TableValueParameter Ignores any Custom TypeHandlers

I have some custom classes that hides some logic but is basically only a simple .Net Type. Think Email, there you can create a EMail class, that validates the string so it's a valid email address, and it could extract the domain from the email address. But in the end it's still only a string that is stored down to the database.

By using a custom TypeHandler, it's easy to store and retrieve this Email class to a single column in the database. The issue is if I use the TableValueParameter function my Custom TypeHandler is "ignored".
I can create a DataTable with a DataColumn with the Type of my EMail class, but as soon as I try to execute my SQL statement with the SqlMapper.ICustomQueryParameter it fails since dapper can't convert the EMail class into a DbType. And since I already have a CustomTypeHandler specified for my EMail class it would be nice if it was extended to work with TableValueParameters to.

This is how my CustomTypeHandler looks today.

`    public class FlightNumberTypeHandler : SqlMapper.TypeHandler<FlightNumber>
    {
        public override void SetValue(IDbDataParameter parameter, FlightNumber value)
        {
            parameter.DbType = DbType.String;
            parameter.Value = value.ToString();
        }

        public override FlightNumber Parse(object value)
        {
            return FlightNumber.Create(value.ToString());
        }
    }`

By adding 2 abstract properties, one for the Value and one for the DbType it could be changed to look like this instead.

    public class FlightNumberTypeHandler : SqlMapper.TypeHandler<FlightNumber>
    {
        public override DbType DbType => DbType.String;
        public override object GetValue(FlightNumber item)
        {
            return item.ToString();
        }

        public override FlightNumber Parse(object value)
        {
            return FlightNumber.Create(value.ToString());
        }
    }

And the public void SetValue(IDbDataParameter parameter, FlightNumber value) method in the base class could use the newley abstract method/property to set the parameter, so there isn't any need to override that method if these are the only 2 things set in the method. And the logic that converts the DataTable to a TableValueParameter could also use the Custom TypeHandler to convert the FlightNumber type to a String.

Best Regards
Magnus

Start by tracing TableValueParameter conversion and how SqlMapper.TypeHandler currently sets values and DbType. Check the surrounding parameter-mapping code and existing tests, if present, to determine where custom handlers are bypassed. Done means a custom handler converts the value and database type for table-valued parameters without breaking existing handlers.