Fix asp-page-handler to generate form tags correctly.

- Prior to this change using asp-page-handler on its own did not create correct `<form>` elements. There were multiple issues, one as that the `FormTagHelper` would purposefully drop into a no-op code path. Second is the `DefaultHtmlGenerator` didn't call through to the `UrlHelper` correctly.
- Added functional test cases to validate asp-page-handler can live on its own on a form tag. This also included adding a variant where method="post".
- Added a `FormTagHelper` unit test to validate the `PageHandler` property is consumed properly.

#6208

src/Microsoft.AspNetCore.Mvc.TagHelpers/FormTagHelper.cs

@@ -159,18 +159,19 @@ namespace Microsoft.AspNetCore.Mvc.TagHelpers
             }
 
             var antiforgeryDefault = true;
+            var routeableParametersProvided = Action != null ||
+                Controller != null ||
+                Area != null ||
+                Page != null ||
+                PageHandler != null ||
+                Fragment != null ||
+                Route != null ||
+                (_routeValues != null && _routeValues.Count > 0);
 
             // If "action" is already set, it means the user is attempting to use a normal <form>.
             if (output.Attributes.TryGetAttribute(HtmlActionAttributeName, out var actionAttribute))
             {
-                if (Action != null ||
-                    Controller != null ||
-                    Area != null ||
-                    Page != null ||
-                    PageHandler != null ||
-                    Fragment != null ||
-                    Route != null ||
-                    (_routeValues != null && _routeValues.Count > 0))
+                if (routeableParametersProvided)
                 {
                     // User also specified bound attributes we cannot use.
                     throw new InvalidOperationException(
@@ -254,13 +255,8 @@ namespace Microsoft.AspNetCore.Mvc.TagHelpers
                 }
 
                 TagBuilder tagBuilder = null;
-                if (Action == null &&
-                    Controller == null &&
-                    Route == null &&
+                if (!routeableParametersProvided &&
                     _routeValues == null &&
-                    Fragment == null &&
-                    Area == null &&
-                    Page == null &&
                     // Antiforgery will sometime be set globally via TagHelper Initializers, verify it was provided in the cshtml. 
                     !context.AllAttributes.ContainsName(AntiforgeryAttributeName))
                 {

src/Microsoft.AspNetCore.Mvc.ViewFeatures/ViewFeatures/DefaultHtmlGenerator.cs

@@ -323,34 +323,8 @@ namespace Microsoft.AspNetCore.Mvc.ViewFeatures
                 throw new ArgumentNullException(nameof(viewContext));
             }
 
-            var defaultMethod = false;
-            if (string.IsNullOrEmpty(method))
-            {
-                defaultMethod = true;
-            }
-            else if (string.Equals(method, "post", StringComparison.OrdinalIgnoreCase))
-            {
-                defaultMethod = true;
-            }
-
-            string action;
-            if (pageName == null && routeValues == null && defaultMethod)
-            {
-                // Submit to the original URL in the special case that user called the BeginForm() overload without
-                // parameters (except for the htmlAttributes parameter). Also reachable in the even-more-unusual case
-                // that user called another BeginForm() overload with default argument values.
-                var request = viewContext.HttpContext.Request;
-                action = request.PathBase + request.Path + request.QueryString;
-                if (fragment != null)
-                {
-                    action += "#" + fragment;
-                }
-            }
-            else
-            {
-                var urlHelper = _urlHelperFactory.GetUrlHelper(viewContext);
-                action = urlHelper.Page(pageName, pageHandler, routeValues, protocol: null, host: null, fragment: fragment);
-            }
+            var urlHelper = _urlHelperFactory.GetUrlHelper(viewContext);
+            var action = urlHelper.Page(pageName, pageHandler, routeValues, protocol: null, host: null, fragment: fragment);
 
             return GenerateFormCore(viewContext, action, method, htmlAttributes);
         }

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

@@ -156,6 +156,26 @@ namespace Microsoft.AspNetCore.Mvc.FunctionalTests
             Assert.Contains(expected, response.Trim());
         }
 
+        [Fact]
+        public async Task FormTagHelper_WithPageHandler_AllowsPostingToSelf()
+        {
+            //Arrange
+            var expected =
+@"<form action=""/TagHelper/PostWithHandler/Edit"" method=""post""><input name=""__RequestVerificationToken"" type=""hidden"" value=""{0}"" /></form>
+<form method=""post"" action=""/TagHelper/PostWithHandler/Edit""><input name=""__RequestVerificationToken"" type=""hidden"" value=""{0}"" /></form>
+<form method=""post"" action=""/TagHelper/PostWithHandler/Edit/10""></form>";
+
+            // Act
+            var response = await Client.GetStringAsync("/TagHelper/PostWithHandler");
+
+            // Assert
+            var responseContent = response.Trim();
+            var forgeryToken = AntiforgeryTestHelper.RetrieveAntiforgeryToken(responseContent, "/TagHelper/PostWithHandler");
+            var expectedContent = string.Format(expected, forgeryToken);
+
+            Assert.Equal(expectedContent, responseContent, ignoreLineEndingDifferences: true);
+        }
+
         [Fact]
         public async Task FormTagHelper_WithPage_AllowsPostingToAnotherPage()
         {

test/Microsoft.AspNetCore.Mvc.TagHelpers.Test/FormTagHelperTest.cs

@@ -25,6 +25,53 @@ namespace Microsoft.AspNetCore.Mvc.TagHelpers
 {
     public class FormTagHelperTest
     {
+        [Fact]
+        public async Task ProcessAsync_InvokesGeneratePageForm_WithOnlyPageHandler()
+        {
+            // Arrange
+            var viewContext = CreateViewContext();
+            var context = new TagHelperContext(
+                tagName: "form",
+                allAttributes: new TagHelperAttributeList()
+                {
+                    { "asp-handler", "page-handler" },
+                    { "method", "get" }
+                },
+                items: new Dictionary<object, object>(),
+                uniqueId: "test");
+            var output = new TagHelperOutput(
+                "form",
+                attributes: new TagHelperAttributeList(),
+                getChildContentAsync: (useCachedResult, encoder) =>
+                {
+                    var tagHelperContent = new DefaultTagHelperContent();
+                    tagHelperContent.SetContent("Something");
+                    return Task.FromResult<TagHelperContent>(tagHelperContent);
+                });
+            var generator = new Mock<IHtmlGenerator>(MockBehavior.Strict);
+            generator
+                .Setup(mock => mock.GeneratePageForm(
+                    viewContext,
+                    null,
+                    "page-handler",
+                    null,
+                    null,
+                    null,
+                    null))
+                .Returns(new TagBuilder("form"))
+                .Verifiable();
+            var formTagHelper = new FormTagHelper(generator.Object)
+            {
+                ViewContext = viewContext,
+                PageHandler = "page-handler",
+                Method = "get"
+            };
+
+            // Act & Assert
+            await formTagHelper.ProcessAsync(context, output);
+            generator.Verify();
+        }
+
         [Fact]
         public async Task ProcessAsync_ActionAndControllerGenerateAntiforgery()
         {

test/WebSites/RazorPagesWebSite/Pages/TagHelper/PostWithHandler.cshtml

@@ -1,6 +1,6 @@
 @page "{handler?}/{id?}"
 
-@function
+@functions
 {
     public IActionResult OnPostEdit(int id)
     {
@@ -13,4 +13,6 @@
     }
 }
 
+<form asp-page-handler="Edit"></form>
+<form method="post" asp-page-handler="Edit"></form>
 <form method="post" asp-page-handler="Edit" asp-route-id="10" asp-antiforgery="false"></form>