LINQ GroupBy with a dynamic group of columns

If you need this to be truly dynamic, use Scott Gu's Dynamic LINQ library.

You just need to figure out what columns to include in your result and group by them.

public static IQueryable GroupByColumns(this IQueryable source,
    bool includeVariety = false,
    bool includeCategory = false)
{
    var columns = new List<string>();
    if (includeVariety) columns.Add("Variety");
    if (includeCategory) columns.Add("Category");
    return source.GroupBy($"new({String.Join(",", columns)})", "it");
}

Then you could just group them.

var query = rows.GroupByColumns(includeVariety: true, includeCategory: true);

var results=items
  .Select(i=>
    new {
      variety=includevariety?t.variety:null,
      category=includecategory?t.category:null,
      ...
    })
  .GroupBy(g=>
    new { variety, category, ... }, g=>g.quantity)
  .Select(i=>new {
    variety=i.Key.variety,
    category=i.Key.category,
    ...
    quantity=i.Sum()
  });

shortened:

var results=items
  .GroupBy(g=>
    new {
      variety=includevariety?t.variety:null,
      category=includecategory?t.category:null,
      ... 
    }, g=>g.quantity)
  .Select(i=>new {
    variety=i.Key.variety,
    category=i.Key.category,
    ...
    quantity=i.Sum()
  });