Add ability to add component frame using runtime type object instead of generic param

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

@@ -148,16 +148,27 @@ namespace Microsoft.AspNetCore.Blazor.RenderTree
         /// <typeparam name="TComponent">The type of the child component.</typeparam>
         /// <param name="sequence">An integer that represents the position of the instruction in the source code.</param>
         public void OpenComponent<TComponent>(int sequence) where TComponent : IComponent
+            => OpenComponentUnchecked(sequence, typeof(TComponent));
+
+        /// <summary>
+        /// Appends a frame representing a child component.
+        /// </summary>
+        /// <param name="sequence">An integer that represents the position of the instruction in the source code.</param>
+        /// <param name="componentType">The type of the child component.</param>
+        public void OpenComponent(int sequence, Type componentType)
+        {
+            if (!typeof(IComponent).IsAssignableFrom(componentType))
+            {
+                throw new ArgumentException($"The component type must implement {typeof(IComponent).FullName}.");
+            }
+
+            OpenComponentUnchecked(sequence, componentType);
+        }
+
+        private void OpenComponentUnchecked(int sequence, Type componentType)
         {
-            // Currently, child components can't have further grandchildren of their own, so it would
-            // technically be possible to skip their CloseElement calls and not track them in _openElementIndices.
-            // However at some point we might want to have the grandchildren frames available at runtime
-            // (rather than being parsed as attributes at compile time) so that we could have APIs for
-            // components to query the complete hierarchy of transcluded frames instead of forcing the
-            // transcluded subtree to be in a particular shape such as representing key/value pairs.
-            // So it's more flexible if we track open/close frames for components explicitly.
             _openElementIndices.Push(_entries.Count);
-            Append(RenderTreeFrame.ChildComponent<TComponent>(sequence));
+            Append(RenderTreeFrame.ChildComponent(sequence, componentType));
         }
 
         /// <summary>

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

@@ -201,8 +201,8 @@ namespace Microsoft.AspNetCore.Blazor.RenderTree
         internal static RenderTreeFrame Attribute(int sequence, string name, object value)
             => new RenderTreeFrame(sequence, attributeName: name, attributeValue: value);
 
-        internal static RenderTreeFrame ChildComponent<T>(int sequence) where T : IComponent
-            => new RenderTreeFrame(sequence, typeof(T), 0);
+        internal static RenderTreeFrame ChildComponent(int sequence, Type componentType)
+            => new RenderTreeFrame(sequence, componentType, 0);
 
         internal static RenderTreeFrame Region(int sequence)
             => new RenderTreeFrame(sequence, regionSubtreeLength: 0);

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

@@ -17,7 +17,7 @@ namespace Microsoft.AspNetCore.Blazor.Test
             // Arrange
             var frames = new[]
             {
-                RenderTreeFrame.ChildComponent<FakeComponent>(0).WithComponentSubtreeLength(1)
+                RenderTreeFrame.ChildComponent(0, typeof(FakeComponent)).WithComponentSubtreeLength(1)
             };
             var parameterCollection = new ParameterCollection(frames, 0);
 

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

@@ -260,7 +260,7 @@ namespace Microsoft.AspNetCore.Blazor.Test
         }
 
         [Fact]
-        public void CanAddChildComponents()
+        public void CanAddChildComponentsUsingGenericParam()
         {
             // Arrange
             var builder = new RenderTreeBuilder(new TestRenderer());
@@ -286,6 +286,34 @@ namespace Microsoft.AspNetCore.Blazor.Test
                 frame => AssertFrame.Attribute(frame, "child2attribute", "C"));
         }
 
+        [Fact]
+        public void CanAddChildComponentsUsingTypeArgument()
+        {
+            // Arrange
+            var builder = new RenderTreeBuilder(new TestRenderer());
+
+            // Act
+            var componentType = typeof(TestComponent);
+            builder.OpenElement(10, "parent");                   //  0: <parent>
+            builder.OpenComponent(11, componentType);            //  1:     <testcomponent
+            builder.AddAttribute(12, "child1attribute1", "A");   //  2:       child1attribute1="A"
+            builder.AddAttribute(13, "child1attribute2", "B");   //  3:       child1attribute2="B">
+            builder.CloseComponent();                            //         </testcomponent>
+            builder.OpenComponent(14, componentType);            //  4:     <testcomponent
+            builder.AddAttribute(15, "child2attribute", "C");    //  5:       child2attribute="C">
+            builder.CloseComponent();                            //         </testcomponent>
+            builder.CloseElement();                              //     </parent>
+
+            // Assert
+            Assert.Collection(builder.GetFrames(),
+                frame => AssertFrame.Element(frame, "parent", 6),
+                frame => AssertFrame.Component<TestComponent>(frame),
+                frame => AssertFrame.Attribute(frame, "child1attribute1", "A"),
+                frame => AssertFrame.Attribute(frame, "child1attribute2", "B"),
+                frame => AssertFrame.Component<TestComponent>(frame),
+                frame => AssertFrame.Attribute(frame, "child2attribute", "C"));
+        }
+
         [Fact]
         public void CanAddRegions()
         {