Add DeclaredModelType to Razor pages

* This allows razor pages to override their model type with a model that
  extends the declared model type through the page application model.

src/Microsoft.AspNetCore.Mvc.Razor/Internal/RazorPagePropertyActivator.cs

@@ -24,14 +24,14 @@ namespace Microsoft.AspNetCore.Mvc.Razor.Internal
 
         public RazorPagePropertyActivator(
             Type pageType,
-            Type modelType,
+            Type declaredModelType,
             IModelMetadataProvider metadataProvider,
             PropertyValueAccessors propertyValueAccessors)
         {
             _metadataProvider = metadataProvider;
 
             // In the absence of a model on the current type, we'll attempt to use ViewDataDictionary<object> on the current type.
-            var viewDataDictionaryModelType = modelType ?? typeof(object);
+            var viewDataDictionaryModelType = declaredModelType ?? typeof(object);
 
             if (viewDataDictionaryModelType != null)
             {

src/Microsoft.AspNetCore.Mvc.RazorPages/ApplicationModels/PageApplicationModel.cs

@@ -23,8 +23,21 @@ namespace Microsoft.AspNetCore.Mvc.ApplicationModels
             PageActionDescriptor actionDescriptor,
             TypeInfo handlerType,
             IReadOnlyList<object> handlerAttributes)
+            : this(actionDescriptor, handlerType, handlerType, handlerAttributes)
+        {
+        }
+
+        /// <summary>
+        /// Initializes a new instance of <see cref="PageApplicationModel"/>.
+        /// </summary>
+        public PageApplicationModel(
+            PageActionDescriptor actionDescriptor,
+            TypeInfo declaredModelType,
+            TypeInfo handlerType,
+            IReadOnlyList<object> handlerAttributes)
         {
             ActionDescriptor = actionDescriptor ?? throw new ArgumentNullException(nameof(actionDescriptor));
+            DeclaredModelType = declaredModelType;
             HandlerType = handlerType;
 
             Filters = new List<IFilterMetadata>();
@@ -56,7 +69,7 @@ namespace Microsoft.AspNetCore.Mvc.ApplicationModels
             Properties = new Dictionary<object, object>(other.Properties);
 
             HandlerMethods = new List<PageHandlerModel>(other.HandlerMethods.Select(m => new PageHandlerModel(m)));
-            HandlerProperties  = new List<PagePropertyModel>(other.HandlerProperties.Select(p => new PagePropertyModel(p)));
+            HandlerProperties = new List<PagePropertyModel>(other.HandlerProperties.Select(p => new PagePropertyModel(p)));
             HandlerTypeAttributes = other.HandlerTypeAttributes;
         }
 
@@ -109,7 +122,16 @@ namespace Microsoft.AspNetCore.Mvc.ApplicationModels
         public TypeInfo PageType { get; set; }
 
         /// <summary>
-        /// Gets or sets the <see cref="TypeInfo"/> of the Razor page model.
+        /// Gets the declared model <see cref="TypeInfo"/> of the model for the page.
+        /// Typically this <see cref="TypeInfo"/> will be the type specified by the @model directive
+        /// in the razor page.
+        /// </summary>
+        public TypeInfo DeclaredModelType { get; }
+
+        /// <summary>
+        /// Gets or sets the runtime model <see cref="TypeInfo"/> of the model for the razor page.
+        /// This is the <see cref="TypeInfo"/> that will be used at runtime to instantiate and populate
+        /// the model property of the page.
         /// </summary>
         public TypeInfo ModelType { get; set; }
 

src/Microsoft.AspNetCore.Mvc.RazorPages/CompiledPageActionDescriptor.cs

@@ -42,7 +42,16 @@ namespace Microsoft.AspNetCore.Mvc.RazorPages
         public TypeInfo HandlerTypeInfo { get; set; }
 
         /// <summary>
-        /// Gets or sets the <see cref="TypeInfo"/> of the model.
+        /// Gets or sets the declared model <see cref="TypeInfo"/> of the model for the page.
+        /// Typically this <see cref="TypeInfo"/> will be the type specified by the @model directive
+        /// in the razor page.
+        /// </summary>
+        public TypeInfo DeclaredModelTypeInfo { get; set; }
+
+        /// <summary>
+        /// Gets or sets the runtime model <see cref="TypeInfo"/> of the model for the razor page.
+        /// This is the <see cref="TypeInfo"/> that will be used at runtime to instantiate and populate
+        /// the model property of the page.
         /// </summary>
         public TypeInfo ModelTypeInfo { get; set; }
 

src/Microsoft.AspNetCore.Mvc.RazorPages/Infrastructure/DefaultPageFactoryProvider.cs

