Custom event args

src/Components/Components/src/PublicAPI.Unshipped.txt

@@ -15,6 +15,7 @@ Microsoft.AspNetCore.Components.DynamicComponent.Type.set -> void
 Microsoft.AspNetCore.Components.CascadingTypeParameterAttribute
 Microsoft.AspNetCore.Components.CascadingTypeParameterAttribute.CascadingTypeParameterAttribute(string! name) -> void
 Microsoft.AspNetCore.Components.CascadingTypeParameterAttribute.Name.get -> string!
+Microsoft.AspNetCore.Components.RenderTree.Renderer.GetEventArgsType(ulong eventHandlerId) -> System.Type!
 static Microsoft.AspNetCore.Components.ParameterView.FromDictionary(System.Collections.Generic.IDictionary<string!, object?>! parameters) -> Microsoft.AspNetCore.Components.ParameterView
 virtual Microsoft.AspNetCore.Components.RenderTree.Renderer.DispatchEventAsync(ulong eventHandlerId, Microsoft.AspNetCore.Components.RenderTree.EventFieldInfo? fieldInfo, System.EventArgs! eventArgs) -> System.Threading.Tasks.Task!
 *REMOVED*readonly Microsoft.AspNetCore.Components.RenderTree.RenderTreeEdit.RemovedAttributeName -> string

src/Components/Components/src/RenderTree/EventArgsTypeCache.cs

@@ -0,0 +1,42 @@
+// Copyright (c) .NET Foundation. All rights reserved.
+// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
+
+using System;
+using System.Collections.Concurrent;
+using System.Reflection;
+
+namespace Microsoft.AspNetCore.Components.RenderTree
+{
+    internal static class EventArgsTypeCache
+    {
+        private static ConcurrentDictionary<MethodInfo, Type> Cache = new ConcurrentDictionary<MethodInfo, Type>();
+
+        public static Type GetEventArgsType(MethodInfo methodInfo)
+        {
+            return Cache.GetOrAdd(methodInfo, methodInfo =>
+            {
+                var parameterInfos = methodInfo.GetParameters();
+                if (parameterInfos.Length == 0)
+                {
+                    return typeof(EventArgs);
+                }
+                else if (parameterInfos.Length > 1)
+                {
+                    throw new InvalidOperationException($"The method {methodInfo} cannot be used as an event handler because it declares more than one parameter.");
+                }
+                else
+                {
+                    var declaredType = parameterInfos[0].ParameterType;
+                    if (typeof(EventArgs).IsAssignableFrom(declaredType))
+                    {
+                        return declaredType;
+                    }
+                    else
+                    {
+                        throw new InvalidOperationException($"The event handler parameter type {declaredType.FullName} for event must inherit from {typeof(EventArgs).FullName}.");
+                    }
+                }
+            });
+        }
+    }
+}

src/Components/Components/src/RenderTree/Renderer.cs

@@ -249,11 +249,7 @@ public virtual Task DispatchEventAsync(ulong eventHandlerId, EventFieldInfo? fie
         {
             Dispatcher.AssertAccess();
 
-            if (!_eventBindings.TryGetValue(eventHandlerId, out var callback))
-            {
-                throw new ArgumentException($"There is no event handler associated with this event. EventId: '{eventHandlerId}'.", nameof(eventHandlerId));
-            }
-
+            var callback = GetRequiredEventCallback(eventHandlerId);
             Log.HandlingEvent(_logger, eventHandlerId, eventArgs);
 
             if (fieldInfo != null)
@@ -291,6 +287,24 @@ public virtual Task DispatchEventAsync(ulong eventHandlerId, EventFieldInfo? fie
             return result;
         }
 
+        /// <summary>
+        /// Gets the event arguments type for the specified event handler.
+        /// </summary>
+        /// <param name="eventHandlerId">The <see cref="RenderTreeFrame.AttributeEventHandlerId"/> value from the original event attribute.</param>
+        /// <returns>The parameter type expected by the event handler. Normally this is a subclass of <see cref="EventArgs"/>.</returns>
+        public Type GetEventArgsType(ulong eventHandlerId)
+        {
+            var methodInfo = GetRequiredEventCallback(eventHandlerId).Delegate?.Method;
+
+            // The DispatchEventAsync code paths allow for the case where Delegate or its method
+            // is null, and in this case the event receiver just receives null. This won't happen
+            // under normal circumstances, but to avoid creating a new failure scenario, allow for
+            // that edge case here too.
+            return methodInfo == null
+                ? typeof(EventArgs)
+                : EventArgsTypeCache.GetEventArgsType(methodInfo);
+        }
+
         internal void InstantiateChildComponentOnFrame(ref RenderTreeFrame frame, int parentComponentId)
         {
             if (frame.FrameTypeField != RenderTreeFrameType.Component)
@@ -404,6 +418,16 @@ internal void TrackReplacedEventHandlerId(ulong oldEventHandlerId, ulong newEven
             _eventHandlerIdReplacements.Add(oldEventHandlerId, newEventHandlerId);
         }
 
