Add support for Action event handlers

This change adds `Action` to the set of types that have an overload on
RenderTreeBuilder. Additionally, we special case `Action` in the runtime
because passing the event args via DynamicInvoke() would throw.

Finally, reverted some of the clutter introduced by the first pass of
the event handler feature.

samples/StandaloneApp/Pages/Counter.cshtml

@@ -1,5 +1,4 @@
 @page "/counter"
-@using Microsoft.AspNetCore.Blazor
 
 <h1>Counter</h1>
 
@@ -10,7 +9,7 @@
 @functions {
     int currentCount = 0;
 
-    void IncrementCount(UIMouseEventArgs e)
+    void IncrementCount()
     {
         currentCount++;
     }

src/Microsoft.AspNetCore.Blazor/Components/BindMethods.cs

@@ -32,6 +32,15 @@ namespace Microsoft.AspNetCore.Blazor.Components
             return value;
         }
 
+        /// <summary>
+        /// Not intended to be used directly.
+        /// </summary>
+        public static MulticastDelegate GetEventHandlerValue<T>(Action value)
+            where T : UIEventArgs
+        {
+            return value;
+        }
+
         /// <summary>
         /// Not intended to be used directly.
         /// </summary>

src/Microsoft.AspNetCore.Blazor/RenderTree/RenderTreeBuilder.cs

@@ -148,6 +148,24 @@ namespace Microsoft.AspNetCore.Blazor.RenderTree
             }
         }
 
+        /// <summary>
+        /// <para>
+        /// Appends a frame representing an <see cref="Action"/>-valued attribute.
+        /// </para>
+        /// <para>
+        /// The attribute is associated with the most recently added element. If the value is <c>null</c> and the
+        /// current element is not a component, the frame will be omitted.
+        /// </para>
+        /// </summary>
+        /// <param name="builder">The <see cref="RenderTreeBuilder"/>.</param>
+        /// <param name="sequence">An integer that represents the position of the instruction in the source code.</param>
+        /// <param name="name">The name of the attribute.</param>
+        /// <param name="value">The value of the attribute.</param>
+        public void AddAttribute(int sequence, string name, Action value)
+        {
+            AddAttribute(sequence, name, (MulticastDelegate)value);
+        }
+
         /// <summary>
         /// <para>
         /// Appends a frame representing an <see cref="UIEventHandler"/>-valued attribute.

src/Microsoft.AspNetCore.Blazor/Rendering/Renderer.cs

@@ -125,15 +125,20 @@ namespace Microsoft.AspNetCore.Blazor.Rendering
             {
                 _eventHandlersById.Add(id, wrapper);
             }
+            // IMPORTANT: we're creating an additional delegate when necessary. This is
+            // going to get cached in _eventHandlersById, but the render tree diff
+            // will operate on 'AttributeValue' which means that we'll only create a new
+            // wrapper delegate when the underlying delegate changes.
+            //
+            // TLDR: If the component uses a method group or a non-capturing lambda
+            // we don't allocate much.
+            else if (frame.AttributeValue is Action action)
+            {
+                _eventHandlersById.Add(id, (UIEventArgs e) => action());
+            }
             else if (frame.AttributeValue is MulticastDelegate @delegate)
             {
-                // IMPORTANT: we're creating an additional delegate when necessary. This is
-                // going to get cached in _eventHandlersById, but the render tree diff
-                // will operate on 'AttributeValue' which means that we'll only create a new
-                // wrapper delegate when the underlying delegate changes.
-                //
-                // TLDR: If the component uses a method group or a non-capturing lambda
-                // we don't allocate much.
+
                _eventHandlersById.Add(id, (UIEventArgs e) => @delegate.DynamicInvoke(e));
             }
 

test/Microsoft.AspNetCore.Blazor.Build.Test/RuntimeCodeGenerationTest.cs

@@ -3,7 +3,6 @@
 
 using Microsoft.CodeAnalysis.CSharp;
 using Xunit;
-using Xunit.Abstractions;
 
 namespace Microsoft.AspNetCore.Blazor.Build.Test
 {
@@ -276,7 +275,23 @@ namespace Test
         }
 
         [Fact]
-        public void EventHandler_OnElement_WithLambdaDelegate()
+        public void EventHandler_OnElement_WithNoArgsLambdaDelegate()
+        {
+            // Arrange
+
+            // Act
+            var generated = CompileToCSharp(@"
+@using Microsoft.AspNetCore.Blazor
+<input onclick=""@(() => { })"" />");
+
+            // Assert
+            AssertDocumentNodeMatchesBaseline(generated.CodeDocument);
+            AssertCSharpDocumentMatchesBaseline(generated.CodeDocument);
+            CompileToAssembly(generated);
+        }
+
+        [Fact]
+        public void EventHandler_OnElement_WithEventArgsLambdaDelegate()
         {
             // Arrange
 
