Provide a tooling gesture that suggests values for required component parameters are not specified.

src/Components/Components/src/DynamicComponent.cs

@@ -32,6 +32,7 @@ public DynamicComponent()
         /// </summary>
         [Parameter]
         [DynamicallyAccessedMembers(DynamicallyAccessedMemberTypes.All)]
+        [EditorRequired]
         public Type Type { get; set; } = default!;
 
         /// <summary>

src/Components/Components/src/EditorRequiredAttribute.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 System;
+
+namespace Microsoft.AspNetCore.Components
+{
+    /// <summary>
+    /// Specifies that the component parameter is required to be provided by the user when authoring it in the editor.
+    /// <para>
+    /// If a value for this parameter is not provided, editors or build tools may provide warnings indicating the user to
+    /// specify a value. This attribute is only valid on properties marked with <see cref="ParameterAttribute"/>.
+    /// </para>
+    /// </summary>
+    [AttributeUsage(AttributeTargets.Property, AllowMultiple = false)]
+    public sealed class EditorRequiredAttribute : Attribute
+    {
+    }
+}

src/Components/Components/src/PublicAPI.Unshipped.txt

@@ -9,6 +9,8 @@ Microsoft.AspNetCore.Components.ComponentApplicationState.PersistAsJson<TValue>(
 Microsoft.AspNetCore.Components.ComponentApplicationState.PersistState(string! key, byte[]! value) -> void
 Microsoft.AspNetCore.Components.ComponentApplicationState.TryTakeAsJson<TValue>(string! key, out TValue? instance) -> bool
 Microsoft.AspNetCore.Components.ComponentApplicationState.TryTakePersistedState(string! key, out byte[]? value) -> bool
+Microsoft.AspNetCore.Components.EditorRequiredAttribute
+Microsoft.AspNetCore.Components.EditorRequiredAttribute.EditorRequiredAttribute() -> void
 Microsoft.AspNetCore.Components.ErrorBoundaryBase
 Microsoft.AspNetCore.Components.ErrorBoundaryBase.ChildContent.get -> Microsoft.AspNetCore.Components.RenderFragment?
 Microsoft.AspNetCore.Components.ErrorBoundaryBase.ChildContent.set -> void

src/Components/Components/src/RouteView.cs

@@ -25,6 +25,7 @@ public class RouteView : IComponent
         /// displayed and the parameter values that will be supplied to the page.
         /// </summary>
         [Parameter]
+        [EditorRequired]
         public RouteData RouteData { get; set; }
 
         /// <summary>

src/Components/Components/src/Routing/Router.cs

@@ -47,7 +47,9 @@ static readonly IReadOnlyDictionary<string, object> _emptyParametersDictionary
         /// <summary>
         /// Gets or sets the assembly that should be searched for components matching the URI.
         /// </summary>
-        [Parameter] public Assembly AppAssembly { get; set; }
+        [Parameter]
+        [EditorRequired]
+        public Assembly AppAssembly { get; set; }
 
         /// <summary>
         /// Gets or sets a collection of additional assemblies that should be searched for components
@@ -58,12 +60,16 @@ static readonly IReadOnlyDictionary<string, object> _emptyParametersDictionary
         /// <summary>
         /// Gets or sets the content to display when no match is found for the requested route.
         /// </summary>
-        [Parameter] public RenderFragment NotFound { get; set; }
+        [Parameter]
+        [EditorRequired]
+        public RenderFragment NotFound { get; set; }
 
         /// <summary>
         /// Gets or sets the content to display when a match is found for the requested route.
         /// </summary>
-        [Parameter] public RenderFragment<RouteData> Found { get; set; }
+        [Parameter]
+        [EditorRequired]
+        public RenderFragment<RouteData> Found { get; set; }
 
         /// <summary>
         /// Get or sets the content to display when asynchronous navigation is in progress.

src/Razor/Microsoft.AspNetCore.Razor.Language/src/BoundAttributeDescriptor.cs

@@ -29,6 +29,8 @@ protected BoundAttributeDescriptor(string kind)
 
         public bool IsBooleanProperty { get; protected set; }
 
+        internal bool IsEditorRequired { get; set; }
+
         public string Name { get; protected set; }
 
         public string IndexerNamePrefix { get; protected set; }
@@ -81,4 +83,4 @@ public override int GetHashCode()
             return BoundAttributeDescriptorComparer.Default.GetHashCode(this);
         }
     }
-}
+}

src/Razor/Microsoft.AspNetCore.Razor.Language/src/BoundAttributeDescriptorBuilder.cs

