// 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;
using System.Collections.Generic;
using System.IO;
using System.Threading.Tasks;
using Microsoft.AspNetCore.Html;
using Microsoft.AspNetCore.Http;
using Microsoft.AspNetCore.Mvc.Abstractions;
using Microsoft.AspNetCore.Mvc.ModelBinding;
using Microsoft.AspNetCore.Mvc.Rendering;
using Microsoft.AspNetCore.Mvc.ViewEngines;
using Microsoft.AspNetCore.Mvc.ViewFeatures;
using Microsoft.AspNetCore.Razor.TagHelpers;
using Microsoft.AspNetCore.Razor.TagHelpers.Testing;
using Microsoft.AspNetCore.Routing;
using Microsoft.AspNetCore.Testing;
using Moq;
using Xunit;

namespace Microsoft.AspNetCore.Mvc.TagHelpers
{
    public class ValidationSummaryTagHelperTest
    {
        public static TheoryData<ModelStateDictionary> ProcessAsync_GeneratesExpectedOutput_WithNoErrorsData
        {
            get
            {
                var emptyModelState = new ModelStateDictionary();

                var modelState = new ModelStateDictionary();
                SetValidModelState(modelState);

                return new TheoryData<ModelStateDictionary>
                {
                    emptyModelState,
                    modelState,
                };
            }
        }

        [Theory]
        [MemberData(nameof(ProcessAsync_GeneratesExpectedOutput_WithNoErrorsData))]
        public async Task ProcessAsync_GeneratesExpectedOutput_WithNoErrors(ModelStateDictionary modelState)
        {
            // Arrange
            var expectedTagName = "not-div";
            var metadataProvider = new TestModelMetadataProvider();
            var htmlGenerator = new TestableHtmlGenerator(metadataProvider);
            var expectedAttributes = new TagHelperAttributeList
            {
                new TagHelperAttribute("class", "form-control validation-summary-valid"),
                new TagHelperAttribute("data-valmsg-summary", "true"),
            };

            var expectedPreContent = "original pre-content";
            var expectedContent = "original content";
            var tagHelperContext = new TagHelperContext(
                tagName: "not-div",
                allAttributes: new TagHelperAttributeList(),
                items: new Dictionary<object, object>(),
                uniqueId: "test");
            var output = new TagHelperOutput(
                expectedTagName,
                attributes: new TagHelperAttributeList
                {
                    { "class", "form-control" }
                },
                getChildContentAsync: (useCachedResult, encoder) =>
                {
                    throw new InvalidOperationException("getChildContentAsync called unexpectedly");
                });
            output.PreContent.SetContent(expectedPreContent);
            output.Content.SetContent(expectedContent);
            output.PostContent.SetContent("Custom Content");

            var model = new Model();
            var viewContext = TestableHtmlGenerator.GetViewContext(model, htmlGenerator, metadataProvider, modelState);
            var validationSummaryTagHelper = new ValidationSummaryTagHelper(htmlGenerator)
            {
                ValidationSummary = ValidationSummary.All,
                ViewContext = viewContext,
            };

            // Act
            await validationSummaryTagHelper.ProcessAsync(tagHelperContext, output);

            // Assert
            Assert.Equal(expectedAttributes, output.Attributes, CaseSensitiveTagHelperAttributeComparer.Default);
            Assert.Equal(expectedPreContent, output.PreContent.GetContent());
            Assert.Equal(expectedContent, output.Content.GetContent());
            Assert.Equal(
                $"Custom Content<ul><li style=\"display:none\"></li>{Environment.NewLine}</ul>",
                output.PostContent.GetContent());
            Assert.Equal(expectedTagName, output.TagName);
        }

        [Theory]
        [MemberData(nameof(ProcessAsync_GeneratesExpectedOutput_WithNoErrorsData))]
        public async Task ProcessAsync_SuppressesOutput_IfClientSideValidationDisabled_WithNoErrorsData(
            ModelStateDictionary modelStateDictionary)
        {
            // Arrange
            var metadataProvider = new TestModelMetadataProvider();
            var htmlGenerator = new TestableHtmlGenerator(metadataProvider);
            var viewContext = CreateViewContext();
            viewContext.ClientValidationEnabled = false;

            var validationSummaryTagHelper = new ValidationSummaryTagHelper(htmlGenerator)
            {
                ValidationSummary = ValidationSummary.All,
                ViewContext = viewContext,
            };

            var output = new TagHelperOutput(
                "div",
                new TagHelperAttributeList(),
                (useCachedResult, encoder) =>
                {
                    throw new InvalidOperationException("getChildContentAsync called unexpectedly.");
                });

            var context = new TagHelperContext(
                tagName: "div",
                allAttributes: new TagHelperAttributeList(),
                items: new Dictionary<object, object>(),
                uniqueId: "test");