+        private EventCallback GetRequiredEventCallback(ulong eventHandlerId)
+        {
+            if (!_eventBindings.TryGetValue(eventHandlerId, out var callback))
+            {
+                throw new ArgumentException($"There is no event handler associated with this event. EventId: '{eventHandlerId}'.", nameof(eventHandlerId));
+            }
+
+            return callback;
+        }
+
         private ulong FindLatestEventHandlerIdInChain(ulong eventHandlerId)
         {
             while (_eventHandlerIdReplacements.TryGetValue(eventHandlerId, out var replacementEventHandlerId))

src/Components/Components/test/RendererTest.cs

@@ -484,6 +484,98 @@ public void CanDispatchEventsToTopLevelComponents()
             Assert.Same(eventArgs, receivedArgs);
         }
 
+        [Fact]
+        public void CanGetEventArgsTypeForHandler()
+        {
+            // Arrange: Render a component with an event handler
+            var renderer = new TestRenderer();
+
+            var component = new EventComponent
+            {
+                OnArbitraryDelegateEvent = (Func<DerivedEventArgs, Task>)(args => Task.CompletedTask),
+            };
+            var componentId = renderer.AssignRootComponentId(component);
+            component.TriggerRender();
+
+            var eventHandlerId = renderer.Batches.Single()
+                .ReferenceFrames
+                .First(frame => frame.AttributeValue != null)
+                .AttributeEventHandlerId;
+
+            // Assert: Can determine event args type
+            var eventArgsType = renderer.GetEventArgsType(eventHandlerId);
+            Assert.Same(typeof(DerivedEventArgs), eventArgsType);
+        }
+
+        [Fact]
+        public void CanGetEventArgsTypeForParameterlessHandler()
+        {
+            // Arrange: Render a component with an event handler
+            var renderer = new TestRenderer();
+
+            var component = new EventComponent
+            {
+                OnArbitraryDelegateEvent = (Func<Task>)(() => Task.CompletedTask),
+            };
+            var componentId = renderer.AssignRootComponentId(component);
+            component.TriggerRender();
+
+            var eventHandlerId = renderer.Batches.Single()
+                .ReferenceFrames
+                .First(frame => frame.AttributeValue != null)
+                .AttributeEventHandlerId;
+
+            // Assert: Can determine event args type
+            var eventArgsType = renderer.GetEventArgsType(eventHandlerId);
+            Assert.Same(typeof(EventArgs), eventArgsType);
+        }
+
+        [Fact]
+        public void CannotGetEventArgsTypeForMultiParameterHandler()
+        {
+            // Arrange: Render a component with an event handler
+            var renderer = new TestRenderer();
+
+            var component = new EventComponent
+            {
+                OnArbitraryDelegateEvent = (Action<EventArgs, string>)((x, y) => { }),
+            };
+            var componentId = renderer.AssignRootComponentId(component);
+            component.TriggerRender();
+
+            var eventHandlerId = renderer.Batches.Single()
+                .ReferenceFrames
+                .First(frame => frame.AttributeValue != null)
+                .AttributeEventHandlerId;
+
+            // Assert: Cannot determine event args type
+            var ex = Assert.Throws<InvalidOperationException>(() => renderer.GetEventArgsType(eventHandlerId));
+            Assert.Contains("declares more than one parameter", ex.Message);
+        }
+
+        [Fact]
+        public void CannotGetEventArgsTypeForHandlerWithNonEventArgsParameter()
+        {
+            // Arrange: Render a component with an event handler
+            var renderer = new TestRenderer();
+
+            var component = new EventComponent
+            {
+                OnArbitraryDelegateEvent = (Action<DateTime>)(arg => { }),
+            };
+            var componentId = renderer.AssignRootComponentId(component);
+            component.TriggerRender();
+
+            var eventHandlerId = renderer.Batches.Single()
+                .ReferenceFrames
+                .First(frame => frame.AttributeValue != null)
+                .AttributeEventHandlerId;
+
+            // Assert: Cannot determine event args type
+            var ex = Assert.Throws<InvalidOperationException>(() => renderer.GetEventArgsType(eventHandlerId));
+            Assert.Contains($"must inherit from {typeof(EventArgs).FullName}", ex.Message);
+        }
+
         [Fact]
         public void DispatchEventHandlesSynchronousExceptionsFromEventHandlers()
         {
@@ -4224,6 +4316,9 @@ private class EventComponent : AutoRenderComponent, IComponent, IHandleEvent
             [Parameter]
             public EventCallback<DerivedEventArgs> OnClickEventCallbackOfT { get; set; }
 
+            [Parameter]
+            public Delegate OnArbitraryDelegateEvent { get; set; }
+
             public bool SkipElement { get; set; }
             private int renderCount = 0;
 
@@ -4269,6 +4364,12 @@ protected override void BuildRenderTree(RenderTreeBuilder builder)
                     {
                         builder.AddAttribute(5, "onclickaction", OnClickAsyncAction);
                     }
+
+                    if (OnArbitraryDelegateEvent != null)
+                    {
+                        builder.AddAttribute(6, "onarbitrarydelegateevent", OnArbitraryDelegateEvent);
+                    }
+
                     builder.CloseElement();
                     builder.CloseElement();
                 }

src/Components/Server/src/Circuits/CircuitHost.cs

@@ -399,7 +399,7 @@ public async Task DispatchEvent(string eventDescriptorJson, string eventArgsJson
             WebEventData webEventData;
             try
             {
-                webEventData = WebEventData.Parse(eventDescriptorJson, eventArgsJson);
+                webEventData = WebEventData.Parse(Renderer, eventDescriptorJson, eventArgsJson);
             }
             catch (Exception ex)
             {