[Solved]-Grouping using multiple columns, then summing a specific column using method syntax-LINQ,C#

for the count you just need this query:

int total = pieces
    .where(x => x.count > 0)
    .groupby(x => new { x.count, x.date, x.description })
    .sum(g => g.key.count);

so you can access all key properties of the grouping.

this returns 85 for your initial sample and 108 if you add the new piece.