            // Act
            await validationSummaryTagHelper.ProcessAsync(context, output);

            // Assert
            Assert.Null(output.TagName);
            Assert.Empty(output.Attributes);
            Assert.Empty(HtmlContentUtilities.HtmlContentToString(output));
        }

        public static TheoryData<string, ModelStateDictionary> ProcessAsync_SuppressesOutput_IfModelOnlyWithNoModelErrorData
        {
            get
            {
                var emptyModelState = new ModelStateDictionary();

                var modelState = new ModelStateDictionary();
                SetValidModelState(modelState);

                var invalidModelState = new ModelStateDictionary();
                SetValidModelState(invalidModelState);
                invalidModelState.AddModelError($"{nameof(Model.Strings)}[1]", "This value is invalid.");

                return new TheoryData<string, ModelStateDictionary>
                {
                    { string.Empty, emptyModelState },
                    { string.Empty, modelState },
                    { nameof(Model.Text), modelState },
                    { "not-a-key", modelState },
                    { string.Empty, invalidModelState },
                    { $"{nameof(Model.Strings)}[2]", invalidModelState },
                    { nameof(Model.Text), invalidModelState },
                    { "not-a-key", invalidModelState },
                };
            }
        }

        [Theory]
        [MemberData(nameof(ProcessAsync_SuppressesOutput_IfModelOnlyWithNoModelErrorData))]
        public async Task ProcessAsync_SuppressesOutput_IfModelOnly_WithNoModelError(
            string prefix,
            ModelStateDictionary modelStateDictionary)
        {
            // Arrange
            var metadataProvider = new TestModelMetadataProvider();
            var htmlGenerator = new TestableHtmlGenerator(metadataProvider);
            var viewContext = CreateViewContext();
            viewContext.ViewData.TemplateInfo.HtmlFieldPrefix = prefix;

            var validationSummaryTagHelper = new ValidationSummaryTagHelper(htmlGenerator)
            {
                ValidationSummary = ValidationSummary.ModelOnly,
                ViewContext = viewContext,
            };

            var output = new TagHelperOutput(
                "div",
                new TagHelperAttributeList(),
                (useCachedResult, encoder) =>
                {
                    throw new InvalidOperationException("getChildContentAsync called unexpectedly.");
                });

            var context = new TagHelperContext(
                tagName: "div",
                allAttributes: new TagHelperAttributeList(),
                items: new Dictionary<object, object>(),
                uniqueId: "test");

            // Act
            await validationSummaryTagHelper.ProcessAsync(context, output);

            // Assert
            Assert.Null(output.TagName);
            Assert.Empty(output.Attributes);
            Assert.Empty(HtmlContentUtilities.HtmlContentToString(output));
        }

        [Theory]
        [InlineData(ValidationSummary.All)]
        [InlineData(ValidationSummary.ModelOnly)]
        public async Task ProcessAsync_GeneratesExpectedOutput_WithModelError(ValidationSummary validationSummary)
        {
            // Arrange
            var expectedError = "I am an error.";
            var expectedTagName = "not-div";
            var metadataProvider = new TestModelMetadataProvider();
            var htmlGenerator = new TestableHtmlGenerator(metadataProvider);

            var validationSummaryTagHelper = new ValidationSummaryTagHelper(htmlGenerator)
            {
                ValidationSummary = validationSummary,
            };

            var expectedPreContent = "original pre-content";
            var expectedContent = "original content";
            var tagHelperContext = new TagHelperContext(
                tagName: "not-div",
                allAttributes: new TagHelperAttributeList(),
                items: new Dictionary<object, object>(),
                uniqueId: "test");
            var output = new TagHelperOutput(
                expectedTagName,
                attributes: new TagHelperAttributeList
                {
                    { "class", "form-control" }
                },
                getChildContentAsync: (useCachedResult, encoder) =>
                {
                    var tagHelperContent = new DefaultTagHelperContent();
                    tagHelperContent.SetContent("Something");
                    return Task.FromResult<TagHelperContent>(tagHelperContent);
                });
            output.PreContent.SetContent(expectedPreContent);
            output.Content.SetContent(expectedContent);
            output.PostContent.SetContent("Custom Content");

            var model = new Model();
            var viewContext = TestableHtmlGenerator.GetViewContext(model, htmlGenerator, metadataProvider);
            validationSummaryTagHelper.ViewContext = viewContext;

            var modelState = viewContext.ModelState;
            SetValidModelState(modelState);
            modelState.AddModelError(string.Empty, expectedError);

