Respect JsonSerializerOptions casing for property names in validation errors

src/Http/Http.Abstractions/src/PublicAPI.Unshipped.txt

@@ -28,6 +28,8 @@ Microsoft.AspNetCore.Http.Validation.ValidateContext.ValidationContext.get -> Sy
 Microsoft.AspNetCore.Http.Validation.ValidateContext.ValidationContext.set -> void
 Microsoft.AspNetCore.Http.Validation.ValidateContext.ValidationErrors.get -> System.Collections.Generic.Dictionary<string!, string![]!>?
 Microsoft.AspNetCore.Http.Validation.ValidateContext.ValidationErrors.set -> void
+Microsoft.AspNetCore.Http.Validation.ValidateContext.SerializerOptions.get -> System.Text.Json.JsonSerializerOptions?
+Microsoft.AspNetCore.Http.Validation.ValidateContext.SerializerOptions.set -> void
 Microsoft.AspNetCore.Http.Validation.ValidateContext.ValidationOptions.get -> Microsoft.AspNetCore.Http.Validation.ValidationOptions!
 Microsoft.AspNetCore.Http.Validation.ValidateContext.ValidationOptions.set -> void
 Microsoft.AspNetCore.Http.Validation.ValidationOptions

src/Http/Http.Abstractions/src/Validation/ValidatablePropertyInfo.cs

@@ -3,6 +3,9 @@
 
 using System.ComponentModel.DataAnnotations;
 using System.Diagnostics.CodeAnalysis;
+using System.Reflection;
+using System.Text.Json;
+using System.Text.Json.Serialization;
 
 namespace Microsoft.AspNetCore.Http.Validation;
 
@@ -13,12 +16,13 @@ namespace Microsoft.AspNetCore.Http.Validation;
 public abstract class ValidatablePropertyInfo : IValidatableInfo
 {
     private RequiredAttribute? _requiredAttribute;
+    private readonly bool _hasDisplayAttribute;
 
     /// <summary>
     /// Creates a new instance of <see cref="ValidatablePropertyInfo"/>.
     /// </summary>
     protected ValidatablePropertyInfo(
-        [param: DynamicallyAccessedMembers(DynamicallyAccessedMemberTypes.PublicProperties)]
+        [param: DynamicallyAccessedMembers(DynamicallyAccessedMemberTypes.PublicProperties | DynamicallyAccessedMemberTypes.PublicConstructors)]
         Type declaringType,
         Type propertyType,
         string name,
@@ -28,12 +32,16 @@ protected ValidatablePropertyInfo(
         PropertyType = propertyType;
         Name = name;
         DisplayName = displayName;
+
+        // Cache the HasDisplayAttribute result to avoid repeated reflection calls
+        var property = DeclaringType.GetProperty(Name);
+        _hasDisplayAttribute = property is not null && HasDisplayAttribute(property);
     }
 
     /// <summary>
     /// Gets the member type.
     /// </summary>
-    [DynamicallyAccessedMembers(DynamicallyAccessedMemberTypes.PublicProperties)]
+    [DynamicallyAccessedMembers(DynamicallyAccessedMemberTypes.PublicProperties | DynamicallyAccessedMemberTypes.PublicConstructors)]
     internal Type DeclaringType { get; }
 
     /// <summary>
@@ -65,18 +73,23 @@ public virtual async Task ValidateAsync(object? value, ValidateContext context,
         var validationAttributes = GetValidationAttributes();
 
         // Calculate and save the current path
+        var memberName = GetJsonPropertyName(Name, property, context.SerializerOptions?.PropertyNamingPolicy);
         var originalPrefix = context.CurrentValidationPath;
         if (string.IsNullOrEmpty(originalPrefix))
         {
-            context.CurrentValidationPath = Name;
+            context.CurrentValidationPath = memberName;
         }
         else
         {
-            context.CurrentValidationPath = $"{originalPrefix}.{Name}";
+            context.CurrentValidationPath = $"{originalPrefix}.{memberName}";
         }
 
-        context.ValidationContext.DisplayName = DisplayName;
-        context.ValidationContext.MemberName = Name;
+        // Format the display name and member name according to JsonPropertyName attribute first, then naming policy
+        // If the property has a [Display] attribute (either on property or record parameter), use DisplayName directly without formatting
+        context.ValidationContext.DisplayName = _hasDisplayAttribute
+            ? DisplayName
+            : GetJsonPropertyName(DisplayName, property, context.SerializerOptions?.PropertyNamingPolicy);
+        context.ValidationContext.MemberName = memberName;
 
