CurrencyMustMeetCultureAttribute.cs

using System;
using System.Collections.Generic;
using System.ComponentModel.DataAnnotations;
using System.Globalization;
using System.Linq;
using System.Text;
using System.Text.RegularExpressions;
using System.Threading.Tasks;
using Microsoft.AspNetCore.Mvc.ModelBinding.Validation;
using Microsoft.Extensions.Localization;

[AttributeUsage(AttributeTargets.Property, AllowMultiple = false, Inherited = false)]
public sealed class CurrencyMustMeetCultureAttribute : ValidationAttribute, IClientModelValidator
{
    public override bool IsValid(object value)
    {
        var cc = CultureInfo.CurrentCulture;
        //var thSep = cc.NumberFormat.CurrencyGroupSeparator;
        var decSep = cc.NumberFormat.CurrencyDecimalSeparator;      

        var separator = decSep.Equals(".") ? @"\." : decSep; // have it's a dot we have to escape it, otherwise it gets a diff meaning in regex

        string pattern = @"^\d{1,}" + separator + @"?\d{0,2}$";
        var isMatch = Regex.IsMatch(value.ToString(), pattern);
        return isMatch;
    }

    /// <summary>
    /// This exposes the single method AddValidation to create all 
    /// Html5 data - attributes needed by the client validation rule
    /// </summary>
    /// <param name="context"></param>
    public void AddValidation(ClientModelValidationContext context)
    {
        var rs = context.ActionContext.HttpContext.RequestServices;
        var loc = (IStringLocalizer<SharedResource>)rs.GetService(typeof(IStringLocalizer<SharedResource>));

        MergeAttribute(context.Attributes, "data-val", "true");

        var errorMessage = loc[this.ErrorMessage].Value;
        MergeAttribute(context.Attributes, "data-val-currencymustmeetculture", errorMessage);
    }

    private bool MergeAttribute(IDictionary<string, string> attributes, string key, string value)
    {
        if (attributes.ContainsKey(key))
        {
            return false;
        }
        attributes.Add(key, value);
        return true;
    }
}