            // Act
            await validationSummaryTagHelper.ProcessAsync(tagHelperContext, output);

            // Assert
            Assert.InRange(output.Attributes.Count, low: 1, high: 2);
            var attribute = Assert.Single(output.Attributes, attr => attr.Name.Equals("class"));
            Assert.Equal(
                new TagHelperAttribute("class", "form-control validation-summary-errors"),
                attribute,
                CaseSensitiveTagHelperAttributeComparer.Default);

            Assert.Equal(expectedPreContent, output.PreContent.GetContent());
            Assert.Equal(expectedContent, output.Content.GetContent());
            Assert.Equal(
                $"Custom Content<ul><li>{expectedError}</li>{Environment.NewLine}</ul>",
                output.PostContent.GetContent());
            Assert.Equal(expectedTagName, output.TagName);
        }

        [Fact]
        public async Task ProcessAsync_GeneratesExpectedOutput_WithPropertyErrors()
        {
            // Arrange
            var expectedError0 = "I am an error.";
            var expectedError2 = "I am also an error.";
            var expectedTagName = "not-div";
            var expectedAttributes = new TagHelperAttributeList
            {
                new TagHelperAttribute("class", "form-control validation-summary-errors"),
                new TagHelperAttribute("data-valmsg-summary", "true"),
            };

            var metadataProvider = new TestModelMetadataProvider();
            var htmlGenerator = new TestableHtmlGenerator(metadataProvider);
            var validationSummaryTagHelper = new ValidationSummaryTagHelper(htmlGenerator)
            {
                ValidationSummary = ValidationSummary.All,
            };

            var expectedPreContent = "original pre-content";
            var expectedContent = "original content";
            var tagHelperContext = new TagHelperContext(
                tagName: "not-div",
                allAttributes: new TagHelperAttributeList(),
                items: new Dictionary<object, object>(),
                uniqueId: "test");
            var output = new TagHelperOutput(
                expectedTagName,
                attributes: new TagHelperAttributeList
                {
                    { "class", "form-control" }
                },
                getChildContentAsync: (useCachedResult, encoder) =>
                {
                    var tagHelperContent = new DefaultTagHelperContent();
                    tagHelperContent.SetContent("Something");
                    return Task.FromResult<TagHelperContent>(tagHelperContent);
                });
            output.PreContent.SetContent(expectedPreContent);
            output.Content.SetContent(expectedContent);
            output.PostContent.SetContent("Custom Content");

            var model = new Model();
            var viewContext = TestableHtmlGenerator.GetViewContext(model, htmlGenerator, metadataProvider);
            validationSummaryTagHelper.ViewContext = viewContext;

            var modelState = viewContext.ModelState;
            SetValidModelState(modelState);
            modelState.AddModelError(key: $"{nameof(Model.Strings)}[0]", errorMessage: expectedError0);
            modelState.AddModelError(key: $"{nameof(Model.Strings)}[2]", errorMessage: expectedError2);

            // Act
            await validationSummaryTagHelper.ProcessAsync(tagHelperContext, output);

            // Assert
            Assert.Equal(expectedAttributes, output.Attributes, CaseSensitiveTagHelperAttributeComparer.Default);
            Assert.Equal(expectedPreContent, output.PreContent.GetContent());
            Assert.Equal(expectedContent, output.Content.GetContent());
            Assert.Equal(
                $"Custom Content<ul><li>{expectedError0}</li>{Environment.NewLine}" +
                $"<li>{expectedError2}</li>{Environment.NewLine}</ul>",
                output.PostContent.GetContent());
            Assert.Equal(expectedTagName, output.TagName);
        }