@@ -50,10 +50,10 @@ namespace Microsoft.AspNetCore.Mvc.RazorPages.Infrastructure
             }
 
             var activatorFactory = _pageActivator.CreateActivator(actionDescriptor);
-            var modelType = actionDescriptor.ModelTypeInfo?.AsType() ?? actionDescriptor.PageTypeInfo.AsType();
+            var declaredModelType = actionDescriptor.DeclaredModelTypeInfo?.AsType() ?? actionDescriptor.PageTypeInfo.AsType();
             var propertyActivator = new RazorPagePropertyActivator(
                 actionDescriptor.PageTypeInfo.AsType(),
-                modelType,
+                declaredModelType,
                 _modelMetadataProvider,
                 _propertyAccessors);
 

src/Microsoft.AspNetCore.Mvc.RazorPages/Internal/CompiledPageActionDescriptorBuilder.cs

@@ -1,6 +1,7 @@
 // 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;
 using System.Collections.Generic;
 using System.Linq;
 using Microsoft.AspNetCore.Mvc.ApplicationModels;
@@ -31,6 +32,17 @@ namespace Microsoft.AspNetCore.Mvc.RazorPages.Internal
                 .ToArray();
             var handlerMethods = CreateHandlerMethods(applicationModel);
 
+            if (applicationModel.ModelType != null && applicationModel.DeclaredModelType != null &&
+                !applicationModel.DeclaredModelType.IsAssignableFrom(applicationModel.ModelType))
+            {
+                var message = Resources.FormatInvalidActionDescriptorModelType(
+                    applicationModel.ActionDescriptor.DisplayName,
+                    applicationModel.ModelType.Name,
+                    applicationModel.DeclaredModelType.Name);
+
+                throw new InvalidOperationException(message);
+            }
+
             var actionDescriptor = applicationModel.ActionDescriptor;
             return new CompiledPageActionDescriptor(actionDescriptor)
             {
@@ -40,6 +52,7 @@ namespace Microsoft.AspNetCore.Mvc.RazorPages.Internal
                 FilterDescriptors = filters,
                 HandlerMethods = handlerMethods,
                 HandlerTypeInfo = applicationModel.HandlerType,
+                DeclaredModelTypeInfo = applicationModel.DeclaredModelType,
                 ModelTypeInfo = applicationModel.ModelType,
                 RouteValues = actionDescriptor.RouteValues,
                 PageTypeInfo = applicationModel.PageType,

src/Microsoft.AspNetCore.Mvc.RazorPages/Internal/DefaultPageApplicationModelProvider.cs

@@ -75,6 +75,7 @@ namespace Microsoft.AspNetCore.Mvc.RazorPages.Internal
             }
 
             var modelTypeInfo = modelProperty.PropertyType.GetTypeInfo();
+            var declaredModelType = modelTypeInfo;
 
             // Now we want figure out which type is the handler type.
             TypeInfo handlerType;
@@ -90,6 +91,7 @@ namespace Microsoft.AspNetCore.Mvc.RazorPages.Internal
             var handlerTypeAttributes = handlerType.GetCustomAttributes(inherit: true);
             var pageModel = new PageApplicationModel(
                 actionDescriptor,
+                declaredModelType,
                 handlerType,
                 handlerTypeAttributes)
             {

src/Microsoft.AspNetCore.Mvc.RazorPages/Internal/PageActionInvokerProvider.cs

@@ -168,7 +168,7 @@ namespace Microsoft.AspNetCore.Mvc.RazorPages.Internal
         {
             var compiledActionDescriptor = (CompiledPageActionDescriptor)context.ActionContext.ActionDescriptor;
 
-            var viewDataFactory = ViewDataDictionaryFactory.CreateFactory(compiledActionDescriptor.ModelTypeInfo);
+            var viewDataFactory = ViewDataDictionaryFactory.CreateFactory(compiledActionDescriptor.DeclaredModelTypeInfo);
 
             var pageFactory = _pageFactoryProvider.CreatePageFactory(compiledActionDescriptor);
             var pageDisposer = _pageFactoryProvider.CreatePageDisposer(compiledActionDescriptor);

src/Microsoft.AspNetCore.Mvc.RazorPages/Properties/Resources.Designer.cs

@@ -10,20 +10,6 @@ namespace Microsoft.AspNetCore.Mvc.RazorPages
         private static readonly ResourceManager _resourceManager
             = new ResourceManager("Microsoft.AspNetCore.Mvc.RazorPages.Resources", typeof(Resources).GetTypeInfo().Assembly);
 