@@ -292,7 +307,27 @@ namespace Test
         }
 
         [Fact]
-        public void EventHandler_OnElement_WithDelegate()
+        public void EventHandler_OnElement_WithNoArgMethodGroup()
+        {
+            // Arrange
+
+            // Act
+            var generated = CompileToCSharp(@"
+@using Microsoft.AspNetCore.Blazor
+<input onclick=""@OnClick"" />
+@functions {
+    void OnClick() {
+    }
+}");
+
+            // Assert
+            AssertDocumentNodeMatchesBaseline(generated.CodeDocument);
+            AssertCSharpDocumentMatchesBaseline(generated.CodeDocument);
+            CompileToAssembly(generated);
+        }
+
+        [Fact]
+        public void EventHandler_OnElement_WithEventArgsMethodGroup()
         {
             // Arrange
 
@@ -310,5 +345,25 @@ namespace Test
             AssertCSharpDocumentMatchesBaseline(generated.CodeDocument);
             CompileToAssembly(generated);
         }
+
+        [Fact]
+        public void EventHandler_OnElement_ArbitraryEventName_WithEventArgsMethodGroup()
+        {
+            // Arrange
+
+            // Act
+            var generated = CompileToCSharp(@"
+@using Microsoft.AspNetCore.Blazor
+<input onclick=""@OnClick"" />
+@functions {
+    void OnClick(UIEventArgs e) {
+    }
+}");
+
+            // Assert
+            AssertDocumentNodeMatchesBaseline(generated.CodeDocument);
+            AssertCSharpDocumentMatchesBaseline(generated.CodeDocument);
+            CompileToAssembly(generated);
+        }
     }
 }

test/Microsoft.AspNetCore.Blazor.Build.Test/TestFiles/RuntimeCodeGenerationTest/EventHandler_OnElement_ArbitraryEventName_WithEventArgsMethodGroup/TestComponent.codegen.cs

@@ -0,0 +1,32 @@
+// <auto-generated/>
+#pragma warning disable 1591
+namespace Test
+{
+    #line hidden
+    using System;
+    using System.Collections.Generic;
+    using System.Linq;
+    using System.Threading.Tasks;
+    using Microsoft.AspNetCore.Blazor;
+    public class TestComponent : Microsoft.AspNetCore.Blazor.Components.BlazorComponent
+    {
+        #pragma warning disable 1998
+        protected override void BuildRenderTree(Microsoft.AspNetCore.Blazor.RenderTree.RenderTreeBuilder builder)
+        {
+            base.BuildRenderTree(builder);
+            builder.OpenElement(0, "input");
+            builder.AddAttribute(1, "onclick", Microsoft.AspNetCore.Blazor.Components.BindMethods.GetEventHandlerValue<Microsoft.AspNetCore.Blazor.UIMouseEventArgs>(OnClick));
+            builder.CloseElement();
+            builder.AddContent(2, "\n");
+        }
+        #pragma warning restore 1998
+#line 3 "x:\dir\subdir\Test\TestComponent.cshtml"
+            
+    void OnClick(UIEventArgs e) {
+    }
+
+#line default
+#line hidden
+    }
+}
+#pragma warning restore 1591

test/Microsoft.AspNetCore.Blazor.Build.Test/TestFiles/RuntimeCodeGenerationTest/EventHandler_OnElement_ArbitraryEventName_WithEventArgsMethodGroup/TestComponent.ir.txt

@@ -0,0 +1,24 @@
+Document - 
+    NamespaceDeclaration -  - Test
+        UsingDirective - (3:1,1 [14] ) - System
+        UsingDirective - (18:2,1 [34] ) - System.Collections.Generic
+        UsingDirective - (53:3,1 [19] ) - System.Linq
+        UsingDirective - (73:4,1 [30] ) - System.Threading.Tasks
+        UsingDirective - (1:0,1 [35] x:\dir\subdir\Test\TestComponent.cshtml) - Microsoft.AspNetCore.Blazor
+        ClassDeclaration -  - public - TestComponent - Microsoft.AspNetCore.Blazor.Components.BlazorComponent - 
+            MethodDeclaration -  - protected override - void - BuildRenderTree
+                CSharpCode - 
+                    IntermediateToken -  - CSharp - base.BuildRenderTree(builder);
+                HtmlContent - 
+                    IntermediateToken -  - Html - <input 
+                HtmlAttribute -  - =" - "
+                    CSharpExpressionAttributeValue -  - 
+                        IntermediateToken -  - CSharp - Microsoft.AspNetCore.Blazor.Components.BindMethods.GetEventHandlerValue<Microsoft.AspNetCore.Blazor.UIMouseEventArgs>(
+                        IntermediateToken - (53:1,17 [7] x:\dir\subdir\Test\TestComponent.cshtml) - CSharp - OnClick
+                        IntermediateToken -  - CSharp - )
+                HtmlContent - 
+                    IntermediateToken -  - Html - />
+                HtmlContent - (64:1,28 [2] x:\dir\subdir\Test\TestComponent.cshtml)
+                    IntermediateToken - (64:1,28 [2] x:\dir\subdir\Test\TestComponent.cshtml) - Html - \n
+            CSharpCode - (78:2,12 [44] x:\dir\subdir\Test\TestComponent.cshtml)
+                IntermediateToken - (78:2,12 [44] x:\dir\subdir\Test\TestComponent.cshtml) - CSharp - \n    void OnClick(UIEventArgs e) {\n    }\n

