Fixed display names loading for validated properties

Author: Sergio0694

Microsoft.Toolkit.Mvvm/ComponentModel/ObservableValidator.cs

@@ -26,6 +26,18 @@ public abstract class ObservableValidator : ObservableObject, INotifyDataErrorIn
         /// </summary>
         private static readonly ConditionalWeakTable<Type, Action<object>> EntityValidatorMap = new();
 
+        /// <summary>
+        /// The <see cref="ConditionalWeakTable{TKey, TValue}"/> instance used to track display names for properties to validate.
+        /// </summary>
+        /// <remarks>
+        /// This is necessary because we want to reuse the same <see cref="ValidationContext"/> instance for all validations, but
+        /// with the same behavior with repsect to formatted names that new instances would have provided. The issue is that the
+        /// <see cref="ValidationContext.DisplayName"/> property is not refreshed when we set <see cref="ValidationContext.MemberName"/>,
+        /// so we need to replicate the same logic to retrieve the right display name for properties to validate and update that
+        /// property manually right before passing the context to <see cref="Validator"/> and proceed with the normal functionality.
+        /// </remarks>
+        private static readonly ConditionalWeakTable<Type, Dictionary<string, string>> DisplayNamesMap = new();
+
         /// <summary>
         /// The cached <see cref="PropertyChangedEventArgs"/> for <see cref="HasErrors"/>.
         /// </summary>
@@ -541,6 +553,7 @@ protected void ValidateProperty(object? value, [CallerMemberName] string? proper
 
             // Validate the property, by adding new errors to the existing list
             this.validationContext.MemberName = propertyName;
+            this.validationContext.DisplayName = GetDisplayNameForProperty(propertyName!);
 
             bool isValid = Validator.TryValidateProperty(value, this.validationContext, propertyErrors);
 
@@ -611,6 +624,7 @@ private bool TryValidateProperty(object? value, string? propertyName, out IReadO
 
             // Validate the property, by adding new errors to the local list
             this.validationContext.MemberName = propertyName;
+            this.validationContext.DisplayName = GetDisplayNameForProperty(propertyName!);
 
             bool isValid = Validator.TryValidateProperty(value, this.validationContext, localErrors);
 
@@ -688,6 +702,36 @@ private void ClearErrorsForProperty(string propertyName)
             ErrorsChanged?.Invoke(this, new DataErrorsChangedEventArgs(propertyName));
         }
 
+        /// <summary>
+        /// Gets the display name for a given property. It could be a custom name or just the property name.
+        /// </summary>
+        /// <param name="propertyName">The target property name being validated.</param>
+        /// <returns>The display name for the property.</returns>
+        private string GetDisplayNameForProperty(string propertyName)
+        {
+            static Dictionary<string, string> GetDisplayNames(Type type)
+            {
+                Dictionary<string, string> displayNames = new();
+
+                foreach (PropertyInfo property in type.GetProperties(BindingFlags.Instance | BindingFlags.Public))
+                {
+                    if (property.GetCustomAttribute<DisplayAttribute>() is DisplayAttribute attribute &&
+                        attribute.GetName() is string displayName)
+                    {
+                        displayNames.Add(property.Name, displayName);
+                    }
+                }
+
+                return displayNames;
+            }
+
+            // This method replicates the logic of DisplayName and GetDisplayName from the
+            // ValidationContext class. See the original source in the BCL for more details.
+            DisplayNamesMap.GetValue(GetType(), static t => GetDisplayNames(t)).TryGetValue(propertyName, out string? displayName);
+
+            return displayName ?? propertyName;
+        }
+
 #pragma warning disable SA1204
         /// <summary>
         /// Throws an <see cref="ArgumentNullException"/> when a property name given as input is <see langword="null"/>.