c# - Converting foreach into single pass Linq Group By -

i have collection of items may have 1 or more properties.

i want use linq statement (query syntax) filter , group list in single pass such checks mandatory property first, , looks optional property.

i can't figure out how without first filtering list , going on again optional property.

here's how in foreach statement. (not efficient, illustrating need.)

var usefulboxes = new dictionary<box, int>;  foreach (box in cart) {   bool boxneeded = false;   int somecounter = 0;    foreach (prop in box.properties)   {     if (prop == neededprop)       boxneeded = true;     else if (boxneeded && prop == optionalprop)       somecounter += 1;   }    if (boxneeded)     usefulboxes.add(box, somecounter) } 

  var usefulboxes= box.where(b=>b.boxproperties.prop==neededprop).tolist();