test/Microsoft.AspNetCore.Blazor.Build.Test/TestFiles/RuntimeCodeGenerationTest/EventHandler_OnElement_ArbitraryEventName_WithEventArgsMethodGroup/TestComponent.mappings.txt

@@ -0,0 +1,11 @@
+Source Location: (78:2,12 [44] x:\dir\subdir\Test\TestComponent.cshtml)
+|
+    void OnClick(UIEventArgs e) {
+    }
+|
+Generated Location: (964:23,12 [44] )
+|
+    void OnClick(UIEventArgs e) {
+    }
+|
+

test/Microsoft.AspNetCore.Blazor.Build.Test/TestFiles/RuntimeCodeGenerationTest/EventHandler_OnElement_WithEventArgsLambdaDelegate/TestComponent.codegen.cs

@@ -0,0 +1,24 @@
+// <auto-generated/>
+#pragma warning disable 1591
+namespace Test
+{
+    #line hidden
+    using System;
+    using System.Collections.Generic;
+    using System.Linq;
+    using System.Threading.Tasks;
+    using Microsoft.AspNetCore.Blazor;
+    public class TestComponent : Microsoft.AspNetCore.Blazor.Components.BlazorComponent
+    {
+        #pragma warning disable 1998
+        protected override void BuildRenderTree(Microsoft.AspNetCore.Blazor.RenderTree.RenderTreeBuilder builder)
+        {
+            base.BuildRenderTree(builder);
+            builder.OpenElement(0, "input");
+            builder.AddAttribute(1, "onclick", Microsoft.AspNetCore.Blazor.Components.BindMethods.GetEventHandlerValue<Microsoft.AspNetCore.Blazor.UIMouseEventArgs>(x => { }));
+            builder.CloseElement();
+        }
+        #pragma warning restore 1998
+    }
+}
+#pragma warning restore 1591

test/Microsoft.AspNetCore.Blazor.Build.Test/TestFiles/RuntimeCodeGenerationTest/EventHandler_OnElement_WithEventArgsLambdaDelegate/TestComponent.ir.txt

@@ -0,0 +1,20 @@
+Document - 
+    NamespaceDeclaration -  - Test
+        UsingDirective - (3:1,1 [14] ) - System
+        UsingDirective - (18:2,1 [34] ) - System.Collections.Generic
+        UsingDirective - (53:3,1 [19] ) - System.Linq
+        UsingDirective - (73:4,1 [30] ) - System.Threading.Tasks
+        UsingDirective - (1:0,1 [35] x:\dir\subdir\Test\TestComponent.cshtml) - Microsoft.AspNetCore.Blazor
+        ClassDeclaration -  - public - TestComponent - Microsoft.AspNetCore.Blazor.Components.BlazorComponent - 
+            MethodDeclaration -  - protected override - void - BuildRenderTree
+                CSharpCode - 
+                    IntermediateToken -  - CSharp - base.BuildRenderTree(builder);
+                HtmlContent - 
+                    IntermediateToken -  - Html - <input 
+                HtmlAttribute -  - =" - "
+                    CSharpExpressionAttributeValue -  - 
+                        IntermediateToken -  - CSharp - Microsoft.AspNetCore.Blazor.Components.BindMethods.GetEventHandlerValue<Microsoft.AspNetCore.Blazor.UIMouseEventArgs>(
+                        IntermediateToken - (54:1,18 [8] x:\dir\subdir\Test\TestComponent.cshtml) - CSharp - x => { }
+                        IntermediateToken -  - CSharp - )
+                HtmlContent - 
+                    IntermediateToken -  - Html - />

test/Microsoft.AspNetCore.Blazor.Build.Test/TestFiles/RuntimeCodeGenerationTest/EventHandler_OnElement_WithEventArgsMethodGroup/TestComponent.codegen.cs

