MustBeTrueAttribute.cs

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

[AttributeUsage(AttributeTargets.Property, AllowMultiple = false, Inherited = false)]
public sealed class MustBeTrueAttribute : ValidationAttribute, IClientModelValidator  //, IModelValidator
{

     string _PropertyNameAttributeIsAppliedTo;
    public MustBeTrueAttribute([CallerMemberName]string propertyName = null) // gets the property name
    {
        _PropertyNameAttributeIsAppliedTo = propertyName;
    }

    public override bool IsValid(object value)
    {
        var isvalid = (bool)value;
        return isvalid;
    }

    public void AddValidation(ClientModelValidationContext context)
    {
        var loc = (IStringLocalizer<SharedResource>)context.ActionContext.HttpContext.RequestServices.GetService(typeof(IStringLocalizer<SharedResource>));
        MergeAttribute(context.Attributes, "data-val", "true");
        //var errorMessage = FormatErrorMessage(loc[this.ErrorMessage].Value);
        var errorMessage = loc[this.ErrorMessage].Value;
        MergeAttribute(context.Attributes, "data-val-mustbetrue", errorMessage);
    }

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