-        /// <summary>
-        /// The route for the page at '{0}' cannot start with / or ~/. Pages do not support overriding the file path of the page.
-        /// </summary>
-        internal static string PageActionDescriptorProvider_RouteTemplateCannotBeOverrideable
-        {
-            get => GetString("PageActionDescriptorProvider_RouteTemplateCannotBeOverrideable");
-        }
-
-        /// <summary>
-        /// The route for the page at '{0}' cannot start with / or ~/. Pages do not support overriding the file path of the page.
-        /// </summary>
-        internal static string FormatPageActionDescriptorProvider_RouteTemplateCannotBeOverrideable(object p0)
-            => string.Format(CultureInfo.CurrentCulture, GetString("PageActionDescriptorProvider_RouteTemplateCannotBeOverrideable"), p0);
-
         /// <summary>
         /// The '{0}' property of '{1}' must not be null.
         /// </summary>
@@ -178,6 +164,20 @@ namespace Microsoft.AspNetCore.Mvc.RazorPages
         internal static string FormatInvalidValidPageName(object p0)
             => string.Format(CultureInfo.CurrentCulture, GetString("InvalidValidPageName"), p0);
 
+        /// <summary>
+        /// The model type for '{0}' is of type '{1}' which is not assignable to its declared model type '{2}'.
+        /// </summary>
+        internal static string InvalidActionDescriptorModelType
+        {
+            get => GetString("InvalidActionDescriptorModelType");
+        }
+
+        /// <summary>
+        /// The model type for '{0}' is of type '{1}' which is not assignable to its declared model type '{2}'.
+        /// </summary>
+        internal static string FormatInvalidActionDescriptorModelType(object p0, object p1, object p2)
+            => string.Format(CultureInfo.CurrentCulture, GetString("InvalidActionDescriptorModelType"), p0, p1, p2);
+
         private static string GetString(string name, params string[] formatterNames)
         {
             var value = _resourceManager.GetString(name);

src/Microsoft.AspNetCore.Mvc.RazorPages/Resources.resx

@@ -150,4 +150,7 @@
   <data name="InvalidValidPageName" xml:space="preserve">
     <value>'{0}' is not a valid page name. A page name is path relative to the Razor Pages root directory that starts with a leading forward slash ('/') and does not contain the file extension e.g "/Users/Edit".</value>
   </data>
+  <data name="InvalidActionDescriptorModelType" xml:space="preserve">
+    <value>The model type for '{0}' is of type '{1}' which is not assignable to its declared model type '{2}'.</value>
+  </data>
 </root>

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

@@ -1,6 +1,7 @@
 // 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.Collections.Generic;
 using System.Net;
 using System.Net.Http;
 using System.Threading.Tasks;
@@ -114,7 +115,7 @@ namespace Microsoft.AspNetCore.Mvc.FunctionalTests
             Assert.Equal("/Login?ReturnUrl=%2FConventions%2FAuthFolder", response.Headers.Location.PathAndQuery);
         }
 
-         [Fact]
+        [Fact]
         public async Task AuthConvention_AppliedToFolders_CanByOverridenByFiltersOnModel()
         {
             // Act
@@ -391,5 +392,76 @@ Hello from /Pages/Shared/";
             // Assert
             Assert.Equal("Hello from AllowAnonymous", response.Trim());
         }
+
+        // These test is important as it covers a feature that allows razor pages to use a different
+        // model at runtime that wasn't known at compile time. Like a non-generic model used at compile
+        // time and overrided at runtime with a closed-generic model that performs the actual implementation.
+        // An example of this is how the Identity UI library defines a base page model in their views,
+        // like how the Register.cshtml view defines its model as RegisterModel and then, at runtime it replaces
+        // that model with RegisterModel<TUser> where TUser is the type of the user used to configure identity.
+        [Fact]
+        public async Task PageConventions_CanBeUsedToCustomizeTheModelType()
+        {
+            // Act
+            var response = await Client.GetAsync("/CustomModelTypeModel");
+
+            // Assert
+            Assert.Equal(HttpStatusCode.OK, response.StatusCode);
+            var content = await response.Content.ReadAsStringAsync();
+            Assert.Contains("<h2>User</h2>", content);
+        }
+
+        [Fact]
+        public async Task PageConventions_CustomizedModelCanPostToHandlers()
+        {
+            // Arrange
+            var getPage = await Client.GetAsync("/CustomModelTypeModel");
+            var token = AntiforgeryTestHelper.RetrieveAntiforgeryToken(await getPage.Content.ReadAsStringAsync(), "");
+            var cookie = AntiforgeryTestHelper.RetrieveAntiforgeryCookie(getPage);
+
+            var message = new HttpRequestMessage(HttpMethod.Post, "/CustomModelTypeModel");
+            message.Content = new FormUrlEncodedContent(new Dictionary<string, string>
+            {
+                ["__RequestVerificationToken"] = token,
+                ["ConfirmPassword"] = "",
+                ["Password"] = "",
+                ["Email"] = ""
+            });
+            message.Headers.TryAddWithoutValidation("Cookie", $"{cookie.Key}={cookie.Value}");
+
+            // Act
+            var response = await Client.SendAsync(message);
+
+            // Assert
+            Assert.Equal(HttpStatusCode.OK, response.StatusCode);
+            var content = await response.Content.ReadAsStringAsync();
+            Assert.Contains("is required.", content);
+        }
+
+        [Fact]
+        public async Task PageConventions_CustomizedModelCanWorkWithModelState()
+        {
+            // Arrange
+            var getPage = await Client.GetAsync("/CustomModelTypeModel");
+            var token = AntiforgeryTestHelper.RetrieveAntiforgeryToken(await getPage.Content.ReadAsStringAsync(), "");
+            var cookie = AntiforgeryTestHelper.RetrieveAntiforgeryCookie(getPage);
+
+            var message = new HttpRequestMessage(HttpMethod.Post, "/CustomModelTypeModel");
+            message.Content = new FormUrlEncodedContent(new Dictionary<string, string>
+            {
+                ["__RequestVerificationToken"] = token,
+                ["Email"] = "javi@example.com",
+                ["Password"] = "Password.12$",
+                ["ConfirmPassword"] = "Password.12$",
+            });
+            message.Headers.TryAddWithoutValidation("Cookie", $"{cookie.Key}={cookie.Value}");
+
+            // Act
+            var response = await Client.SendAsync(message);
+
+            // Assert
+            Assert.Equal(HttpStatusCode.Redirect, response.StatusCode);
+            Assert.Equal("/", response.Headers.Location.ToString());
+        }
     }
 }