@@ -0,0 +1,32 @@
+// <auto-generated/>
+#pragma warning disable 1591
+namespace Test
+{
+    #line hidden
+    using System;
+    using System.Collections.Generic;
+    using System.Linq;
+    using System.Threading.Tasks;
+    using Microsoft.AspNetCore.Blazor;
+    public class TestComponent : Microsoft.AspNetCore.Blazor.Components.BlazorComponent
+    {
+        #pragma warning disable 1998
+        protected override void BuildRenderTree(Microsoft.AspNetCore.Blazor.RenderTree.RenderTreeBuilder builder)
+        {
+            base.BuildRenderTree(builder);
+            builder.OpenElement(0, "input");
+            builder.AddAttribute(1, "onclick", Microsoft.AspNetCore.Blazor.Components.BindMethods.GetEventHandlerValue<Microsoft.AspNetCore.Blazor.UIMouseEventArgs>(OnClick));
+            builder.CloseElement();
+            builder.AddContent(2, "\n");
+        }
+        #pragma warning restore 1998
+#line 3 "x:\dir\subdir\Test\TestComponent.cshtml"
+            
+    void OnClick(UIMouseEventArgs e) {
+    }
+
+#line default
+#line hidden
+    }
+}
+#pragma warning restore 1591

test/Microsoft.AspNetCore.Blazor.Build.Test/TestFiles/RuntimeCodeGenerationTest/EventHandler_OnElement_WithEventArgsMethodGroup/TestComponent.ir.txt

@@ -0,0 +1,24 @@
+Document - 
+    NamespaceDeclaration -  - Test
+        UsingDirective - (3:1,1 [14] ) - System
+        UsingDirective - (18:2,1 [34] ) - System.Collections.Generic
+        UsingDirective - (53:3,1 [19] ) - System.Linq
+        UsingDirective - (73:4,1 [30] ) - System.Threading.Tasks
+        UsingDirective - (1:0,1 [35] x:\dir\subdir\Test\TestComponent.cshtml) - Microsoft.AspNetCore.Blazor
+        ClassDeclaration -  - public - TestComponent - Microsoft.AspNetCore.Blazor.Components.BlazorComponent - 
+            MethodDeclaration -  - protected override - void - BuildRenderTree
+                CSharpCode - 
+                    IntermediateToken -  - CSharp - base.BuildRenderTree(builder);
+                HtmlContent - 
+                    IntermediateToken -  - Html - <input 
+                HtmlAttribute -  - =" - "
+                    CSharpExpressionAttributeValue -  - 
+                        IntermediateToken -  - CSharp - Microsoft.AspNetCore.Blazor.Components.BindMethods.GetEventHandlerValue<Microsoft.AspNetCore.Blazor.UIMouseEventArgs>(
+                        IntermediateToken - (53:1,17 [7] x:\dir\subdir\Test\TestComponent.cshtml) - CSharp - OnClick
+                        IntermediateToken -  - CSharp - )
+                HtmlContent - 
+                    IntermediateToken -  - Html - />
+                HtmlContent - (64:1,28 [2] x:\dir\subdir\Test\TestComponent.cshtml)
+                    IntermediateToken - (64:1,28 [2] x:\dir\subdir\Test\TestComponent.cshtml) - Html - \n
+            CSharpCode - (78:2,12 [49] x:\dir\subdir\Test\TestComponent.cshtml)
+                IntermediateToken - (78:2,12 [49] x:\dir\subdir\Test\TestComponent.cshtml) - CSharp - \n    void OnClick(UIMouseEventArgs e) {\n    }\n

test/Microsoft.AspNetCore.Blazor.Build.Test/TestFiles/RuntimeCodeGenerationTest/EventHandler_OnElement_WithEventArgsMethodGroup/TestComponent.mappings.txt

@@ -0,0 +1,11 @@
+Source Location: (78:2,12 [49] x:\dir\subdir\Test\TestComponent.cshtml)
+|
+    void OnClick(UIMouseEventArgs e) {
+    }
+|
+Generated Location: (964:23,12 [49] )
+|
+    void OnClick(UIMouseEventArgs e) {
+    }
+|
+

test/Microsoft.AspNetCore.Blazor.Build.Test/TestFiles/RuntimeCodeGenerationTest/EventHandler_OnElement_WithNoArgMethodGroup/TestComponent.codegen.cs