        [Theory]
        [InlineData(ValidationSummary.All, false)]
        [InlineData(ValidationSummary.ModelOnly, true)]
        public async Task ProcessAsync_CallsIntoGenerateValidationSummaryWithExpectedParameters(
            ValidationSummary validationSummary,
            bool expectedExcludePropertyErrors)
        {
            // Arrange
            var expectedViewContext = CreateViewContext();

            var generator = new Mock<IHtmlGenerator>();
            generator
                .Setup(mock => mock.GenerateValidationSummary(
                    expectedViewContext,
                    expectedExcludePropertyErrors,
                    null,   // message
                    null,   // headerTag
                    null))  // htmlAttributes
                .Returns(new TagBuilder("div"))
                .Verifiable();

            var validationSummaryTagHelper = new ValidationSummaryTagHelper(generator.Object)
            {
                ValidationSummary = validationSummary,
            };

            var expectedPreContent = "original pre-content";
            var expectedContent = "original content";
            var expectedPostContent = "original post-content";
            var output = new TagHelperOutput(
                tagName: "div",
                attributes: new TagHelperAttributeList(),
                getChildContentAsync: (useCachedResult, encoder) => Task.FromResult<TagHelperContent>(
                    new DefaultTagHelperContent()));
            output.PreContent.SetContent(expectedPreContent);
            output.Content.SetContent(expectedContent);
            output.PostContent.SetContent(expectedPostContent);

            validationSummaryTagHelper.ViewContext = expectedViewContext;

            var context = new TagHelperContext(
                tagName: "div",
                allAttributes: new TagHelperAttributeList(),
                items: new Dictionary<object, object>(),
                uniqueId: "test");

            // Act & Assert
            await validationSummaryTagHelper.ProcessAsync(context, output);

            generator.Verify();
            Assert.Equal("div", output.TagName);
            Assert.Empty(output.Attributes);
            Assert.Equal(expectedPreContent, output.PreContent.GetContent());
            Assert.Equal(expectedContent, output.Content.GetContent());
            Assert.Equal(expectedPostContent, output.PostContent.GetContent());
        }

        [Fact]
        public async Task ProcessAsync_MergesTagBuilderFromGenerateValidationSummary()
        {
            // Arrange
            var tagBuilder = new TagBuilder("span2");
            tagBuilder.InnerHtml.SetHtmlContent("New HTML");
            tagBuilder.Attributes.Add("anything", "something");
            tagBuilder.Attributes.Add("data-foo", "bar");
            tagBuilder.Attributes.Add("data-hello", "world");

            var generator = new Mock<IHtmlGenerator>(MockBehavior.Strict);
            generator
                .Setup(mock => mock.GenerateValidationSummary(
                    It.IsAny<ViewContext>(),
                    It.IsAny<bool>(),
                    It.IsAny<string>(),
                    It.IsAny<string>(),
                    It.IsAny<object>()))
                .Returns(tagBuilder);

            var validationSummaryTagHelper = new ValidationSummaryTagHelper(generator.Object)
            {
                ValidationSummary = ValidationSummary.ModelOnly,
            };

            var expectedPreContent = "original pre-content";
            var expectedContent = "original content";
            var output = new TagHelperOutput(
                tagName: "div",
                attributes: new TagHelperAttributeList(),
                getChildContentAsync: (useCachedResult, encoder) => Task.FromResult<TagHelperContent>(
                    new DefaultTagHelperContent()));
            output.PreContent.SetContent(expectedPreContent);
            output.Content.SetContent(expectedContent);
            output.PostContent.SetContent("Content of validation summary");

            var viewContext = CreateViewContext();
            validationSummaryTagHelper.ViewContext = viewContext;

            var context = new TagHelperContext(
                tagName: "div",
                allAttributes: new TagHelperAttributeList(),
                items: new Dictionary<object, object>(),
                uniqueId: "test");

            // Act
            await validationSummaryTagHelper.ProcessAsync(context, output);

            // Assert
            Assert.Equal("div", output.TagName);
            Assert.Collection(
                output.Attributes,
                attribute =>
                {
                    Assert.Equal("anything", attribute.Name);
                    Assert.Equal("something", attribute.Value);
                },
                attribute =>
                {
                    Assert.Equal("data-foo", attribute.Name);
                    Assert.Equal("bar", attribute.Value);
                },
                attribute =>
                {
                    Assert.Equal("data-hello", attribute.Name);
                    Assert.Equal("world", attribute.Value);
                });
            Assert.Equal(expectedPreContent, output.PreContent.GetContent());
            Assert.Equal(expectedContent, output.Content.GetContent());
            Assert.Equal("Content of validation summaryNew HTML", output.PostContent.GetContent());
        }