@@ -1,4 +1,4 @@
-// Copyright (c) .NET Foundation. All rights reserved.
+// 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 System;
@@ -28,6 +28,8 @@ public abstract class BoundAttributeDescriptorBuilder
 
         public abstract RazorDiagnosticCollection Diagnostics { get; }
 
+        internal bool IsEditorRequired { get; set; }
+
         public virtual IReadOnlyList<BoundAttributeParameterDescriptorBuilder> BoundAttributeParameters { get; }
 
         public virtual void BindAttributeParameter(Action<BoundAttributeParameterDescriptorBuilder> configure)

src/Razor/Microsoft.AspNetCore.Razor.Language/src/Components/ComponentLoweringPass.cs

@@ -67,7 +67,7 @@ protected override void ExecuteCore(RazorCodeDocument codeDocument, DocumentInte
             }
         }
 
-        private ComponentIntermediateNode RewriteAsComponent(TagHelperIntermediateNode node, TagHelperDescriptor tagHelper)
+        private static ComponentIntermediateNode RewriteAsComponent(TagHelperIntermediateNode node, TagHelperDescriptor tagHelper)
         {
             var component = new ComponentIntermediateNode()
             {
@@ -89,13 +89,54 @@ private ComponentIntermediateNode RewriteAsComponent(TagHelperIntermediateNode n
             // because we see the nodes in the wrong order.
             foreach (var childContent in component.ChildContents)
             {
-                childContent.ParameterName = childContent.ParameterName ?? component.ChildContentParameterName ?? ComponentMetadata.ChildContent.DefaultParameterName;
+                childContent.ParameterName ??= component.ChildContentParameterName ?? ComponentMetadata.ChildContent.DefaultParameterName;
             }
 
+            ValidateRequiredAttributes(node, tagHelper, component);
+
             return component;
         }
 
-        private MarkupElementIntermediateNode RewriteAsElement(TagHelperIntermediateNode node)
+        private static void ValidateRequiredAttributes(TagHelperIntermediateNode node, TagHelperDescriptor tagHelper, ComponentIntermediateNode intermediateNode)
+        {
+            if (intermediateNode.Children.Any(c => c is TagHelperDirectiveAttributeIntermediateNode node && (node.TagHelper?.IsSplatTagHelper() ?? false)))
+            {
+                // If there are any splat attributes, assume the user may have provided all values.
+                // This pass runs earlier than ComponentSplatLoweringPass, so we cannot rely on the presence of SplatIntermediateNode to make this check.
+                return;
+            }
+
+            foreach (var requiredAttribute in tagHelper.EditorRequiredAttributes)
+            {
+                if (!IsPresentAsAttribute(requiredAttribute.Name, intermediateNode))
+                {
+                    intermediateNode.Diagnostics.Add(
+                      RazorDiagnosticFactory.CreateComponent_EditorRequiredParameterNotSpecified(
+                          node.Source ?? SourceSpan.Undefined,
+                          intermediateNode.TagName,
+                          requiredAttribute.Name));
+                }
+            }
+
+            static bool IsPresentAsAttribute(string attributeName, ComponentIntermediateNode intermediateNode)
+            {
+                foreach (var child in intermediateNode.Children)
+                {
+                    if (child is ComponentAttributeIntermediateNode attributeNode && attributeName == attributeNode.AttributeName)
+                    {
+                        return true;
+                    }
+                    else if (child is ComponentChildContentIntermediateNode childContent && attributeName == childContent.AttributeName)
+                    {
+                        return true;
+                    }
+                }
+
+                return false;
+            }
+        }
+
+        private static MarkupElementIntermediateNode RewriteAsElement(TagHelperIntermediateNode node)
         {
             var result = new MarkupElementIntermediateNode()
             {

src/Razor/Microsoft.AspNetCore.Razor.Language/src/DefaultBoundAttributeDescriptorBuilder.cs