@@ -0,0 +1,32 @@
+// <auto-generated/>
+#pragma warning disable 1591
+namespace Test
+{
+    #line hidden
+    using System;
+    using System.Collections.Generic;
+    using System.Linq;
+    using System.Threading.Tasks;
+    using Microsoft.AspNetCore.Blazor;
+    public class TestComponent : Microsoft.AspNetCore.Blazor.Components.BlazorComponent
+    {
+        #pragma warning disable 1998
+        protected override void BuildRenderTree(Microsoft.AspNetCore.Blazor.RenderTree.RenderTreeBuilder builder)
+        {
+            base.BuildRenderTree(builder);
+            builder.OpenElement(0, "input");
+            builder.AddAttribute(1, "onclick", Microsoft.AspNetCore.Blazor.Components.BindMethods.GetEventHandlerValue<Microsoft.AspNetCore.Blazor.UIMouseEventArgs>(OnClick));
+            builder.CloseElement();
+            builder.AddContent(2, "\n");
+        }
+        #pragma warning restore 1998
+#line 3 "x:\dir\subdir\Test\TestComponent.cshtml"
+            
+    void OnClick() {
+    }
+
+#line default
+#line hidden
+    }
+}
+#pragma warning restore 1591

test/Microsoft.AspNetCore.Blazor.Build.Test/TestFiles/RuntimeCodeGenerationTest/EventHandler_OnElement_WithNoArgMethodGroup/TestComponent.ir.txt

@@ -0,0 +1,24 @@
+Document - 
+    NamespaceDeclaration -  - Test
+        UsingDirective - (3:1,1 [14] ) - System
+        UsingDirective - (18:2,1 [34] ) - System.Collections.Generic
+        UsingDirective - (53:3,1 [19] ) - System.Linq
+        UsingDirective - (73:4,1 [30] ) - System.Threading.Tasks
+        UsingDirective - (1:0,1 [35] x:\dir\subdir\Test\TestComponent.cshtml) - Microsoft.AspNetCore.Blazor
+        ClassDeclaration -  - public - TestComponent - Microsoft.AspNetCore.Blazor.Components.BlazorComponent - 
+            MethodDeclaration -  - protected override - void - BuildRenderTree
+                CSharpCode - 
+                    IntermediateToken -  - CSharp - base.BuildRenderTree(builder);
+                HtmlContent - 
+                    IntermediateToken -  - Html - <input 
+                HtmlAttribute -  - =" - "
+                    CSharpExpressionAttributeValue -  - 
+                        IntermediateToken -  - CSharp - Microsoft.AspNetCore.Blazor.Components.BindMethods.GetEventHandlerValue<Microsoft.AspNetCore.Blazor.UIMouseEventArgs>(
+                        IntermediateToken - (53:1,17 [7] x:\dir\subdir\Test\TestComponent.cshtml) - CSharp - OnClick
+                        IntermediateToken -  - CSharp - )
+                HtmlContent - 
+                    IntermediateToken -  - Html - />
+                HtmlContent - (64:1,28 [2] x:\dir\subdir\Test\TestComponent.cshtml)
+                    IntermediateToken - (64:1,28 [2] x:\dir\subdir\Test\TestComponent.cshtml) - Html - \n
+            CSharpCode - (78:2,12 [31] x:\dir\subdir\Test\TestComponent.cshtml)
+                IntermediateToken - (78:2,12 [31] x:\dir\subdir\Test\TestComponent.cshtml) - CSharp - \n    void OnClick() {\n    }\n

test/Microsoft.AspNetCore.Blazor.Build.Test/TestFiles/RuntimeCodeGenerationTest/EventHandler_OnElement_WithNoArgMethodGroup/TestComponent.mappings.txt

@@ -0,0 +1,11 @@
+Source Location: (78:2,12 [31] x:\dir\subdir\Test\TestComponent.cshtml)
+|
+    void OnClick() {
+    }
+|
+Generated Location: (964:23,12 [31] )
+|
+    void OnClick() {
+    }
+|
+

test/Microsoft.AspNetCore.Blazor.Build.Test/TestFiles/RuntimeCodeGenerationTest/EventHandler_OnElement_WithNoArgsLambdaDelegate/TestComponent.codegen.cs

@@ -0,0 +1,24 @@
+// <auto-generated/>
+#pragma warning disable 1591
+namespace Test
+{
+    #line hidden
+    using System;
+    using System.Collections.Generic;
+    using System.Linq;
+    using System.Threading.Tasks;
+    using Microsoft.AspNetCore.Blazor;
+    public class TestComponent : Microsoft.AspNetCore.Blazor.Components.BlazorComponent
+    {
+        #pragma warning disable 1998
+        protected override void BuildRenderTree(Microsoft.AspNetCore.Blazor.RenderTree.RenderTreeBuilder builder)
+        {
+            base.BuildRenderTree(builder);
+            builder.OpenElement(0, "input");
+            builder.AddAttribute(1, "onclick", Microsoft.AspNetCore.Blazor.Components.BindMethods.GetEventHandlerValue<Microsoft.AspNetCore.Blazor.UIMouseEventArgs>(() => { }));
+            builder.CloseElement();
+        }
+        #pragma warning restore 1998
+    }
+}
+#pragma warning restore 1591

test/Microsoft.AspNetCore.Blazor.Build.Test/TestFiles/RuntimeCodeGenerationTest/EventHandler_OnElement_WithNoArgsLambdaDelegate/TestComponent.ir.txt

