如何在规范中生成动态数量的ThenBy子句

Question

I'm building a Spec Evaluator which must consider multiple possible OrderBy, as in the next example:

if (spec.Order != null)
{
    var count = spec.Order.Count;

    if (count == 1)
    {
        query = query.OrderBy(spec.Order[0]);
    }
    else if (count == 2)
    {
        query = query.OrderBy(spec.Order[0])
            .ThenBy(spec.Order[1]);
    }
    else if (count == 3)
    {
        query = query.OrderBy(spec.Order[0])
            .ThenBy(spec.Order[1])
            .ThenBy(spec.Order[2]);
    }
    // And so on...
}

Query is an IQueryable , spec.Order is a list of clauses: List<Expression<Func<T, object>>> .

I know that I can use an OrderBy with all the clauses passed as string. And I guess I can just project all the Order clauses to a new string comma-separated. But that solution doesn't seem clean.

Is there any other way to dynamically generate one new ThenBy for every item of the Order list, above 1?

Answer 1

You could use a for loop. Basically loop through all of the Order values, use OrderBy for the first one, and ThenBy for subsequent items. Since you've said that you're using IQueryable , I've modified this to use a temporary IOrderedQueryable<T> variable.

if (spec.Order != null)
{
    var count = spec.Order.Count;

    IOrderedQueryable<T> orderedQuery = null;
    for (int i = 0; i < count; ++i)
    {
        if (i == 0)
        {
            orderedQuery = query.OrderBy(spec.Order[i]);
        }
        else
        {
            orderedQuery = orderedQuery.ThenBy(spec.Order[i]);
        }
    }
    query = orderedQuery ?? query;
}

You could also approach it like this, although I'm not sure how the performance differs between the two methods, if it does at all:

if (spec.Order != null)
{
    var count = spec.Order.Count;

    for (int i = 0; i < count; ++i)
    {
        if (query is IOrderedQueryable<T> orderedQuery)
        {
            query = orderedQuery.ThenBy(spec.Order[i]);
        }
        else
        {
            query = query.OrderBy(spec.Order[i]);
        }
    }
}