<property name="ObjectProperty" column="variant_column" type="MyNamespace.SqlVariant, MyAssembly" />

public class SqlVariant : IUserType
{

    private SqlType[] variantSqlType = new SqlType[1] { new SqlType(DbType.Object) };

    public SqlType[] SqlTypes
    {
        get
        {
            return variantSqlType;
        }
    }

    public System.Type ReturnedType
    {
        get { return typeof(object); }
    }

    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)
    {
        int ordinal = rs.GetOrdinal(names[0]);
        if (rs.IsDBNull(ordinal))
        {
            return null;
        }
        else
        {
            return rs[ordinal];
        }
    }

    public void NullSafeSet(IDbCommand cmd, object value, int index)
    {

        object valueToSet = (value == null) ? DBNull.Value : value;
        ((IDbDataParameter) cmd.Parameters[index]).Value = valueToSet;
    }

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

    public bool IsMutable
    {
        get { return false; }
    }

    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;
    }

}