@@ -0,0 +1,20 @@
+Document - 
+    NamespaceDeclaration -  - Test
+        UsingDirective - (3:1,1 [14] ) - System
+        UsingDirective - (18:2,1 [34] ) - System.Collections.Generic
+        UsingDirective - (53:3,1 [19] ) - System.Linq
+        UsingDirective - (73:4,1 [30] ) - System.Threading.Tasks
+        UsingDirective - (1:0,1 [35] x:\dir\subdir\Test\TestComponent.cshtml) - Microsoft.AspNetCore.Blazor
+        ClassDeclaration -  - public - TestComponent - Microsoft.AspNetCore.Blazor.Components.BlazorComponent - 
+            MethodDeclaration -  - protected override - void - BuildRenderTree
+                CSharpCode - 
+                    IntermediateToken -  - CSharp - base.BuildRenderTree(builder);
+                HtmlContent - 
+                    IntermediateToken -  - Html - <input 
+                HtmlAttribute -  - =" - "
+                    CSharpExpressionAttributeValue -  - 
+                        IntermediateToken -  - CSharp - Microsoft.AspNetCore.Blazor.Components.BindMethods.GetEventHandlerValue<Microsoft.AspNetCore.Blazor.UIMouseEventArgs>(
+                        IntermediateToken - (54:1,18 [9] x:\dir\subdir\Test\TestComponent.cshtml) - CSharp - () => { }
+                        IntermediateToken -  - CSharp - )
+                HtmlContent - 
+                    IntermediateToken -  - Html - />

test/Microsoft.AspNetCore.Blazor.Test/RenderTreeBuilderTest.cs

@@ -498,7 +498,7 @@ namespace Microsoft.AspNetCore.Blazor.Test
         }
 
         [Fact]
-        public void AddAttribute_Element_EventHandler_AddsFrame()
+        public void AddAttribute_Element_UIEventHandler_AddsFrame()
         {
             // Arrange
             var builder = new RenderTreeBuilder(new TestRenderer());
@@ -518,7 +518,7 @@ namespace Microsoft.AspNetCore.Blazor.Test
         }
 
         [Fact]
-        public void AddAttribute_Element_NullEventHandler_IgnoresFrame()
+        public void AddAttribute_Element_NullUIEventHandler_IgnoresFrame()
         {
             // Arrange
             var builder = new RenderTreeBuilder(new TestRenderer());
@@ -534,6 +534,43 @@ namespace Microsoft.AspNetCore.Blazor.Test
                 frame => AssertFrame.Element(frame, "elem", 1, 0));
         }
 
+        [Fact]
+        public void AddAttribute_Element_Action_AddsFrame()
+        {
+            // Arrange
+            var builder = new RenderTreeBuilder(new TestRenderer());
+
+            var value = new Action(() => { });
+
+            // Act
+            builder.OpenElement(0, "elem");
+            builder.AddAttribute(1, "attr", value);
+            builder.CloseElement();
+
+            // Assert
+            Assert.Collection(
+                builder.GetFrames(),
+                frame => AssertFrame.Element(frame, "elem", 2, 0),
+                frame => AssertFrame.Attribute(frame, "attr", value, 1));
+        }
+
+        [Fact]
+        public void AddAttribute_Element_NullAction_IgnoresFrame()
+        {
+            // Arrange
+            var builder = new RenderTreeBuilder(new TestRenderer());
+
+            // Act
+            builder.OpenElement(0, "elem");
+            builder.AddAttribute(1, "attr", (Action)null);
+            builder.CloseElement();
+
+            // Assert
+            Assert.Collection(
+                builder.GetFrames(),
+                frame => AssertFrame.Element(frame, "elem", 1, 0));
+        }
+
         public static TheoryData<UIEventHandler> UIEventHandlerValues => new TheoryData<UIEventHandler>
         {
             null,
@@ -651,7 +688,7 @@ namespace Microsoft.AspNetCore.Blazor.Test
         }
 
         [Fact]
-        public void AddAttribute_Element_ObjectEventHandler_AddsFrame()
+        public void AddAttribute_Element_ObjectUIEventHandler_AddsFrame()
         {
             // Arrange
             var builder = new RenderTreeBuilder(new TestRenderer());
@@ -671,7 +708,7 @@ namespace Microsoft.AspNetCore.Blazor.Test
         }
 