test/Microsoft.AspNetCore.Mvc.Razor.Test/Internal/RazorPagePropertyActivatorTest.cs

@@ -35,7 +35,7 @@ namespace Microsoft.AspNetCore.Mvc.Razor.Internal
             // Arrange
             var activator = new RazorPagePropertyActivator(
                 typeof(TestPage),
-                modelType: null,
+                declaredModelType: null,
                 metadataProvider: new TestModelMetadataProvider(),
                 propertyValueAccessors: null);
             var viewContext = new ViewContext();
@@ -55,7 +55,36 @@ namespace Microsoft.AspNetCore.Mvc.Razor.Internal
             var modelMetadataProvider = new TestModelMetadataProvider();
             var activator = new RazorPagePropertyActivator(
                 typeof(TestPage),
-                modelType: typeof(TestModel),
+                declaredModelType: typeof(TestModel),
+                metadataProvider: modelMetadataProvider,
+                propertyValueAccessors: null);
+            var original = new ViewDataDictionary(modelMetadataProvider, new ModelStateDictionary())
+            {
+                {  "test-key", "test-value" },
+            };
+            var viewContext = new ViewContext
+            {
+                ViewData = original,
+            };
+
+            // Act
+            var viewDataDictionary = activator.CreateViewDataDictionary(viewContext);
+
+            // Assert
+            Assert.NotNull(viewDataDictionary);
+            Assert.NotSame(original, viewDataDictionary);
+            Assert.IsType<ViewDataDictionary<TestModel>>(viewDataDictionary);
+            Assert.Equal("test-value", viewDataDictionary["test-key"]);
+        }
+
+        [Fact]
+        public void CreateViewDataDictionary_UsesDeclaredTypeOverModelType_WhenCreatingTheViewDataDictionary()
+        {
+            // Arrange
+            var modelMetadataProvider = new TestModelMetadataProvider();
+            var activator = new RazorPagePropertyActivator(
+                typeof(TestPage),
+                declaredModelType: typeof(TestModel),
                 metadataProvider: modelMetadataProvider,
                 propertyValueAccessors: null);
             var original = new ViewDataDictionary(modelMetadataProvider, new ModelStateDictionary())
@@ -84,7 +113,7 @@ namespace Microsoft.AspNetCore.Mvc.Razor.Internal
             var modelMetadataProvider = new TestModelMetadataProvider();
             var activator = new RazorPagePropertyActivator(
                 typeof(TestPage),
-                modelType: typeof(TestModel),
+                declaredModelType: typeof(TestModel),
                 metadataProvider: modelMetadataProvider,
                 propertyValueAccessors: null);
             var original = new ViewDataDictionary<object>(modelMetadataProvider, new ModelStateDictionary())
@@ -113,7 +142,7 @@ namespace Microsoft.AspNetCore.Mvc.Razor.Internal
             var modelMetadataProvider = new TestModelMetadataProvider();
             var activator = new RazorPagePropertyActivator(
                 typeof(TestPage),
-                modelType: null,
+                declaredModelType: null,
                 metadataProvider: modelMetadataProvider,
                 propertyValueAccessors: null);
             var original = new ViewDataDictionary<TestModel>(modelMetadataProvider, new ModelStateDictionary())
@@ -142,7 +171,7 @@ namespace Microsoft.AspNetCore.Mvc.Razor.Internal
             var modelMetadataProvider = new TestModelMetadataProvider();
             var activator = new RazorPagePropertyActivator(
                 typeof(TestPage),
-                modelType: typeof(TestModel),
+                declaredModelType: typeof(TestModel),
                 metadataProvider: modelMetadataProvider,
                 propertyValueAccessors: null);
             var original = new ViewDataDictionary<TestModel>(modelMetadataProvider, new ModelStateDictionary())
@@ -169,7 +198,7 @@ namespace Microsoft.AspNetCore.Mvc.Razor.Internal
             var modelMetadataProvider = new TestModelMetadataProvider();
             var activator = new RazorPagePropertyActivator(
                 typeof(TestPage),
-                modelType: null,
+                declaredModelType: null,
                 metadataProvider: modelMetadataProvider,
                 propertyValueAccessors: null);
             var original = new ViewDataDictionary<object>(modelMetadataProvider, new ModelStateDictionary());
@@ -193,5 +222,9 @@ namespace Microsoft.AspNetCore.Mvc.Razor.Internal
         private class TestModel
         {
         }
+
+        private class DerivedTestModel : TestModel
+        {
+        }
     }
 }

