gistfile1.cs

public static IQueryOver<TResult, TResult> ApplyFilter<TFilter, TResult>(
    this IQueryOver<TResult, TResult> query,
    TFilter field,
    Func<TFilter, Expression<Func<TResult, bool>>> predicate)
{
    field.ToMaybe().Do(x => query.Where(predicate(x)));
    return query;
}

public static IQueryOver<TResult, TResult> ApplyFilter<TField, TResult>(
    this IQueryOver<TResult, TResult> query,
    TField? field,
    Func<TField, Expression<Func<TResult, bool>>> predicate)
    where TField : struct
{
    field.ToMaybe().Do(x => query.Where(predicate(x)));
    return query;
}