在fluent-nhibernate映射中将字符串转换为double

public class StringToDoubleConverter : IUserType
{
    public new bool Equals(object x, object y)
    {
        return object.Equals(x, y);
    }

    public int GetHashCode(object x)
    {
        return x.GetHashCode();
    }

    public object NullSafeGet(IDataReader rs, string[] names, object owner)
    {
        object obj = NHibernateUtil.String.NullSafeGet(rs, names[0]);
        if (obj == null)
        {
            return null;
        }
        return Convert.ToDouble(obj.ToString(), CultureInfo.InvariantCulture);
    }

    public void NullSafeSet(IDbCommand cmd, object value, int index)
    {
        if (value == null)
        {
            ((IDataParameter)cmd.Parameters[index]).Value = DBNull.Value;
        }
        else
        {
            ((IDataParameter)cmd.Parameters[index]).Value = value.ToString();
        }
    }

    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
    {
        get
        {
            return new[] { new SqlType(DbType.String) };
        }
    }

    public Type ReturnedType
    {
        get
        {
            return typeof(double);
        }
    }

    public bool IsMutable
    {
        get
        {
            return false;
        }
    }
}

public class MyEntityMap : ClassMap<MyEntity>
{
    public MyEntityMap()
    {
        Table("MyEntity");
        Id(x => x.Id).GeneratedBy.Identity();
        Map(x => x.MyDoubleProperty).CustomType<StringToDoubleConverter>().Not.Nullable();
    }
}