test/Microsoft.AspNetCore.Mvc.RazorPages.Test/Infrastructure/DefaultPageFactoryProviderTest.cs

@@ -110,6 +110,7 @@ namespace Microsoft.AspNetCore.Mvc.RazorPages.Infrastructure
             var descriptor = new CompiledPageActionDescriptor
             {
                 PageTypeInfo = typeof(ViewDataTestPage).GetTypeInfo(),
+                DeclaredModelTypeInfo = typeof(ViewDataTestPageModel).GetTypeInfo(),
                 ModelTypeInfo = typeof(ViewDataTestPageModel).GetTypeInfo()
             };
             descriptor.RelativePath = "/this/is/a/path.cshtml";
@@ -139,6 +140,7 @@ namespace Microsoft.AspNetCore.Mvc.RazorPages.Infrastructure
                 ActionDescriptor = new CompiledPageActionDescriptor
                 {
                     PageTypeInfo = typeof(ViewDataTestPage).GetTypeInfo(),
+                    DeclaredModelTypeInfo = typeof(ViewDataTestPageModel).GetTypeInfo(),
                     ModelTypeInfo = typeof(ViewDataTestPageModel).GetTypeInfo(),
                 },
             };
@@ -156,6 +158,33 @@ namespace Microsoft.AspNetCore.Mvc.RazorPages.Infrastructure
             Assert.NotNull(testPage.ViewData);
         }
 
+        [Fact]
+        public void PageFactorySetViewDataWithDeclaredModelTypeWhenNotNull()
+        {
+            // Arrange
+            var pageContext = new PageContext
+            {
+                ActionDescriptor = new CompiledPageActionDescriptor
+                {
+                    PageTypeInfo = typeof(ViewDataTestPage).GetTypeInfo(),
+                    DeclaredModelTypeInfo = typeof(ViewDataTestPageModel).GetTypeInfo(),
+                    ModelTypeInfo = typeof(DerivedViewDataTestPageModel).GetTypeInfo(),
+                },
+            };
+
+            var viewContext = new ViewContext();
+
+            var factoryProvider = CreatePageFactory();
+
+            // Act
+            var factory = factoryProvider.CreatePageFactory(pageContext.ActionDescriptor);
+            var instance = factory(pageContext, viewContext);
+
+            // Assert
+            var testPage = Assert.IsType<ViewDataTestPage>(instance);
+            Assert.NotNull(testPage.ViewData);
+        }
+
         [Fact]
         public void PageFactorySetsNonGenericViewDataDictionary()
         {
@@ -334,6 +363,11 @@ namespace Microsoft.AspNetCore.Mvc.RazorPages.Infrastructure
         {
         }
 
+        private class DerivedViewDataTestPageModel : ViewDataTestPageModel
+        {
+        }
+
+
         private class PropertiesWithoutRazorInject : Page
         {
             public IModelExpressionProvider ModelExpressionProviderWithoutInject { get; set; }

test/Microsoft.AspNetCore.Mvc.RazorPages.Test/Internal/CompiledPageActionDescriptorBuilderTest.cs

