Conditional "orderby" sort order in LINQ

If you build the expression incrementally you can do this. Generally easier using expressions rather than comprehension expressions:

var x = widgets.Where(w => w.Name.Contains("xyz"));
if (flag) {
  x = x.OrderBy(w => w.property);
} else {
  x = x.OrderByDescending(w => w.property);
}

(Assuming the Widget's property is basis of sort since you don't list one.)

...Or do it all in one statement

bool flag;

var result = from w in widgets where w.Name.Contains("xyz")
  orderby
    flag ? w.Id : 0,
    flag ? 0 : w.Id descending
  select w;

Here is a more general solution, that can be used for various conditional lambda expressions without breaking the flow of the expression.

public static IEnumerable<T> IfThenElse<T>(
    this IEnumerable<T> elements,
    Func<bool> condition,
    Func<IEnumerable<T>, IEnumerable<T>> thenPath,
    Func<IEnumerable<T>, IEnumerable<T>> elsePath)
{
    return condition()
        ? thenPath(elements)
        : elsePath(elements);
}

e.g.

var result = widgets
    .Where(w => w.Name.Contains("xyz"))
    .IfThenElse(
        () => flag,
        e => e.OrderBy(w => w.Id),
        e => e.OrderByDescending(w => w.Id));