Define ILayoutComponent so there's compile-time validation that layouts are defined correctly

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

@@ -14,6 +14,7 @@
  */
 
 using Microsoft.AspNetCore.Blazor.Components;
+using Microsoft.AspNetCore.Blazor.Layouts;
 using System;
 
 namespace Microsoft.AspNetCore.Mvc
@@ -28,7 +29,7 @@ namespace Microsoft.AspNetCore.Mvc.Razor
     {
         // This is temporary and exists only to support TemporaryLayoutPass.
         // It will be removed when we can add Blazor-specific directives.
-        public object Layout<TLayout>() where TLayout : IComponent
+        public object Layout<TLayout>() where TLayout : ILayoutComponent
             => throw new NotImplementedException();
 
         // Similar temporary mechanism as above

src/Microsoft.AspNetCore.Blazor/Layouts/ILayoutComponent.cs

@@ -0,0 +1,19 @@
+// 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.AspNetCore.Blazor.Components;
+using Microsoft.AspNetCore.Blazor.RenderTree;
+
+namespace Microsoft.AspNetCore.Blazor.Layouts
+{
+    /// <summary>
+    /// Indicates that the type represents a layout.
+    /// </summary>
+    public interface ILayoutComponent : IComponent
+    {
+        /// <summary>
+        /// Gets or sets the content to be rendered inside the layout.
+        /// </summary>
+        RenderFragment Body { get; set; }
+    }
+}

src/Microsoft.AspNetCore.Blazor/Layouts/LayoutAttribute.cs

@@ -1,7 +1,6 @@
 // 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.AspNetCore.Blazor.Components;
 using System;
 
 namespace Microsoft.AspNetCore.Blazor.Layouts
@@ -13,22 +12,22 @@ namespace Microsoft.AspNetCore.Blazor.Layouts
     public class LayoutAttribute : Attribute
     {
         /// <summary>
-        /// The type of the layout. The type always implements <see cref="IComponent"/>.
+        /// The type of the layout. The type always implements <see cref="ILayoutComponent"/>.
         /// </summary>
         public Type LayoutType { get; private set; }
 
         /// <summary>
         /// Constructs an instance of <see cref="LayoutAttribute"/>.
         /// </summary>
-        /// <param name="layoutType">The type of the layout. This must implement <see cref="IComponent"/>.</param>
+        /// <param name="layoutType">The type of the layout. This must implement <see cref="ILayoutComponent"/>.</param>
         public LayoutAttribute(Type layoutType)
         {
             LayoutType = layoutType ?? throw new ArgumentNullException(nameof(layoutType));
 
-            if (!typeof(IComponent).IsAssignableFrom(layoutType))
+            if (!typeof(ILayoutComponent).IsAssignableFrom(layoutType))
             {
                 throw new ArgumentException($"Invalid layout type: {layoutType.FullName} " +
-                    $"does not implement {typeof(IComponent).FullName}.");
+                    $"does not implement {typeof(ILayoutComponent).FullName}.");
             }
         }
     }

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

@@ -375,7 +375,7 @@ namespace Microsoft.AspNetCore.Blazor.Build.Test
         public void SupportsLayoutDeclarationsViaTemporarySyntax()
         {
             // Arrange/Act
-            var testComponentTypeName = typeof(TestComponent).FullName.Replace('+', '.');
+            var testComponentTypeName = typeof(TestLayout).FullName.Replace('+', '.');
             var component = CompileToComponent(
                 $"@(Layout<{testComponentTypeName}>())" +
                 $"Hello");
@@ -384,7 +384,7 @@ namespace Microsoft.AspNetCore.Blazor.Build.Test
             // Assert
             var layoutAttribute = component.GetType().GetCustomAttribute<LayoutAttribute>();
             Assert.NotNull(layoutAttribute);
-            Assert.Equal(typeof(TestComponent), layoutAttribute.LayoutType);
+            Assert.Equal(typeof(TestLayout), layoutAttribute.LayoutType);
             Assert.Collection(frames,
                 frame => AssertFrame.Text(frame, "Hello"));
         }
@@ -540,6 +540,19 @@ namespace Microsoft.AspNetCore.Blazor.Build.Test
             }
         }
 
+        public class TestLayout : ILayoutComponent
+        {
+            public RenderFragment Body { get; set; }
+
+            public void Init(RenderHandle renderHandle)
+            {
+            }
+
+            public void SetParameters(ParameterCollection parameters)
+            {
+            }
+        }
+
         public interface ITestInterface { }
     }
 }