@@ -1,6 +1,7 @@
 // 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;
 using System.Collections.Generic;
 using System.Linq;
 using System.Reflection;
@@ -58,7 +59,7 @@ namespace Microsoft.AspNetCore.Mvc.RazorPages.Internal
                 ViewEnginePath = "/Pages/Foo",
             };
             var handlerTypeInfo = typeof(TestModel).GetTypeInfo();
-            var pageApplicationModel = new PageApplicationModel(actionDescriptor, handlerTypeInfo, new object[0])
+            var pageApplicationModel = new PageApplicationModel(actionDescriptor, typeof(TestModel).GetTypeInfo(), handlerTypeInfo, new object[0])
             {
                 PageType = typeof(TestPage).GetTypeInfo(),
                 ModelType = typeof(TestModel).GetTypeInfo(),
@@ -86,6 +87,7 @@ namespace Microsoft.AspNetCore.Mvc.RazorPages.Internal
 
             // Assert
             Assert.Same(pageApplicationModel.PageType, actual.PageTypeInfo);
+            Assert.Same(pageApplicationModel.DeclaredModelType, actual.DeclaredModelTypeInfo);
             Assert.Same(pageApplicationModel.ModelType, actual.ModelTypeInfo);
             Assert.Same(pageApplicationModel.HandlerType, actual.HandlerTypeInfo);
             Assert.Same(pageApplicationModel.Properties, actual.Properties);
@@ -94,6 +96,48 @@ namespace Microsoft.AspNetCore.Mvc.RazorPages.Internal
             Assert.Equal(pageApplicationModel.HandlerProperties.Select(p => p.PropertyName), actual.BoundProperties.Select(p => p.Name));
         }
 
+        [Fact]
+        public void CreateDescriptor_ThrowsIfModelIsNotCompatibleWithDeclaredModel()
+        {
+            // Arrange
+            var actionDescriptor = new PageActionDescriptor
+            {
+                ActionConstraints = new List<IActionConstraintMetadata>(),
+                AttributeRouteInfo = new AttributeRouteInfo(),
+                FilterDescriptors = new List<FilterDescriptor>(),
+                RelativePath = "/Foo",
+                RouteValues = new Dictionary<string, string>(),
+                ViewEnginePath = "/Pages/Foo",
+            };
+            var handlerTypeInfo = typeof(TestModel).GetTypeInfo();
+            var pageApplicationModel = new PageApplicationModel(actionDescriptor, typeof(TestModel).GetTypeInfo(), handlerTypeInfo, new object[0])
+            {
+                PageType = typeof(TestPage).GetTypeInfo(),
+                ModelType = typeof(string).GetTypeInfo(),
+                Filters =
+                {
+                    Mock.Of<IFilterMetadata>(),
+                    Mock.Of<IFilterMetadata>(),
+                },
+                HandlerMethods =
+                {
+                    new PageHandlerModel(handlerTypeInfo.GetMethod(nameof(TestModel.OnGet)), new object[0]),
+                },
+                HandlerProperties =
+                {
+                    new PagePropertyModel(handlerTypeInfo.GetProperty(nameof(TestModel.Property)), new object[0])
+                    {
+                        BindingInfo = new BindingInfo(),
+                    },
+                }
+            };
+            var globalFilters = new FilterCollection();
+
+            // Act & Assert
+            var actual = Assert.Throws<InvalidOperationException>(() => 
+                CompiledPageActionDescriptorBuilder.Build(pageApplicationModel, globalFilters));
+        }
+
         [Fact]
         public void CreateDescriptor_AddsGlobalFiltersWithTheRightScope()
         {

test/Microsoft.AspNetCore.Mvc.RazorPages.Test/Internal/DefaultPageApplicationModelProviderTest.cs

@@ -470,8 +470,9 @@ namespace Microsoft.AspNetCore.Mvc.RazorPages.Internal
             var pageModel = context.PageApplicationModel;
             Assert.Empty(pageModel.HandlerProperties.Where(p => p.BindingInfo != null));
             Assert.Empty(pageModel.HandlerMethods);
-            Assert.Same(typeof(EmptyPageModel).GetTypeInfo(), pageModel.HandlerType);
+            Assert.Same(typeof(EmptyPageModel).GetTypeInfo(), pageModel.DeclaredModelType);
             Assert.Same(typeof(EmptyPageModel).GetTypeInfo(), pageModel.ModelType);
+            Assert.Same(typeof(EmptyPageModel).GetTypeInfo(), pageModel.HandlerType);
             Assert.Same(typeof(EmptyPageWithPageModel).GetTypeInfo(), pageModel.PageType);
         }
 

test/Microsoft.AspNetCore.Mvc.RazorPages.Test/Internal/PageActionInvokerProviderTest.cs

