Add mechanism of overriding name used for date input error messages

Closes #282

CHANGELOG.md

@@ -2,6 +2,13 @@
 
 ## Unreleased
 
+### `DateInputAttribute`
+This attribute can be added to properties that are model bound from date input components. It allows overriding the prefix used for error messages e.g.
+```cs
+[DateInput(ErrorMessagePrefix = "Your date of birth")]
+public DateOnly? DateOfBirth { get; set; }
+```
+
 ### `asp-for` attributes
 The `asp-for` attributes have been deprecated; the `for` attribute should be used instead.
 

samples/Samples.DateInput/Pages/Index.cshtml

@@ -5,7 +5,11 @@
 }
 
 <form asp-page="Index">
-    <govuk-date-input asp-for="DateOfBirth" />
+    <govuk-date-input asp-for="DateOfBirth">
+        <govuk-date-input-fieldset>
+            <govuk-date-input-fieldset-legend is-page-heading="true" class="govuk-fieldset__legend--l" />
+        </govuk-date-input-fieldset>
+    </govuk-date-input>
 
     <govuk-button type="submit">Save</govuk-button>
 </form>

samples/Samples.DateInput/Pages/Index.cshtml.cs

@@ -1,4 +1,5 @@
 using System.ComponentModel.DataAnnotations;
+using GovUk.Frontend.AspNetCore;
 using Microsoft.AspNetCore.Mvc;
 using Microsoft.AspNetCore.Mvc.RazorPages;
 using NodaTime;
@@ -8,7 +9,8 @@ namespace Samples.DateInput.Pages;
 public class IndexModel : PageModel
 {
     [BindProperty]
-    [Display(Name = "Date of birth")]
+    [Display(Name = "What is your date of birth?")]
+    [DateInput(ErrorMessagePrefix = "Your date of birth")]
     [Required(ErrorMessage = "Enter your date of birth")]
     public LocalDate? DateOfBirth { get; set; }
 

src/GovUk.Frontend.AspNetCore/DateInputAttribute.cs

@@ -0,0 +1,20 @@
+using System;
+using GovUk.Frontend.AspNetCore.ModelBinding;
+
+namespace GovUk.Frontend.AspNetCore;
+
+/// <summary>
+/// An attribute that can specify the error message prefix to use in model binding from date input components.
+/// </summary>
+[AttributeUsage(AttributeTargets.Property | AttributeTargets.Parameter, AllowMultiple = false)]
+public sealed class DateInputAttribute : Attribute
+{
+    /// <summary>
+    /// Gets or sets the prefix used in error messages.
+    /// </summary>
+    /// <remarks>
+    /// This prefix is used at the start of error messages produced by <see cref="DateInputModelBinder"/>
+    /// e.g. <c>{ErrorMessagePrefix} must be a real date</c>
+    /// </remarks>
+    public string? ErrorMessagePrefix { get; set; }
+}

src/GovUk.Frontend.AspNetCore/GovUkFrontendAspNetCoreExtensions.cs

@@ -95,6 +95,7 @@ public ConfigureMvcOptions(IOptions<GovUkFrontendAspNetCoreOptions> gfaOptionsAc
         public void Configure(MvcOptions options)
         {
             options.ModelBinderProviders.Insert(2, new DateInputModelBinderProvider(_gfaOptionsAccessor));
+            options.ModelMetadataDetailsProviders.Add(new GovUkFrontendAspNetCoreMetadataDetailsProvider());
         }
     }
 }

src/GovUk.Frontend.AspNetCore/ModelBinding/DateInputModelConverterModelBinder.cs

@@ -10,9 +10,9 @@ namespace GovUk.Frontend.AspNetCore.ModelBinding;
 
 internal class DateInputModelConverterModelBinder : IModelBinder
 {
-    private const string DayComponentName = "Day";
-    private const string MonthComponentName = "Month";
-    private const string YearComponentName = "Year";
+    private const string DayInputName = "Day";
+    private const string MonthInputName = "Month";
+    private const string YearInputName = "Year";
 
     private readonly DateInputModelConverter _dateInputModelConverter;
     private readonly IOptions<GovUkFrontendAspNetCoreOptions> _optionsAccessor;
@@ -35,9 +35,9 @@ public Task BindModelAsync(ModelBindingContext bindingContext)
             throw new InvalidOperationException($"Cannot bind {modelType.Name}.");
         }
 
