Ensure layouts are not executed without a contents page

src/RazorBlade.Library/HtmlLayout.cs

@@ -1,4 +1,7 @@
 using System;
+using System.ComponentModel;
+using System.Diagnostics.CodeAnalysis;
+using System.IO;
 using System.Threading;
 using System.Threading.Tasks;
 
@@ -9,6 +12,8 @@ namespace RazorBlade;
 /// </summary>
 public abstract class HtmlLayout : HtmlTemplate, IRazorLayout
 {
+    private const string _contentsRequiredErrorMessage = "Layout pages can only be rendered when associated with a contents page.";
+
     private IRazorExecutionResult? _layoutInput;
 
     async Task<IRazorExecutionResult> IRazorLayout.ExecuteLayoutAsync(IRazorExecutionResult input)
@@ -24,6 +29,54 @@ async Task<IRazorExecutionResult> IRazorLayout.ExecuteLayoutAsync(IRazorExecutio
         }
     }
 
+    private protected override Task<IRazorExecutionResult> ExecuteAsyncCore(CancellationToken cancellationToken)
+    {
+        if (_layoutInput is null)
+            throw new InvalidOperationException(_contentsRequiredErrorMessage);
+
+        return base.ExecuteAsyncCore(cancellationToken);
+    }
+
+    /// <summary>
+    /// Should not be used on layout pages.
+    /// </summary>
+    [Obsolete(_contentsRequiredErrorMessage, true)]
+    [EditorBrowsable(EditorBrowsableState.Never)]
+    [SuppressMessage("ReSharper", "UnusedParameter.Global")]
+    [SuppressMessage("ReSharper", "MemberCanBeMadeStatic.Global")]
+    public new void Render(CancellationToken cancellationToken = default)
+        => throw new InvalidOperationException(_contentsRequiredErrorMessage);
+
+    /// <summary>
+    /// Should not be used on layout pages.
+    /// </summary>
+    [Obsolete(_contentsRequiredErrorMessage, true)]
+    [EditorBrowsable(EditorBrowsableState.Never)]
+    [SuppressMessage("ReSharper", "UnusedParameter.Global")]
+    [SuppressMessage("ReSharper", "MemberCanBeMadeStatic.Global")]
+    public new void Render(TextWriter textWriter, CancellationToken cancellationToken = default)
+        => throw new InvalidOperationException(_contentsRequiredErrorMessage);
+
+    /// <summary>
+    /// Should not be used on layout pages.
+    /// </summary>
+    [Obsolete(_contentsRequiredErrorMessage, true)]
+    [EditorBrowsable(EditorBrowsableState.Never)]
+    [SuppressMessage("ReSharper", "UnusedParameter.Global")]
+    [SuppressMessage("ReSharper", "MemberCanBeMadeStatic.Global")]
+    public new Task<string> RenderAsync(CancellationToken cancellationToken = default)
+        => throw new InvalidOperationException(_contentsRequiredErrorMessage);
+
+    /// <summary>
+    /// Should not be used on layout pages.
+    /// </summary>
+    [Obsolete(_contentsRequiredErrorMessage, true)]
+    [EditorBrowsable(EditorBrowsableState.Never)]
+    [SuppressMessage("ReSharper", "UnusedParameter.Global")]
+    [SuppressMessage("ReSharper", "MemberCanBeMadeStatic.Global")]
+    public new Task RenderAsync(TextWriter textWriter, CancellationToken cancellationToken = default)
+        => throw new InvalidOperationException(_contentsRequiredErrorMessage);
+
     /// <summary>
     /// Returns the inner page body.
     /// </summary>

src/RazorBlade.Library/RazorTemplate.cs

@@ -139,7 +139,7 @@ private async Task<StringBuilder> RenderAsyncCore(CancellationToken cancellation
     /// <summary>
     /// Calls the <see cref="ExecuteAsync"/> method in a new <see cref="ExecutionScope"/>.
     /// </summary>
-    private protected async Task<IRazorExecutionResult> ExecuteAsyncCore(CancellationToken cancellationToken)
+    private protected virtual async Task<IRazorExecutionResult> ExecuteAsyncCore(CancellationToken cancellationToken)
     {
         cancellationToken.ThrowIfCancellationRequested();
 

src/RazorBlade.Tests/HtmlLayoutTests.cs

@@ -1,4 +1,6 @@
 using System;
+using System.IO;
+using System.Threading;
 using System.Threading.Tasks;
 using NUnit.Framework;
 using RazorBlade.Tests.Support;
@@ -170,6 +172,18 @@ public void should_throw_when_duplicate_section_is_defined()
         Assert.Throws<InvalidOperationException>(() => page.Render());
     }
 
+    [Test, Obsolete]
+    public void should_throw_on_render()
+    {
+        var layout = new Layout(_ => { });
+
+        Assert.Throws<InvalidOperationException>(() => layout.Render(CancellationToken.None));
+        Assert.Throws<InvalidOperationException>(() => layout.Render(TextWriter.Null, CancellationToken.None));
+        Assert.Throws<InvalidOperationException>(() => layout.RenderAsync(CancellationToken.None).GetAwaiter().GetResult());
+        Assert.Throws<InvalidOperationException>(() => layout.RenderAsync(TextWriter.Null, CancellationToken.None).GetAwaiter().GetResult());
+        Assert.Throws<InvalidOperationException>(() => ((RazorTemplate)layout).Render(CancellationToken.None));
+    }
+
     private class Template : HtmlTemplate
     {
         private readonly Action<Template> _executeAction;