@@ -91,7 +91,7 @@ namespace Microsoft.AspNetCore.Mvc.RazorPages.Internal
             var descriptor = new PageActionDescriptor
             {
                 RelativePath = "Path1",
-                FilterDescriptors = new FilterDescriptor[0],
+                FilterDescriptors = new FilterDescriptor[0]
             };
 
             Func<PageContext, ViewContext, object> factory = (a, b) => null;
@@ -102,7 +102,10 @@ namespace Microsoft.AspNetCore.Mvc.RazorPages.Internal
             var loader = new Mock<IPageLoader>();
             loader
                 .Setup(l => l.Load(It.IsAny<PageActionDescriptor>()))
-                .Returns(CreateCompiledPageActionDescriptor(descriptor, pageType: typeof(PageWithModel)));
+                .Returns(CreateCompiledPageActionDescriptor(
+                    descriptor,
+                    pageType: typeof(PageWithModel),
+                    modelType: typeof(DerivedTestPageModel)));
 
             var pageFactoryProvider = new Mock<IPageFactoryProvider>();
             pageFactoryProvider
@@ -322,7 +325,7 @@ namespace Microsoft.AspNetCore.Mvc.RazorPages.Internal
         }
 
         [Fact]
-        public void GetViewStartFactories_FindsFullHeirarchy()
+        public void GetViewStartFactories_FindsFullHierarchy()
         {
 
             // Arrange
@@ -437,20 +440,27 @@ namespace Microsoft.AspNetCore.Mvc.RazorPages.Internal
 
         private static CompiledPageActionDescriptor CreateCompiledPageActionDescriptor(
             PageActionDescriptor descriptor,
-            Type pageType = null)
+            Type pageType = null,
+            Type modelType = null)
         {
             pageType = pageType ?? typeof(object);
             var pageTypeInfo = pageType.GetTypeInfo();
 
-            TypeInfo modelTypeInfo = null;
+            var modelTypeInfo = modelType?.GetTypeInfo();
+            TypeInfo declaredModelTypeInfo = null;
             if (pageType != null)
             {
-                modelTypeInfo = pageTypeInfo.GetProperty("Model")?.PropertyType.GetTypeInfo();
+                declaredModelTypeInfo = pageTypeInfo.GetProperty("Model")?.PropertyType.GetTypeInfo();
+                if (modelTypeInfo == null)
+                {
+                    modelTypeInfo = declaredModelTypeInfo;
+                }
             }
 
             return new CompiledPageActionDescriptor(descriptor)
             {
                 HandlerTypeInfo = modelTypeInfo ?? pageTypeInfo,
+                DeclaredModelTypeInfo = declaredModelTypeInfo ?? pageTypeInfo,
                 ModelTypeInfo = modelTypeInfo ?? pageTypeInfo,
                 PageTypeInfo = pageTypeInfo,
                 FilterDescriptors = Array.Empty<FilterDescriptor>(),
@@ -522,5 +532,9 @@ namespace Microsoft.AspNetCore.Mvc.RazorPages.Internal
             {
             }
         }
+
+        private class DerivedTestPageModel : TestPageModel
+        {
+        }
     }
 }

test/WebSites/RazorPagesWebSite/Conventions/CustomModelTypeConvention.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 Microsoft.AspNetCore.Mvc.ApplicationModels;
+using System.Reflection;
+
+namespace RazorPagesWebSite.Conventions
+{
+    internal class CustomModelTypeConvention : IPageApplicationModelConvention
+    {
+        public void Apply(PageApplicationModel model)
+        {
+            if (model.ModelType == typeof(CustomModelTypeModel))
+            {
+                model.ModelType = typeof(CustomModelTypeModel<User>).GetTypeInfo();
+            }
+        }
+    }
+}

test/WebSites/RazorPagesWebSite/Pages/CustomModelTypeModel.cshtml

@@ -0,0 +1,36 @@
+@page
+@addTagHelper *, Microsoft.AspNetCore.Mvc.TagHelpers
+@using RazorPagesWebSite
+@model CustomModelTypeModel
+@{
+    ViewData["Title"] = "Register";
+}
+
+<h2>@ViewData["Title"]</h2>
+<h2>@ViewData["UserType"]</h2>
+
+<div class="row">
+    <div class="col-md-4">
+        <form asp-route-returnUrl="@Model.ReturnUrl" method="post">
+            <h4>Create a new account.</h4>
+            <hr />
+            <div asp-validation-summary="All" class="text-danger"></div>
+            <div class="form-group">
+                <label asp-for="Input.Email"></label>
+                <input asp-for="Input.Email" class="form-control" />
+                <span asp-validation-for="Input.Email" class="text-danger"></span>
+            </div>
+            <div class="form-group">
+                <label asp-for="Input.Password"></label>
+                <input asp-for="Input.Password" class="form-control" />
+                <span asp-validation-for="Input.Password" class="text-danger"></span>
+            </div>
+            <div class="form-group">
+                <label asp-for="Input.ConfirmPassword"></label>
+                <input asp-for="Input.ConfirmPassword" class="form-control" />
+                <span asp-validation-for="Input.ConfirmPassword" class="text-danger"></span>
+            </div>
+            <button type="submit" class="btn btn-default">Register</button>
+        </form>
+    </div>
+</div>