@@ -125,7 +125,10 @@ public BoundAttributeDescriptor Build()
                 CaseSensitive,
                 parameters,
                 new Dictionary<string, string>(Metadata),
-                diagnostics.ToArray());
+                diagnostics.ToArray())
+            {
+                IsEditorRequired = IsEditorRequired,
+            };
 
             return descriptor;
         }

src/Razor/Microsoft.AspNetCore.Razor.Language/src/RazorDiagnosticFactory.cs

@@ -2,6 +2,7 @@
 // Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
 
 using System;
+using System.Collections.Generic;
 using Microsoft.AspNetCore.Razor.Language.Legacy;
 
 namespace Microsoft.AspNetCore.Razor.Language
@@ -588,6 +589,17 @@ public static RazorDiagnostic CreateTagHelper_InconsistentTagStructure(SourceSpa
             return RazorDiagnostic.Create(TagHelper_InconsistentTagStructure, location, firstDescriptor, secondDescriptor, tagName, nameof(TagMatchingRuleDescriptor.TagStructure));
         }
 
+        internal static readonly RazorDiagnosticDescriptor Component_EditorRequiredParameterNotSpecified =
+            new RazorDiagnosticDescriptor(
+                $"{DiagnosticPrefix}2012",
+                () => Resources.Component_EditorRequiredParameterNotSpecified,
+                RazorDiagnosticSeverity.Warning);
+
+        public static RazorDiagnostic CreateComponent_EditorRequiredParameterNotSpecified(SourceSpan location, string tagName, string parameterName)
+        {
+            return RazorDiagnostic.Create(Component_EditorRequiredParameterNotSpecified, location, tagName, parameterName);
+        }
+
         #endregion
 
         #region TagHelper Errors

src/Razor/Microsoft.AspNetCore.Razor.Language/src/Resources.resx

@@ -1,4 +1,4 @@
-<?xml version="1.0" encoding="utf-8"?>
+<?xml version="1.0" encoding="utf-8"?>
 <root>
   <!-- 
     Microsoft ResX Schema 
@@ -571,4 +571,7 @@
   <data name="ParseError_Unexpected_Identifier_At_Position" xml:space="preserve">
     <value>'{0}' is not valid in this position. Valid options are '{1}'</value>
   </data>
-</root>
+  <data name="Component_EditorRequiredParameterNotSpecified" xml:space="preserve">
+    <value>Component '{0}' expects a value for the parameter '{1}', but a value may not have been provided.</value>
+  </data>
+</root>

src/Razor/Microsoft.AspNetCore.Razor.Language/src/TagHelperDescriptor.cs

@@ -10,6 +10,7 @@ namespace Microsoft.AspNetCore.Razor.Language
     public abstract class TagHelperDescriptor : IEquatable<TagHelperDescriptor>
     {
         private IEnumerable<RazorDiagnostic> _allDiagnostics;
+        private BoundAttributeDescriptor[] _editorRequiredAttributes;
 
         protected TagHelperDescriptor(string kind)
         {
@@ -45,6 +46,14 @@ protected TagHelperDescriptor(string kind)
         internal bool? IsComponentFullyQualifiedNameMatchCache { get; set; }
         internal bool? IsChildContentTagHelperCache { get; set; }
         internal ParsedTypeInformation? ParsedTypeInfo { get; set; }
+        internal BoundAttributeDescriptor[] EditorRequiredAttributes
+        {
+            get
+            {
+                _editorRequiredAttributes ??= GetEditorRequiredAttributes(BoundAttributes);
+                return _editorRequiredAttributes;
+            }
+        }
 
         public bool HasErrors
         {
@@ -96,6 +105,23 @@ public override int GetHashCode()
             return _hashCode.Value;
         }
 
+        private static BoundAttributeDescriptor[] GetEditorRequiredAttributes(IReadOnlyList<BoundAttributeDescriptor> boundAttributeDescriptors)
+        {
+            List<BoundAttributeDescriptor> editorRequiredAttributes = null;
+            var count = boundAttributeDescriptors.Count;
+            for (var i = 0; i < count; i++)
+            {
+                var attribute = boundAttributeDescriptors[i];
+                if (attribute.IsEditorRequired)
+                {
+                    editorRequiredAttributes ??= new();
+                    editorRequiredAttributes.Add(attribute);
+                }
+            }
+
+            return editorRequiredAttributes?.ToArray() ?? Array.Empty<BoundAttributeDescriptor>();
+        }
+
         internal readonly struct ParsedTypeInformation
         {
             public ParsedTypeInformation(bool success, StringSegment @namespace, StringSegment typeName)