ModelBuilderExtension.cs

using ConverterExample.Converter;
using ConverterExample.Attribute;
using System.Linq;
using System.Reflection;
using Microsoft.EntityFrameworkCore;
using Microsoft.EntityFrameworkCore.Metadata;
using System;

namespace ConverterExample.Extension
{
    public static class ModelBuilderExtension
    {
        public static void UseAlwaysUpper(this ModelBuilder modelBuilder)
        {
            var upperConverter = new UpperConverter();
            foreach (IMutableEntityType entityType in modelBuilder.Model.GetEntityTypes())
            {
                foreach (IMutableProperty property in entityType.GetProperties())
                {
                    if(property.ClrType == typeof(string))
                    {
                        object[] attributes = property.PropertyInfo.GetCustomAttributes(typeof(AlwaysUpperAttribute), false);
                        if(attributes.Any())
                            property.SetValueConverter(upperConverter);
                    }
                }
            }

        }
    }
}