NHibernate, Fluent.NHibernate and PostgreSql: How to map string property into PostgreSql json type using Fluent.NHibernate and IUserType

SomeClassMappingOverrides.cs

public class SomeClassMappingOverrides : IAutoMappingOverride<SomeClass>
{
    public void Override(AutoMapping<SygicPoi> mapping)
    {
        mapping.Map(x => x.Json).CustomSqlType("json").CustomType<StringAsJson>(); // Json property type is string
    }
}

StringAsJson.cs

// inspired by
// https://gist.github.com/bariloce/e65fe5db6c6ddf46e6f8
// https://web.archive.org/web/20150214164507/http://blog.miraclespain.com/archive/2008/Mar-18.html
// https://nhibernate.info/blog/2009/10/15/mapping-different-types-iusertype.html
public class StringAsJson : IUserType
{
    bool IUserType.Equals(object x, object y)
    {
        return string.Equals((string)x, (string)y);
    }

    public int GetHashCode(object x)
    {
        return x == null ? 0 : x.GetHashCode();
    }

    public object NullSafeGet(DbDataReader rs, string[] names, ISessionImplementor session, object owner)
    {
        var value = rs[names[0]];
        return value != DBNull.Value
            ? value
            : null;
    }

    public void NullSafeSet(DbCommand cmd, object value, int index, ISessionImplementor session)
    {
        var parameter = (NpgsqlParameter)cmd.Parameters[index];
        parameter.NpgsqlDbType = NpgsqlDbType.Json;
        parameter.Value = value ?? DBNull.Value;
    }

    public object DeepCopy(object value)
    {
        return value;
    }

    public object Replace(object original, object target, object owner)
    {
        return original;
    }

    public object Assemble(object cached, object owner)
    {
        return cached;
    }

    public object Disassemble(object value)
    {
        return value;
    }

    public SqlType[] SqlTypes => new[] { new SqlType(DbType.String) };

    public Type ReturnedType => typeof(string);

    public bool IsMutable => false;
}

Thanks:
https://gist.github.com/bariloce/e65fe5db6c6ddf46e6f8
https://web.archive.org/web/20150214164507/http://blog.miraclespain.com/archive/2008/Mar-18.html
https://nhibernate.info/blog/2009/10/15/mapping-different-types-iusertype.html