        [Fact]
        public async Task ProcessAsync_DoesNothingIfValidationSummaryNone()
        {
            // Arrange
            var generator = new Mock<IHtmlGenerator>(MockBehavior.Strict);

            var validationSummaryTagHelper = new ValidationSummaryTagHelper(generator.Object)
            {
                ValidationSummary = ValidationSummary.None,
            };

            var expectedPreContent = "original pre-content";
            var expectedContent = "original content";
            var expectedPostContent = "original post-content";
            var output = new TagHelperOutput(
                tagName: "div",
                attributes: new TagHelperAttributeList(),
                getChildContentAsync: (useCachedResult, encoder) => Task.FromResult<TagHelperContent>(
                    new DefaultTagHelperContent()));
            output.PreContent.SetContent(expectedPreContent);
            output.Content.SetContent(expectedContent);
            output.PostContent.SetContent(expectedPostContent);

            var viewContext = CreateViewContext();
            validationSummaryTagHelper.ViewContext = viewContext;

            var context = new TagHelperContext(
                tagName: "div",
                allAttributes: new TagHelperAttributeList(),
                items: new Dictionary<object, object>(),
                uniqueId: "test");

            // Act
            await validationSummaryTagHelper.ProcessAsync(context, output);

            // Assert
            Assert.Equal("div", output.TagName);
            Assert.Empty(output.Attributes);
            Assert.Equal(expectedPreContent, output.PreContent.GetContent());
            Assert.Equal(expectedContent, output.Content.GetContent());
            Assert.Equal(expectedPostContent, output.PostContent.GetContent());
        }

        [Theory]
        [InlineData(ValidationSummary.All)]
        [InlineData(ValidationSummary.ModelOnly)]
        public async Task ProcessAsync_GeneratesValidationSummaryWhenNotNone(ValidationSummary validationSummary)
        {
            // Arrange
            var tagBuilder = new TagBuilder("span2");
            tagBuilder.InnerHtml.SetHtmlContent("New HTML");

            var generator = new Mock<IHtmlGenerator>();
            generator
                .Setup(mock => mock.GenerateValidationSummary(
                    It.IsAny<ViewContext>(),
                    It.IsAny<bool>(),
                    It.IsAny<string>(),
                    It.IsAny<string>(),
                    It.IsAny<object>()))
                .Returns(tagBuilder)
                .Verifiable();

            var validationSummaryTagHelper = new ValidationSummaryTagHelper(generator.Object)
            {
                ValidationSummary = validationSummary,
            };

            var expectedPreContent = "original pre-content";
            var expectedContent = "original content";
            var output = new TagHelperOutput(
                tagName: "div",
                attributes: new TagHelperAttributeList(),
                getChildContentAsync: (useCachedResult, encoder) => Task.FromResult<TagHelperContent>(
                    new DefaultTagHelperContent()));
            output.PreContent.SetContent(expectedPreContent);
            output.Content.SetContent(expectedContent);
            output.PostContent.SetContent("Content of validation message");

            var viewContext = CreateViewContext();
            validationSummaryTagHelper.ViewContext = viewContext;

            var context = new TagHelperContext(
                tagName: "div",
                allAttributes: new TagHelperAttributeList(),
                items: new Dictionary<object, object>(),
                uniqueId: "test");

            // Act
            await validationSummaryTagHelper.ProcessAsync(context, output);

            // Assert
            Assert.Equal("div", output.TagName);
            Assert.Empty(output.Attributes);
            Assert.Equal(expectedPreContent, output.PreContent.GetContent());
            Assert.Equal(expectedContent, output.Content.GetContent());
            Assert.Equal("Content of validation messageNew HTML", output.PostContent.GetContent());
            generator.Verify();
        }

        [Theory]
        [InlineData((ValidationSummary)(-1))]
        [InlineData((ValidationSummary)23)]
        [InlineData(ValidationSummary.All | ValidationSummary.ModelOnly)]
        [ReplaceCulture]
        public void ValidationSummaryProperty_ThrowsWhenSetToInvalidValidationSummaryValue(
            ValidationSummary validationSummary)
        {
            // Arrange
            var generator = new TestableHtmlGenerator(new EmptyModelMetadataProvider());

            var validationSummaryTagHelper = new ValidationSummaryTagHelper(generator);
            var validationTypeName = typeof(ValidationSummary).FullName;
            var expectedMessage = $"The value of argument 'value' ({validationSummary}) is invalid for Enum type '{validationTypeName}'.";

            // Act & Assert
            ExceptionAssert.ThrowsArgument(
                () => validationSummaryTagHelper.ValidationSummary = validationSummary,
                "value",
                expectedMessage);
        }

