Add InvalidTagHelperIndexerAssignment to RazorPageBase

src/Microsoft.AspNetCore.Mvc.Razor/RazorPageBase.cs

@@ -3,6 +3,7 @@
 
 using System;
 using System.Collections.Generic;
+using System.ComponentModel;
 using System.Diagnostics;
 using System.IO;
 using System.Security.Claims;
@@ -134,6 +135,25 @@ namespace Microsoft.AspNetCore.Mvc.Razor
 
         public abstract Task ExecuteAsync();
 
+        /// <summary>
+        /// Format an error message about using an indexer when the tag helper property is <c>null</c>.
+        /// </summary>
+        /// <param name="attributeName">Name of the HTML attribute associated with the indexer.</param>
+        /// <param name="tagHelperTypeName">Full name of the tag helper <see cref="Type"/>.</param>
+        /// <param name="propertyName">Dictionary property in the tag helper.</param>
+        /// <returns>An error message about using an indexer when the tag helper property is <c>null</c>.</returns>
+        [EditorBrowsable(EditorBrowsableState.Never)]
+        public string InvalidTagHelperIndexerAssignment(
+            string attributeName,
+            string tagHelperTypeName,
+            string propertyName)
+        {
+            return Resources.FormatRazorPage_InvalidTagHelperIndexerAssignment(
+                attributeName,
+                tagHelperTypeName,
+                propertyName);
+        }
+
         /// <summary>
         /// Creates and activates a <see cref="ITagHelper"/>.
         /// </summary>
@@ -669,7 +689,6 @@ namespace Microsoft.AspNetCore.Mvc.Razor
         /// <see cref="FlushAsync"/> is called. For example, call <see cref="SetAntiforgeryCookieAndHeader"/> to send
         /// antiforgery cookie token and X-Frame-Options header to client before this method flushes headers out.
         /// </remarks>
-
         public virtual async Task<HtmlString> FlushAsync()
         {
             // If there are active scopes, then we should throw. Cannot flush content that has the potential to change.

test/Microsoft.AspNetCore.Mvc.FunctionalTests/RazorPagesTest.cs

@@ -32,6 +32,24 @@ namespace Microsoft.AspNetCore.Mvc.FunctionalTests
             Assert.Equal("Path: /PathSet.cshtml", content.Trim());
         }
 
+        // Tests that RazorPage includes InvalidTagHelperIndexerAssignment which is called when the page has an indexer
+        // Issue https://github.com/aspnet/Mvc/issues/5920
+        [Fact]
+        public async Task TagHelper_InvalidIndexerDoesNotFail()
+        {
+            // Arrange
+            var request = new HttpRequestMessage(HttpMethod.Get, "http://localhost/TagHelpers");
+
+            // Act
+            var response = await Client.SendAsync(request);
+
+                // Assert
+            var content = await response.Content.ReadAsStringAsync();
+
+            Assert.Equal(HttpStatusCode.OK, response.StatusCode);
+            Assert.Equal("<a href=\"/Show?id=2\">Post title</a>", content.Trim());
+        }
+
         [Fact]
         public async Task NoPage_NotFound()
         {

test/WebSites/RazorPagesWebSite/Show.cshtml

@@ -0,0 +1,5 @@
+@page
+
+@function{ 
+    public int Id {get; set;}
+}

test/WebSites/RazorPagesWebSite/TagHelpers.cshtml

@@ -0,0 +1,16 @@
+@page
+@addTagHelper *, Microsoft.AspNetCore.Mvc.TagHelpers
+
+<!-- Tests that RazorPage includes InvalidTagHelperIndexerAssignment which is called when the page has an indexer
+     Issue https://github.com/aspnet/Mvc/issues/5920 -->
+
+@functions {
+        public class Post
+        {
+            public int Id { get; set; }
+            public string Title { get; set; }
+        }
+        public Post post = new Post { Id = 2, Title = "Post title" };
+}
+
+<a asp-route-page="/Show" asp-route-id="@post.Id">@post.Title</a>