         [Fact]
-        public void AddAttribute_Component_ObjectEventHandleValue_SetsAttributeValue()
+        public void AddAttribute_Component_ObjectUIEventHandleValue_SetsAttributeValue()
         {
             // Arrange
             var builder = new RenderTreeBuilder(new TestRenderer());
@@ -690,6 +727,46 @@ namespace Microsoft.AspNetCore.Blazor.Test
                 frame => AssertFrame.Attribute(frame, "attr", value, 1));
         }
 
+        [Fact]
+        public void AddAttribute_Element_ObjectAction_AddsFrame()
+        {
+            // Arrange
+            var builder = new RenderTreeBuilder(new TestRenderer());
+
+            var value = new Action(() => { });
+
+            // Act
+            builder.OpenElement(0, "elem");
+            builder.AddAttribute(1, "attr", (object)value);
+            builder.CloseElement();
+
+            // Assert
+            Assert.Collection(
+                builder.GetFrames(),
+                frame => AssertFrame.Element(frame, "elem", 2, 0),
+                frame => AssertFrame.Attribute(frame, "attr", value, 1));
+        }
+
+        [Fact]
+        public void AddAttribute_Component_ObjectAction_SetsAttributeValue()
+        {
+            // Arrange
+            var builder = new RenderTreeBuilder(new TestRenderer());
+
+            var value = new Action(() => { });
+
+            // Act
+            builder.OpenComponent<TestComponent>(0);
+            builder.AddAttribute(1, "attr", (object)value);
+            builder.CloseComponent();
+
+            // Assert
+            Assert.Collection(
+                builder.GetFrames(),
+                frame => AssertFrame.Component<TestComponent>(frame, 2, 0),
+                frame => AssertFrame.Attribute(frame, "attr", value, 1));
+        }
+
         [Fact]
         public void AddAttribute_Element_ObjectNull_IgnoresFrame()
         {

test/Microsoft.AspNetCore.Blazor.Test/RendererTest.cs

@@ -217,6 +217,34 @@ namespace Microsoft.AspNetCore.Blazor.Test
             Assert.Same(eventArgs, receivedArgs);
         }
 
+        [Fact]
+        public void CanDispatchActionEventsToTopLevelComponents()
+        {
+            // Arrange: Render a component with an event handler
+            var renderer = new TestRenderer();
+            object receivedArgs = null;
+
+            var component = new EventComponent
+            {
+                OnClickAction = () => { receivedArgs = new object(); }
+            };
+            var componentId = renderer.AssignComponentId(component);
+            component.TriggerRender();
+
+            var eventHandlerId = renderer.Batches.Single()
+                .ReferenceFrames
+                .First(frame => frame.AttributeValue != null)
+                .AttributeEventHandlerId;
+
+            // Assert: Event not yet fired
+            Assert.Null(receivedArgs);
+
+            // Act/Assert: Event can be fired
+            var eventArgs = new UIMouseEventArgs();
+            renderer.DispatchEvent(componentId, eventHandlerId, eventArgs);
+            Assert.NotNull(receivedArgs);
+        }
+
         [Fact]
         public void CanDispatchEventsToNestedComponents()
         {
@@ -955,6 +983,7 @@ namespace Microsoft.AspNetCore.Blazor.Test
         {
             public UIEventHandler OnTest { get; set; }
             public UIMouseEventHandler OnClick { get; set; }
+            public Action OnClickAction { get; set; }
 
             public bool SkipElement { get; set; }
             private int renderCount = 0;
@@ -974,11 +1003,15 @@ namespace Microsoft.AspNetCore.Blazor.Test
                     {
                         builder.AddAttribute(4, "onclick", OnClick);
                     }
+                    if (OnClickAction != null)
+                    {
+                        builder.AddAttribute(5, "onclickaction", OnClickAction);
+                    }
                     builder.CloseElement();
                     builder.CloseElement();
                 }
                 builder.CloseElement();
-                builder.AddContent(5, $"Render count: {++renderCount}");
+                builder.AddContent(6, $"Render count: {++renderCount}");
             }
 
             public void HandleEvent(UIEventHandler handler, UIEventArgs args)

test/testapps/BasicTestApp/AddRemoveChildComponents.cshtml

@@ -1,5 +1,4 @@
 @using System.Collections.Generic
-@using Microsoft.AspNetCore.Blazor
 Child components follow.
 <button class="addChild" onclick="@AddChild">Add</button>
 <button class="removeChild" onclick="@RemoveChild">Remove</button>
@@ -13,13 +12,13 @@ Child components follow.
     int numAdded = 0;
     List<string> currentChildrenMessages = new List<string>();
 
-    void AddChild(UIMouseEventArgs e)
+    void AddChild()
     {
         numAdded++;
         currentChildrenMessages.Add($"Child {numAdded}");
     }
 
