Get ASP.NET ModelState errors (all errors or by a specific field)

ModeStateExtensions.cs

using Microsoft.AspNetCore.Http;
using Microsoft.AspNetCore.Mvc.ModelBinding;
using System;
using System.Collections.Generic;
using System.Linq;

public static class ModelStateExtensions
{
    /// <summary>
    /// Extracts error keys and error messages of all model fields
    /// </summary>
    /// <param name="modelState">ASP.NET ModelState dictionary</param>
    /// <returns>A list of key-value pairs</returns>
    public static List<KeyValuePair<string, string>> GetModelStateErrors(this ModelStateDictionary modelState)
    {
        var errors = new List<KeyValuePair<string, string>>();

        foreach (var item in modelState.Where(i => i.Value.Errors.Any()))
        {
            var key = item.Key;
            var values = item.Value.Errors;

            foreach (var val in values)
            {
                errors.Add(new KeyValuePair<string, string>(key, val.ErrorMessage));
            }
        }

        return errors;
    }

    /// <summary>
    /// Extracts error keys and error messages of a particular field in the model
    /// </summary>
    /// <param name="modelState">ASP.NET ModelState dictionary</param>
    /// <param name="fieldName">Name of the model field for which errors should be extracted</param>
    /// <returns>A list of key-value pairs</returns>
    public static List<KeyValuePair<string, string>> GetModelStateErrorsForField(this ModelStateDictionary modelState, string fieldName)
    {
        var errors = new List<KeyValuePair<string, string>>();

        foreach (var item in modelState.Where(i => i.Key.Equals(fieldName, StringComparison.OrdinalIgnoreCase) && i.Value.Errors.Any()))
        {
            var key = item.Key;
            var values = item.Value.Errors;

            foreach (var val in values)
            {
                errors.Add(new KeyValuePair<string, string>(key, val.ErrorMessage));
            }
        }

        return errors;
    }
}