how to allow the user to choose how to sort his list using a variable?

Question

the information that the user enters can be sorted in many ways ascending and descending and i\'m trying to make the user choose how he want to see the data:

so is there

Answer 1

If the User in your question is some software that calls your function, then the user is aware of the type of the objects in the el sequence: he knows which properties the elements in the el sequence are, and he knows how he wants them ordered.

Why not let this user give you the keySelector to use in the OrderBy?

Creating an extension function:

static class QueryableExtensions
{
    IQueryable SelectAndSort(this IQueryable> selector, 
        Expression> sortKeySelector,
        System.ComponentModel.ListSortDirection sortDirection)
    {
        var selectBeforeOrdering = myRepository.el.Select(selector);
        // note: this is still an IQueryable, only the query has been made,
        // the database is not accessed yet!

        IQueryable result = (sortDirection == ListSortDirectrion.Ascending) ?
            // sort in ascending order:
            selectBeforeOrdering.OrderBy(sortKeySelector) :
            // else: selec in descending order:
            selectBeforeOrdering.OrderByDescending(sortKeySelector);

        return result;
    }
}

Usage: suppose your user knows your el, and he want several fields of it, ordered by one of the properties of your el

using (var myDbContext = new DbContext(...))
{
    IQueryable myEls = myDbContext.el;

    var mySelectedItems = myEls.SelectAndSore(
        // selector: what properties do I as a user want?
        el => new
        {
             Id = el.Id,
             FullName = el.FullName,
             ... // other desired properties
        },
        // sortSelector: how do I want my selected items to be sorted?
        selectedItem => selectedItem.FullName,
        // direction:
        ListSortDirection.Descending);

        // note: until now the database has not been accessed
        // only the Expression of the query has been created.

        // depending on what you want as a result: ToList / ToArray / First / ...
        return mySelectedItems.ToList();
    }

If, on the other hand your user is not software, but an operator, who has to select by which column he wants his items to be ordered, he has to have a method to tell the computer which column should be sorted.

This is usually done by clicking the column. Another method could be by selecting a value in a combo box. Anyway, you'll have to attach something to the column or to the combobox values that holds the sortKeySelector:

class MySortableColumn : DataGridViewColumn 
   // or whatever column class you are using
{
    public Expression> SortKeySelector{get;set;}
}

Now you have several columns that each display one of the properties of MyDisplayedItem:

var columnId = new MySortableColumn()
{
    SortKeySelector = displayedItem => myDisplayedItem.Id,
};
var columnFullName = new MyStortableColumn()
{
    SortKeySelector = displayedItem => myDisplayedItem.Id,
}
// etc.

Whenever the operator clicks on a column, the sortKey is extracted and used as parameter to sort the items:

void SortColumn(SortableColumn column)
{
     var sortedItems = Sort(column.SortKeySelector, ListSortOrder...);
     Display(sortedItems);
}