-        var dayModelName = $"{bindingContext.ModelName}.{DayComponentName}";
-        var monthModelName = $"{bindingContext.ModelName}.{MonthComponentName}";
-        var yearModelName = $"{bindingContext.ModelName}.{YearComponentName}";
+        var dayModelName = ModelNames.CreatePropertyModelName(bindingContext.ModelName, DayInputName);
+        var monthModelName = ModelNames.CreatePropertyModelName(bindingContext.ModelName, MonthInputName);
+        var yearModelName = ModelNames.CreatePropertyModelName(bindingContext.ModelName, YearInputName);
 
         var dayValueProviderResult = bindingContext.ValueProvider.GetValue(dayModelName);
         var monthValueProviderResult = bindingContext.ValueProvider.GetValue(monthModelName);
@@ -97,27 +97,31 @@ internal static string GetModelStateErrorMessage(DateInputParseErrors parseError
         Debug.Assert(parseErrors != DateInputParseErrors.None);
         Debug.Assert(parseErrors != (DateInputParseErrors.MissingDay | DateInputParseErrors.MissingMonth | DateInputParseErrors.MissingYear));
 
-        var displayName = modelMetadata.DisplayName ?? modelMetadata.PropertyName;
+        var dateInputModelMetadata = modelMetadata.AdditionalValues.TryGetValue(typeof(DateInputModelMetadata), out var metadataObj) &&
+            metadataObj is DateInputModelMetadata dimm ? dimm :
+            null;
 
-        var missingComponents = new List<string>();
+        var displayName = dateInputModelMetadata?.ErrorMessagePrefix ?? modelMetadata.DisplayName ?? modelMetadata.PropertyName;
+
+        var missingFields = new List<string>();
 
         if ((parseErrors & DateInputParseErrors.MissingDay) != 0)
         {
-            missingComponents.Add("day");
+            missingFields.Add("day");
         }
         if ((parseErrors & DateInputParseErrors.MissingMonth) != 0)
         {
-            missingComponents.Add("month");
+            missingFields.Add("month");
         }
         if ((parseErrors & DateInputParseErrors.MissingYear) != 0)
         {
-            missingComponents.Add("year");
+            missingFields.Add("year");
         }
 
-        if (missingComponents.Count > 0)
+        if (missingFields.Count > 0)
         {
-            Debug.Assert(missingComponents.Count <= 2);
-            return $"{displayName} must include a {string.Join(" and ", missingComponents)}";
+            Debug.Assert(missingFields.Count <= 2);
+            return $"{displayName} must include a {string.Join(" and ", missingFields)}";
         }
 
         return $"{displayName} must be a real date";
@@ -176,7 +180,7 @@ bool TryParseMonth(string value, out int result)
 
             if (!int.TryParse(value, out result) && acceptMonthNames)
             {
-                result = value.ToLower() switch
+                result = value.ToLowerInvariant() switch
                 {
                     "jan" => 1,
                     "january" => 1,

src/GovUk.Frontend.AspNetCore/ModelBinding/DateInputModelMetadata.cs

@@ -0,0 +1,6 @@
+namespace GovUk.Frontend.AspNetCore.ModelBinding;
+
+internal sealed class DateInputModelMetadata
+{
+    public string? ErrorMessagePrefix { get; set; }
+}

src/GovUk.Frontend.AspNetCore/ModelBinding/DateInputParseErrors.cs

@@ -3,7 +3,7 @@
 namespace GovUk.Frontend.AspNetCore.ModelBinding;
 
 /// <summary>
-/// The errors that occurred when parsing the components of a date input component.
+/// The errors that occurred when parsing the fields of a date input component.
 /// </summary>
 [Flags]
 public enum DateInputParseErrors
@@ -14,32 +14,32 @@ public enum DateInputParseErrors
     None = 0,
 
     /// <summary>
-    /// The year component is missing.
+    /// The year field is missing.
     /// </summary>
     MissingYear = 1,
 
     /// <summary>
-    /// The year component is invalid.
+    /// The year field is invalid.
     /// </summary>
     InvalidYear = 2,
 
     /// <summary>
-    /// The month component is missing.
+    /// The month field is missing.
     /// </summary>
     MissingMonth = 4,
 
     /// <summary>
-    /// The month component is invalid.
+    /// The month field is invalid.
     /// </summary>
     InvalidMonth = 8,
 
     /// <summary>
-    /// The day component is missing.
+    /// The day field is missing.
     /// </summary>
     MissingDay = 16,
 
     /// <summary>
-    /// The day component is invalid.
+    /// The day field is invalid.
     /// </summary>
     InvalidDay = 32
 }

src/GovUk.Frontend.AspNetCore/ModelBinding/GovUkFrontendAspNetCoreMetadataDetailsProvider.cs

