In RazorCompiler, support @using statements

src/Microsoft.Blazor.Build/Core/RazorCompilation/Engine/BlazorIntermediateNodeWriter.cs

@@ -244,7 +244,7 @@ namespace Microsoft.Blazor.Build.Core.RazorCompilation.Engine
 
         public override void WriteUsingDirective(CodeRenderingContext context, UsingDirectiveIntermediateNode node)
         {
-            throw new System.NotImplementedException(nameof(WriteUsingDirective));
+            context.CodeWriter.WriteUsing(node.Content, endLine: true);
         }
 
         private static string GetContent(HtmlContentIntermediateNode node)

test/Microsoft.Blazor.Build.Test/RazorCompilerTest.cs

@@ -286,6 +286,23 @@ namespace Microsoft.Blazor.Build.Test
                 });
         }
 
+        [Fact]
+        public void SupportsUsingStatements()
+        {
+            // Arrange
+            var treeBuilder = new RenderTreeBuilder(new TestRenderer());
+
+            // Arrange/Act
+            var component = CompileToComponent(@"
+                @using System.Collections.Generic
+                @(typeof(List<string>).FullName)");
+            component.BuildRenderTree(treeBuilder);
+
+            // Assert
+            Assert.Collection(treeBuilder.GetNodes().Where(NotWhitespace),
+                node => AssertNode.Text(node, typeof(List<string>).FullName));
+        }
+
         private static bool NotWhitespace(RenderTreeNode node)
             => node.NodeType != RenderTreeNodeType.Text
             || !string.IsNullOrWhiteSpace(node.TextContent);

test/testapps/BasicTestApp/KeyPressEventComponent.cs

@@ -1,36 +0,0 @@
-// 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 Microsoft.Blazor.Components;
-using Microsoft.Blazor.RenderTree;
-using System.Collections.Generic;
-
-namespace BasicTestApp
-{
-    public class KeyPressEventComponent : IComponent
-    {
-        private List<string> keysPressed = new List<string>();
-
-        public void BuildRenderTree(RenderTreeBuilder builder)
-        {
-            builder.AddText("Type here:");
-            builder.OpenElement("input");
-            builder.AddAttribute("onkeypress", OnKeyPressed);
-            builder.CloseElement();
-
-            builder.OpenElement("ul");
-            foreach (var key in keysPressed)
-            {
-                builder.OpenElement("li");
-                builder.AddText(key);
-                builder.CloseElement();
-            }
-            builder.CloseElement();
-        }
-
-        private void OnKeyPressed(UIEventArgs eventInfo)
-        {
-            keysPressed.Add(((UIKeyboardEventArgs)eventInfo).Key);
-        }
-    }
-}

test/testapps/BasicTestApp/KeyPressEventComponent.cshtml

@@ -0,0 +1,18 @@
+@using System.Collections.Generic
+@using Microsoft.Blazor.RenderTree
+Type here: <input onkeypress=@OnKeyPressed />
+<ul>
+    @foreach (var key in keysPressed)
+    {
+        <li>@key</li>
+    }
+</ul>
+
+@functions {
+    List<string> keysPressed = new List<string>();
+
+    void OnKeyPressed(UIEventArgs eventArgs)
+    {
+        keysPressed.Add(((UIKeyboardEventArgs)eventArgs).Key);
+    }
+}