List<PartRequest> partRequests = new List<PartRequest>();

    partRequests.Add(new PartRequest()

    {

        factory = "f1",

        stock = "s1",

        material = "m1",

        quantity = 10

    });

    partRequests.Add(new PartRequest()

    {

        factory = "f1",

        stock = "s1",

        material = "m1",

        quantity = 20

    });

    partRequests.Add(new PartRequest()

    {

        factory = "f1",

        stock = "s2",

        material = "m1",

        quantity = 40

    });

    partRequests.Add(new PartRequest()

    {

        factory = "f2",

        stock = "s1",

        material = "m1",

        quantity = 10

    });

    partRequests.Add(new PartRequest()

    {

        factory = "f1",

        stock = "s1",

        material = "m2",

        quantity = 80

    });

    partRequests.Add(new PartRequest()

    {

        factory = "f2",

        stock = "s2",

        material = "m2",

        quantity = 120

    });

    partRequests.Add(new PartRequest()

    {

        factory = "f2",

        stock = "s2",

        material = "m2",

        quantity = 120

    });

    var materialGroup = partRequests.GroupBy(x => new { x.material }).Select(x =>   x.Key.material).ToList();

     var groupedResult = partRequests.GroupBy(x=>string.Join(x.factory,":",x.stock,":",x.material)).ToList();

    var groupByStckAndFactoryResult = partRequests.GroupBy(x => new { factory = x.factory, stock = x.stock})

        .Select(y => new { factory = y.Key.factory, stock = y.Key.stock, material = y.First().material }).ToList();

    var groupByStckAndFactoryAndMaterialResult = partRequests.GroupBy(x => new { factory = x.factory, stock = x.stock, material = x.material })

        .Select(y => new { factory = y.Key.factory, stock = y.Key.stock, material = y.Key.material, quantity = y.Sum(i => i.quantity) }).ToList();

    Console.WriteLine(materialGroup);