        [Fact]
        public async Task ProcessAsync_GeneratesExpectedOutput_WithModelErrorForIEnumerable()
        {
            // Arrange
            var expectedError = "Something went wrong.";
            var expectedTagName = "not-div";
            var expectedAttributes = new TagHelperAttributeList
            {
                new TagHelperAttribute("class", "form-control validation-summary-errors"),
                new TagHelperAttribute("data-valmsg-summary", "true"),
            };

            var metadataProvider = new TestModelMetadataProvider();
            var htmlGenerator = new TestableHtmlGenerator(metadataProvider);
            var validationSummaryTagHelper = new ValidationSummaryTagHelper(htmlGenerator)
            {
                ValidationSummary = ValidationSummary.All,
            };

            var expectedPreContent = "original pre-content";
            var expectedContent = "original content";
            var tagHelperContext = new TagHelperContext(
                tagName: "not-div",
                allAttributes: new TagHelperAttributeList(),
                items: new Dictionary<object, object>(),
                uniqueId: "test");
            var output = new TagHelperOutput(
                expectedTagName,
                attributes: new TagHelperAttributeList
                {
                    { "class", "form-control" }
                },
                getChildContentAsync: (useCachedResult, encoder) =>
                {
                    var tagHelperContent = new DefaultTagHelperContent();
                    tagHelperContent.SetContent("Something");
                    return Task.FromResult<TagHelperContent>(tagHelperContent);
                });
            output.PreContent.SetContent(expectedPreContent);
            output.Content.SetContent(expectedContent);
            output.PostContent.SetContent("Custom Content");

            var model = new FormMetadata();
            var viewContext = TestableHtmlGenerator.GetViewContext(model, htmlGenerator, metadataProvider);
            validationSummaryTagHelper.ViewContext = viewContext;

            viewContext.ModelState.AddModelError(key: nameof(FormMetadata.ID), errorMessage: expectedError);

            // Act
            await validationSummaryTagHelper.ProcessAsync(tagHelperContext, output);

            // Assert
            Assert.Equal(expectedAttributes, output.Attributes, CaseSensitiveTagHelperAttributeComparer.Default);
            Assert.Equal(expectedPreContent, output.PreContent.GetContent());
            Assert.Equal(expectedContent, output.Content.GetContent());
            Assert.Equal(
                $"Custom Content<ul><li>{expectedError}</li>{Environment.NewLine}</ul>",
                output.PostContent.GetContent());
            Assert.Equal(expectedTagName, output.TagName);
        }

        private static ViewContext CreateViewContext()
        {
            var actionContext = new ActionContext(
                new DefaultHttpContext(),
                new RouteData(),
                new ActionDescriptor());

            return new ViewContext(
                actionContext,
                Mock.Of<IView>(),
                new ViewDataDictionary(
                    new EmptyModelMetadataProvider(),
                    new ModelStateDictionary()),
                Mock.Of<ITempDataDictionary>(),
                TextWriter.Null,
                new HtmlHelperOptions());
        }

        private static void SetValidModelState(ModelStateDictionary modelState)
        {
            modelState.SetModelValue(key: nameof(Model.Empty), rawValue: null, attemptedValue: null);
            modelState.SetModelValue(key: $"{nameof(Model.Strings)}[0]", rawValue: null, attemptedValue: null);
            modelState.SetModelValue(key: $"{nameof(Model.Strings)}[1]", rawValue: null, attemptedValue: null);
            modelState.SetModelValue(key: $"{nameof(Model.Strings)}[2]", rawValue: null, attemptedValue: null);
            modelState.SetModelValue(key: nameof(Model.Text), rawValue: null, attemptedValue: null);

            foreach (var key in modelState.Keys)
            {
                modelState.MarkFieldValid(key);
            }
        }

        private class Model
        {
            public string Text { get; set; }

            public string[] Strings { get; set; }

            // Exists to ensure #4989 does not regress. Issue specific to case where collection has a ModelStateEntry
            // but no element does.
            public byte[] Empty { get; set; }
        }

        private class FormMetadata : IEnumerable<Model>
        {
            private List<Model> _fields = new List<Model>();

            public int ID { get; set; }

            public IEnumerator<Model> GetEnumerator()
            {
                return _fields.GetEnumerator();
            }

            IEnumerator IEnumerable.GetEnumerator()
            {
                return _fields.GetEnumerator();
            }
        }
    }
}