@@ -0,0 +1,22 @@
+using System.Linq;
+using Microsoft.AspNetCore.Mvc.ModelBinding.Metadata;
+
+namespace GovUk.Frontend.AspNetCore.ModelBinding;
+
+internal class GovUkFrontendAspNetCoreMetadataDetailsProvider : IMetadataDetailsProvider, IDisplayMetadataProvider
+{
+    public void CreateDisplayMetadata(DisplayMetadataProviderContext context)
+    {
+        var dateOnlyMetadataAttribute = context.Attributes.OfType<DateInputAttribute>().FirstOrDefault();
+
+        if (dateOnlyMetadataAttribute is not null)
+        {
+            var dateOnlyMetadata = new DateInputModelMetadata()
+            {
+                ErrorMessagePrefix = dateOnlyMetadataAttribute.ErrorMessagePrefix
+            };
+
+            context.DisplayMetadata.AdditionalValues.Add(typeof(DateInputModelMetadata), dateOnlyMetadata);
+        }
+    }
+}

tests/GovUk.Frontend.AspNetCore.Tests/ModelBinding/DateInputModelBinderTests.cs

@@ -1,5 +1,6 @@
 using System;
 using System.Collections.Generic;
+using System.Collections.ObjectModel;
 using System.Threading.Tasks;
 using GovUk.Frontend.AspNetCore.ModelBinding;
 using GovUk.Frontend.AspNetCore.Tests.Infrastructure;
@@ -240,6 +241,41 @@ public void GetModelStateErrorMessage(DateInputParseErrors parseErrors, string e
         Assert.Equal(expectedMessage, result);
     }
 
+    [Theory]
+    [InlineData(DateInputParseErrors.MissingYear, "Your date of birth must include a year")]
+    [InlineData(DateInputParseErrors.InvalidYear, "Your date of birth must be a real date")]
+    [InlineData(DateInputParseErrors.MissingMonth, "Your date of birth must include a month")]
+    [InlineData(DateInputParseErrors.InvalidMonth, "Your date of birth must be a real date")]
+    [InlineData(DateInputParseErrors.InvalidDay, "Your date of birth must be a real date")]
+    [InlineData(DateInputParseErrors.MissingDay, "Your date of birth must include a day")]
+    [InlineData(DateInputParseErrors.MissingYear | DateInputParseErrors.MissingMonth, "Your date of birth must include a month and year")]
+    [InlineData(DateInputParseErrors.MissingYear | DateInputParseErrors.MissingDay, "Your date of birth must include a day and year")]
+    [InlineData(DateInputParseErrors.MissingMonth | DateInputParseErrors.MissingDay, "Your date of birth must include a day and month")]
+    [InlineData(DateInputParseErrors.InvalidYear | DateInputParseErrors.InvalidMonth, "Your date of birth must be a real date")]
+    [InlineData(DateInputParseErrors.InvalidYear | DateInputParseErrors.InvalidMonth | DateInputParseErrors.InvalidDay, "Your date of birth must be a real date")]
+    [InlineData(DateInputParseErrors.InvalidMonth | DateInputParseErrors.InvalidDay, "Your date of birth must be a real date")]
+    public void GetModelStateErrorMessageWithDateInputMetadata(DateInputParseErrors parseErrors, string expectedMessage)
+    {
+        // Arrange
+        var dateInputModelMetadata = new DateInputModelMetadata()
+        {
+            ErrorMessagePrefix = "Your date of birth"
+        };
+
+        var modelMetadata = new DisplayNameModelMetadata(
+            "Date of birth",
+            additionalValues: new Dictionary<object, object>()
+            {
+                { typeof(DateInputModelMetadata), dateInputModelMetadata }
+            });
+
+        // Act
+        var result = DateInputModelConverterModelBinder.GetModelStateErrorMessage(parseErrors, modelMetadata);
+
+        // Assert
+        Assert.Equal(expectedMessage, result);
+    }
+
     [Theory]
     [InlineData("", "4", "2020", DateInputParseErrors.MissingDay)]
     [InlineData("1", "", "2020", DateInputParseErrors.MissingMonth)]
@@ -345,13 +381,14 @@ private static ActionContext CreateActionContextWithServices()
 
     private class DisplayNameModelMetadata : ModelMetadata
     {
-        public DisplayNameModelMetadata(string displayName)
+        public DisplayNameModelMetadata(string displayName, IReadOnlyDictionary<object, object>? additionalValues = null)
             : base(ModelMetadataIdentity.ForType(typeof(DateOnly?)))
         {
             DisplayName = displayName;
+            AdditionalValues = additionalValues ?? new Dictionary<object, object>();
         }
 
-        public override IReadOnlyDictionary<object, object> AdditionalValues => throw new NotImplementedException();
+        public override IReadOnlyDictionary<object, object> AdditionalValues { get; }
 
         public override ModelPropertyCollection Properties => throw new NotImplementedException();