-    void RemoveChild(UIMouseEventArgs e)
+    void RemoveChild()
     {
         if (currentChildrenMessages.Count > 0)
         {

test/testapps/BasicTestApp/BindCasesComponent.cshtml

@@ -64,7 +64,7 @@
     enum SelectableValue { First, Second, Third, Fourth }
     SelectableValue selectValue = SelectableValue.Second;
 
-    void AddAndSelectNewSelectOption(UIMouseEventArgs e)
+    void AddAndSelectNewSelectOption()
     {
         includeFourthOption = true;
         selectValue = SelectableValue.Fourth;

test/testapps/BasicTestApp/CounterComponent.cshtml

@@ -1,7 +1,6 @@
-@using Microsoft.AspNetCore.Blazor
-<h1>Counter</h1>
+<h1>Counter</h1>
 <p>Current count: @currentCount</p>
-<p><button onclick="@((handleClicks ? (Action<UIMouseEventArgs>)IncrementCount : null))">Click me</button></p>
+<p><button onclick="@((handleClicks ? (Action)IncrementCount : null))">Click me</button></p>
 
 <label>
     <input type="checkbox" bind="@handleClicks" />
@@ -12,7 +11,7 @@
     int currentCount = 0;
     bool handleClicks = true;
 
-    void IncrementCount(UIMouseEventArgs e)
+    void IncrementCount()
     {
         currentCount++;
     }

test/testapps/BasicTestApp/CounterComponentUsingChild.cshtml

@@ -1,5 +1,4 @@
-@using Microsoft.AspNetCore.Blazor
-<h1>Counter</h1>
+<h1>Counter</h1>
 
 <!-- Note: passing 'Message' parameter with lowercase name to show it's case insensitive -->
 <p>Current count: <MessageComponent message=@currentCount.ToString() /></p>
@@ -9,7 +8,7 @@
 @functions {
     int currentCount = 0;
 
-    void IncrementCount(UIMouseEventArgs e)
+    void IncrementCount()
     {
         currentCount++;
     }

test/testapps/BasicTestApp/ExternalContentPackage.cshtml

@@ -1,5 +1,4 @@
 @addTagHelper *, TestContentPackage
-@using Microsoft.AspNetCore.Blazor
 @using TestContentPackage
 
 <h1>Functionality and content from an external package</h1>
@@ -32,7 +31,7 @@
 {
     string result;
 
-    void ShowJavaScriptPrompt(UIMouseEventArgs e)
+    void ShowJavaScriptPrompt()
     {
         result = MyPrompt.Show("Hello!");
     }

test/testapps/BasicTestApp/HttpClientTest/CookieCounterComponent.cshtml

@@ -1,5 +1,4 @@
-@using Microsoft.AspNetCore.Blazor
-@inject System.Net.Http.HttpClient Http
+@inject System.Net.Http.HttpClient Http
 
 <h1>Cookie counter</h1>
 <p>The server increments the count by one on each request.</p>
@@ -18,13 +17,13 @@
     string testServerBaseUrl;
     string responseText;
 
-    async void DeleteCookie(UIMouseEventArgs e)
+    async void DeleteCookie()
     {
         await DoRequest("api/cookie/reset");
         StateHasChanged();
     }
 
-    async void GetAndIncrementCounter(UIMouseEventArgs e)
+    async void GetAndIncrementCounter()
     {
         await DoRequest("api/cookie/increment");
         StateHasChanged();

test/testapps/BasicTestApp/HttpClientTest/HttpRequestsComponent.cshtml

@@ -1,6 +1,5 @@
 @using System.Net
 @using System.Net.Http
-@using Microsoft.AspNetCore.Blazor
 @using Microsoft.AspNetCore.Blazor.Browser.Http
 @inject HttpClient Http
 
@@ -72,7 +71,7 @@
     string responseBody;
     string responseHeaders;
 
-    async void DoRequest(UIMouseEventArgs e)
+    async void DoRequest()
     {
         responseStatusCode = null;
 
@@ -125,7 +124,7 @@
         StateHasChanged();
     }
 
-    void AddHeader(UIMouseEventArgs e)
+    void AddHeader()
         => requestHeaders.Add(new RequestHeader());
 
     void RemoveHeader(RequestHeader header)

test/testapps/TestContentPackage/ComponentFromPackage.cshtml

@@ -1,5 +1,4 @@
-@using Microsoft.AspNetCore.Blazor
-<div class="special-style">
+<div class="special-style">
     This component, including the CSS and image required to produce its
     elegant styling, is in an external NuGet package.
     <button onclick="@ChangeLabel">@buttonLabel </button>
@@ -8,7 +7,7 @@
 @functions {
     string buttonLabel = "Click me";
 
-    void ChangeLabel(UIMouseEventArgs e)
+    void ChangeLabel()
     {
         buttonLabel = "It works";
     }