test/WebSites/RazorPagesWebSite/Pages/CustomModelTypeModel.cshtml.cs

@@ -0,0 +1,78 @@
+using System;
+using System.ComponentModel.DataAnnotations;
+using Microsoft.AspNetCore.Mvc;
+using Microsoft.AspNetCore.Mvc.RazorPages;
+using Microsoft.Extensions.Logging;
+
+namespace RazorPagesWebSite
+{
+    public class CustomModelTypeModel : PageModel
+    {
+        [BindProperty]
+        public InputModel Input { get; set; }
+
+        public string ReturnUrl { get; set; }
+
+        public class InputModel
+        {
+            [Required]
+            [EmailAddress]
+            [Display(Name = "Email")]
+            public string Email { get; set; }
+
+            [Required]
+            [StringLength(100, ErrorMessage = "The {0} must be at least {2} and at max {1} characters long.", MinimumLength = 6)]
+            [DataType(DataType.Password)]
+            [Display(Name = "Password")]
+            public string Password { get; set; }
+
+            [DataType(DataType.Password)]
+            [Display(Name = "Confirm password")]
+            [Compare("Password", ErrorMessage = "The password and confirmation password do not match.")]
+            public string ConfirmPassword { get; set; }
+        }
+
+        public virtual void OnGet(string returnUrl = null)
+        {
+            throw new NotImplementedException();
+        }
+
+        public virtual IActionResult OnPostAsync(string returnUrl = null)
+        {
+            throw new NotImplementedException();
+        }
+    }
+
+    public class User
+    {
+    }
+
+    internal class CustomModelTypeModel<TUser> : CustomModelTypeModel where TUser : User
+    {
+        private readonly ILogger<CustomModelTypeModel<TUser>> _logger;
+
+        public CustomModelTypeModel(ILogger<CustomModelTypeModel<TUser>> logger)
+        {
+            _logger = logger;
+        }
+
+        public override void OnGet(string returnUrl = null)
+        {
+            // We only care about being able to resolve the service from DI.
+            // The line below is just to make the compiler happy.
+            _logger.LogInformation(typeof(TUser).Name);
+            ViewData["UserType"] = typeof(TUser).Name;
+            ReturnUrl = returnUrl;
+        }
+
+        public override IActionResult OnPostAsync(string returnUrl = null)
+        {
+            if (!ModelState.IsValid)
+            {
+                return Page();
+            }
+
+            return Redirect("~/");
+        }
+    }
+}

test/WebSites/RazorPagesWebSite/Startup.cs

@@ -2,9 +2,10 @@
 // Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
 
 using System.Globalization;
-using Microsoft.AspNetCore.Builder;
 using Microsoft.AspNetCore.Authentication.Cookies;
+using Microsoft.AspNetCore.Builder;
 using Microsoft.Extensions.DependencyInjection;
+using RazorPagesWebSite.Conventions;
 
 namespace RazorPagesWebSite
 {
@@ -22,6 +23,7 @@ namespace RazorPagesWebSite
                     options.Conventions.AllowAnonymousToPage("/Pages/Admin/Login");
                     options.Conventions.AddPageRoute("/HelloWorldWithRoute", "Different-Route/{text}");
                     options.Conventions.AddPageRoute("/Pages/NotTheRoot", string.Empty);
+                    options.Conventions.Add(new CustomModelTypeConvention());
                 })
                 .WithRazorPagesAtContentRoot();
         }

test/WebSites/RazorPagesWebSite/StartupWithBasePath.cs

@@ -4,6 +4,7 @@
 using Microsoft.AspNetCore.Builder;
 using Microsoft.AspNetCore.Authentication.Cookies;
 using Microsoft.Extensions.DependencyInjection;
+using RazorPagesWebSite.Conventions;
 
 namespace RazorPagesWebSite
 {
@@ -22,6 +23,7 @@ namespace RazorPagesWebSite
                     options.Conventions.AuthorizeFolder("/Conventions/AuthFolder");
                     options.Conventions.AuthorizeAreaFolder("Accounts", "/RequiresAuth");
                     options.Conventions.AllowAnonymousToAreaPage("Accounts", "/RequiresAuth/AllowAnonymous");
+                    options.Conventions.Add(new CustomModelTypeConvention());
                 });
         }