Print a useful error message when render-mode isn't specified

src/Mvc/Mvc.TagHelpers/src/ComponentTagHelper.cs

@@ -14,14 +14,16 @@ namespace Microsoft.AspNetCore.Mvc.TagHelpers
     /// <summary>
     /// A <see cref="TagHelper"/> that renders a Razor component.
     /// </summary>
-    [HtmlTargetElement("component", Attributes = ComponentTypeName, TagStructure = TagStructure.WithoutEndTag)]
+    [HtmlTargetElement(TagHelperName, Attributes = ComponentTypeName, TagStructure = TagStructure.WithoutEndTag)]
     public sealed class ComponentTagHelper : TagHelper
     {
+        private const string TagHelperName = "component";
         private const string ComponentParameterName = "params";
         private const string ComponentParameterPrefix = "param-";
         private const string ComponentTypeName = "type";
         private const string RenderModeName = "render-mode";
         private IDictionary<string, object> _parameters;
+        private RenderMode? _renderMode;
 
         /// <summary>
         /// Gets or sets the <see cref="Rendering.ViewContext"/> for the current request.
@@ -54,7 +56,29 @@ namespace Microsoft.AspNetCore.Mvc.TagHelpers
         /// Gets or sets the <see cref="Rendering.RenderMode"/>
         /// </summary>
         [HtmlAttributeName(RenderModeName)]
-        public RenderMode RenderMode { get; set; }
+        public RenderMode RenderMode
+        {
+            get => _renderMode ?? default;
+            set
+            {
+                switch (value)
+                {
+                    case RenderMode.Server:
+                    case RenderMode.ServerPrerendered:
+                    case RenderMode.Static:
+                        _renderMode = value;
+                        break;
+
+                    default:
+                        throw new ArgumentException(
+                            message: Resources.FormatInvalidEnumArgument(
+                                nameof(value),
+                                value,
+                                typeof(RenderMode).FullName),
+                            paramName: nameof(value));
+                }
+            }
+        }
 
         /// <inheritdoc />
         public async override Task ProcessAsync(TagHelperContext context, TagHelperOutput output)
@@ -69,6 +93,11 @@ namespace Microsoft.AspNetCore.Mvc.TagHelpers
                 throw new ArgumentNullException(nameof(output));
             }
 
+            if (_renderMode is null)
+            {
+                throw new ArgumentException(Resources.FormatAttributeIsRequired(RenderModeName, TagHelperName), nameof(RenderMode));
+            }
+
             var componentRenderer = ViewContext.HttpContext.RequestServices.GetRequiredService<IComponentRenderer>();
             var result = await componentRenderer.RenderComponentAsync(ViewContext, ComponentType, RenderMode, _parameters);
 

src/Mvc/Mvc.TagHelpers/src/Resources.resx

@@ -162,4 +162,7 @@
   <data name="ViewEngine_FallbackViewNotFound" xml:space="preserve">
     <value>The fallback partial view '{0}' was not found. The following locations were searched:{1}</value>
   </data>
+  <data name="AttributeIsRequired" xml:space="preserve">
+    <value>A value for the '{0}' attribute must be supplied to the '{1}' tag helper.</value>
+  </data>
 </root>

src/Mvc/Mvc.TagHelpers/test/ComponentTagHelperTest.cs

@@ -3,13 +3,13 @@
 
 using System;
 using System.Collections.Generic;
-using System.IO.Pipes;
 using System.Threading.Tasks;
 using Microsoft.AspNetCore.Html;
 using Microsoft.AspNetCore.Http;
 using Microsoft.AspNetCore.Mvc.Rendering;
 using Microsoft.AspNetCore.Mvc.ViewFeatures;
 using Microsoft.AspNetCore.Razor.TagHelpers;
+using Microsoft.AspNetCore.Testing;
 using Microsoft.Extensions.DependencyInjection;
 using Moq;
 using Xunit;
@@ -25,6 +25,7 @@ namespace Microsoft.AspNetCore.Mvc.TagHelpers
             var tagHelper = new ComponentTagHelper
             {
                 ViewContext = GetViewContext(),
+                RenderMode = RenderMode.Static,
             };
             var context = GetTagHelperContext();
             var output = GetTagHelperOutput();
@@ -38,6 +39,24 @@ namespace Microsoft.AspNetCore.Mvc.TagHelpers
             Assert.Null(output.TagName);
         }
 
+        [Fact]
+        public async Task ProcessAsync_WithoutSpecifyingRenderMode_ThrowsError()
+        {
+            // Arrange
+            var tagHelper = new ComponentTagHelper
+            {
+                ViewContext = GetViewContext(),
+            };
+            var context = GetTagHelperContext();
+            var output = GetTagHelperOutput();
+
+            // Act & Assert
+            await ExceptionAssert.ThrowsArgumentAsync(
+                () => tagHelper.ProcessAsync(context, output),
+                nameof(RenderMode),
+                "A value for the 'render-mode' attribute must be supplied to the 'component' tag helper.");
+        }
+
         private static TagHelperContext GetTagHelperContext()
         {
             return new TagHelperContext(