [UnconditionalSuppressMessage("ReflectionAnalysis", "IL2026:RequiresUnreferencedCode",
-        Justification = "We are already using the appropriate overload")]
     private static EventArgs ParseEventArgsJson(
         Renderer renderer,
         JsonSerializerOptions jsonSerializerOptions,
@@ -74,9 +72,14 @@ private static EventArgs ParseEventArgsJson(
                 return eventArgs;
             }
 
-            // For custom events, the args type is determined from the associated delegate
+            // For custom events, the args type is determined from the associated delegate. The type
+            // is registered via the [EventHandler] attribute, whose 'eventArgsType' argument is
+            // annotated with DynamicallyAccessedMembers so the members required for JSON
+            // deserialization are preserved when trimming. We deserialize using the JsonTypeInfo
+            // returned by the options, which is the trim-safe API.
             var eventArgsType = renderer.GetEventArgsType(eventHandlerId);
-            return (EventArgs)JsonSerializer.Deserialize(eventArgsJson.GetRawText(), eventArgsType, jsonSerializerOptions)!;
+            var eventArgsTypeInfo = jsonSerializerOptions.GetTypeInfo(eventArgsType);
+            return (EventArgs)JsonSerializer.Deserialize(eventArgsJson.GetRawText(), eventArgsTypeInfo)!;
         }
         catch (Exception e)
         {

src/Components/Web/test/WebEventData/WebEventDataTest.cs

@@ -0,0 +1,102 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+using System.Text.Json;
+using System.Text.Json.Serialization.Metadata;
+using Microsoft.AspNetCore.Components.RenderTree;
+using Microsoft.AspNetCore.Components.Rendering;
+using Microsoft.AspNetCore.Components.Test.Helpers;
+
+namespace Microsoft.AspNetCore.Components.Web;
+
+public class WebEventDataTest
+{
+    private static readonly JsonSerializerOptions _jsonOptions = new(JsonSerializerDefaults.Web)
+    {
+        IncludeFields = true,
+        TypeInfoResolver = new DefaultJsonTypeInfoResolver(),
+    };
+
+    [Fact]
+    public void ParseEventArgsJson_DeserializesCustomEventArgs_UsingHandlerParameterType()
+    {
+        var renderer = new TestRenderer();
+        var component = new CustomEventComponent
+        {
+            Handler = (Func<CustomEventArgs, Task>)(_ => Task.CompletedTask),
+        };
+        renderer.AssignRootComponentId(component);
+        component.TriggerRender();
+
+        var eventHandlerId = renderer.Batches.Single()
+            .ReferenceFrames
+            .First(frame => frame.AttributeValue != null)
+            .AttributeEventHandlerId;
+
+        var descriptor = new WebEventDescriptor
+        {
+            EventHandlerId = eventHandlerId,
+            EventName = "oncustomevent",
+        };
+
+        using var eventArgsJson = JsonDocument.Parse("{\"value\":\"hello\"}");
+
+        var webEventData = WebEventData.Parse(renderer, _jsonOptions, descriptor, eventArgsJson.RootElement);
+
+        var customArgs = Assert.IsType<CustomEventArgs>(webEventData.EventArgs);
+        Assert.Equal("hello", customArgs.Value);
+        Assert.Equal(eventHandlerId, webEventData.EventHandlerId);
+    }
+
+    [Fact]
+    public void ParseEventArgsJson_ReturnsEmptyEventArgs_ForParameterlessHandler()
+    {
+        // Arrange: a handler that takes no EventArgs resolves to EventArgs.
+        var renderer = new TestRenderer();
+        var component = new CustomEventComponent
+        {
+            Handler = (Func<Task>)(() => Task.CompletedTask),
+        };
+        renderer.AssignRootComponentId(component);
+        component.TriggerRender();
+
+        var eventHandlerId = renderer.Batches.Single()
+            .ReferenceFrames
+            .First(frame => frame.AttributeValue != null)
+            .AttributeEventHandlerId;
+
+        var descriptor = new WebEventDescriptor
+        {
+            EventHandlerId = eventHandlerId,
+            EventName = "oncustomevent",
+        };
+
+        using var eventArgsJson = JsonDocument.Parse("{}");
+
+        var webEventData = WebEventData.Parse(renderer, _jsonOptions, descriptor, eventArgsJson.RootElement);
+
+        Assert.NotNull(webEventData.EventArgs);
+        Assert.IsType<EventArgs>(webEventData.EventArgs);
+    }
+
+    private sealed class CustomEventArgs : EventArgs
+    {
+        public string Value { get; set; }
+    }
+
+    private sealed class CustomEventComponent : AutoRenderComponent, IHandleEvent
+    {
+        [Parameter]
+        public Delegate Handler { get; set; }
+
+        protected override void BuildRenderTree(RenderTreeBuilder builder)
+        {
+            builder.OpenElement(0, "element");
+            builder.AddAttribute(1, "oncustomevent", Handler);
+            builder.CloseElement();
+        }
+
+        public Task HandleEventAsync(EventCallbackWorkItem callback, object arg)
+            => callback.InvokeAsync(arg);
+    }
+}