         // Check required attribute first
         if (_requiredAttribute is not null || validationAttributes.TryGetRequiredAttribute(out _requiredAttribute))
@@ -170,4 +183,61 @@ void ValidateValue(object? val, string errorPrefix, ValidationAttribute[] valida
             }
         }
     }
+
+    /// <summary>
+    /// Gets the effective member name for JSON serialization, considering JsonPropertyName attribute and naming policy.
+    /// </summary>
+    /// <param name="targetValue">The target value to get the name for.</param>
+    /// <param name="property">The property info to get the name for.</param>
+    /// <param name="namingPolicy">The JSON naming policy to apply if no JsonPropertyName attribute is present.</param>
+    /// <returns>The effective property name for JSON serialization.</returns>
+    private static string GetJsonPropertyName(string targetValue, PropertyInfo property, JsonNamingPolicy? namingPolicy)
+    {
+        var jsonPropertyName = property.GetCustomAttribute<JsonPropertyNameAttribute>()?.Name;
+
+        if (jsonPropertyName is not null)
+        {
+            return jsonPropertyName;
+        }
+
+        if (namingPolicy is not null)
+        {
+            return namingPolicy.ConvertName(targetValue);
+        }
+
+        return targetValue;
+    }
+
+    /// <summary>
+    /// Determines whether the property has a DisplayAttribute, either directly on the property
+    /// or on the corresponding constructor parameter if the declaring type is a record.
+    /// </summary>
+    /// <param name="property">The property to check.</param>
+    /// <returns>True if the property has a DisplayAttribute, false otherwise.</returns>
+    private bool HasDisplayAttribute(PropertyInfo property)
+    {
+        // Check if the property itself has the DisplayAttribute with a valid Name
+        if (property.GetCustomAttribute<DisplayAttribute>() is { Name: not null })
+        {
+            return true;
+        }
+
+        // Look for a constructor parameter matching the property name (case-insensitive)
+        // to account for the record scenario
+        foreach (var constructor in DeclaringType.GetConstructors())
+        {
+            foreach (var parameter in constructor.GetParameters())
+            {
+                if (string.Equals(parameter.Name, property.Name, StringComparison.OrdinalIgnoreCase))
+                {
+                    if (parameter.GetCustomAttribute<DisplayAttribute>() is { Name: not null })
+                    {
+                        return true;
+                    }
+                }
+            }
+        }
+
+        return false;
+    }
 }

src/Http/Http.Abstractions/src/Validation/ValidatableTypeInfo.cs

@@ -106,9 +106,17 @@ public virtual async Task ValidateAsync(object? value, ValidateContext context,
                         // Create a validation error for each member name that is provided
                         foreach (var memberName in validationResult.MemberNames)
                         {
+                            // Format the member name using JsonSerializerOptions naming policy if available
+                            // Note: we don't respect [JsonPropertyName] here because we have no context of the property being validated.
+                            var formattedMemberName = memberName;
+                            if (context.SerializerOptions?.PropertyNamingPolicy != null)
+                            {
+                                formattedMemberName = context.SerializerOptions.PropertyNamingPolicy.ConvertName(memberName);
+                            }
+
                             var key = string.IsNullOrEmpty(originalPrefix) ?
-                                memberName :
-                                $"{originalPrefix}.{memberName}";
+                                formattedMemberName :
+                                $"{originalPrefix}.{formattedMemberName}";
                             context.AddOrExtendValidationError(key, validationResult.ErrorMessage);
                         }
 

src/Http/Http.Abstractions/src/Validation/ValidateContext.cs

@@ -3,6 +3,8 @@
 
 using System.ComponentModel.DataAnnotations;
 using System.Diagnostics.CodeAnalysis;
+using System.Linq;
+using System.Text.Json;
 
 namespace Microsoft.AspNetCore.Http.Validation;
 
@@ -60,11 +62,18 @@ public sealed class ValidateContext
     /// </summary>
     public int CurrentDepth { get; set; }
 
-    internal void AddValidationError(string key, string[] error)
+    /// <summary>
+    /// Gets or sets the JSON serializer options to use for property name formatting.
+    /// When available, property names in validation errors will be formatted according to the
+    /// PropertyNamingPolicy and JsonPropertyName attributes.
+    /// </summary>
+    public JsonSerializerOptions? SerializerOptions { get; set; }
+
+    internal void AddValidationError(string key, string[] errors)
     {
         ValidationErrors ??= [];
 
-        ValidationErrors[key] = error;
+        ValidationErrors[key] = errors;
     }
 
     internal void AddOrExtendValidationErrors(string key, string[] errors)
@@ -90,7 +99,7 @@ internal void AddOrExtendValidationError(string key, string error)
 
         if (ValidationErrors.TryGetValue(key, out var existingErrors) && !existingErrors.Contains(error))
         {
-            ValidationErrors[key] = [.. existingErrors, error];
+            ValidationErrors[key] = [..existingErrors, error];
         }
         else
         {