FluentValidation log validation errors as json

readme.md

One page sample

public PageResult OnPostSubmit()
{
    // Perform validation
    var result = validator.Validate(Customer);
    if (!result.IsValid)
    {
        Log.Information("Validation failure on page {PageName} {@errors}", PageHelpers.GetCurrentPageName(HttpContext.Request), result.ToJson());
        result.AddToModelState(ModelState);
    }

    return Page();
}

FluentValidationExtensions.cs

using System.Text.Json;
using FluentValidation.Results;

namespace TODO.LanguageExtensions;

/// <summary>
/// Provides extension methods for FluentValidation to simplify validation and error handling.
/// </summary>
public static class FluentValidationExtensions
{

    /// <summary>
    /// Converts the validation failures in the <see cref="ValidationResult"/> into a dictionary
    /// where the key is the property name and the value is a list of error messages associated with that property.
    /// </summary>
    /// <param name="result">The <see cref="ValidationResult"/> containing validation failures. Cannot be <see langword="null"/>.</param>
    /// <returns>A dictionary where each key is a property name and the value is a list of error messages for that property.</returns>
    /// <exception cref="ArgumentNullException">Thrown if <paramref name="result"/> is <see langword="null"/>.</exception>
    public static IDictionary<string, List<string>> ToErrorDictionary(this ValidationResult result) =>
        result is null
            ? throw new ArgumentNullException(nameof(result))
            : result.Errors
                .GroupBy(f => f.PropertyName)
                .ToDictionary(
                    g => g.Key,
                    g => g.Select(f => f.ErrorMessage).ToList());

    private static JsonSerializerOptions JsonSerializerOptions => new() { WriteIndented = true };  
    
    /// <summary>
    /// Converts the validation failures in the <see cref="ValidationResult"/> into a JSON string.
    /// </summary>
    /// <param name="result">The <see cref="ValidationResult"/> containing validation failures. Cannot be <see langword="null"/>.</param>
    /// <returns>A JSON string representing the validation failures, where each property name is associated with its list of error messages.</returns>
    /// <exception cref="ArgumentNullException">Thrown if <paramref name="result"/> is <see langword="null"/>.</exception>
    public static string ToJson(this ValidationResult result) =>
        result is null
            ? "{}"
            : JsonSerializer.Serialize(result.ToErrorDictionary(), JsonSerializerOptions);

}

Log.txt

[2025-10-03 14:51:42.252 [Information] Validation failure on page "Index" "{
  \"DateOfBirth\": [
    \"\u0027Date Of Birth\u0027 is required.\"
  ],
  \"FirstName\": [
    \"First Name is required...\"
  ],
  \"LastName\": [
    \"Last Name is required...\"
  ],
  \"Gender\": [
    \"Please select Male or Female.\"
  ]
}"

PageHelpers.cs

/// <summary>
/// Provides helper methods for working with pages in an ASP.NET Core application.
/// </summary>
/// <remarks>
/// This class contains utility methods that assist in handling page-related operations, 
/// such as retrieving the current page name from an HTTP request.
/// </remarks>
public class PageHelpers
{
    /// <summary>
    /// Retrieves the name of the current page based on the provided HTTP request.
    /// </summary>
    /// <param name="request">The <see cref="HttpRequest"/> object representing the current HTTP request. Cannot be <see langword="null"/>.</param>
    /// <returns>
    /// A <see cref="string"/> representing the name of the current page. 
    /// Returns "Index" if the request path is the root ("/"), otherwise returns the file name without its extension.
    /// </returns>
    /// <exception cref="ArgumentNullException">Thrown if <paramref name="request"/> is <see langword="null"/>.</exception>
    public static string GetCurrentPageName(HttpRequest request)
    {
        string path = request.Path;
        return path == "/" ? "Index" : Path.GetFileNameWithoutExtension(path);
    }
}