Question

I have a Product class:

class Product
{
    public string Name { get; set; }
    public DateTime ProductionDate { get; set; }
    public int CategoryId { get; set; }
}

If I have a List<Product> and I want to GroupBy by ProductionDate and CategoryId. I do:

List<Product> products = GetAllProducts();

var groupedProducts = products.Groupby(p => new { p.ProductionDate, p.CategoryId })
                              .Select(pgroup => new {
                                                  prodDate = pgroup.Key.ProductionDate.ToString(),
                                                  categoryId = pgroup.Key.CategoryId.ToString(),
                                                  amount = pgroup.Count() 
                                                });

For each categoryId I would like to have how many products have been produced in a specific prodDate. So I create a Dictionary object where for each categoryId key, I store the prodDate and the amount. The object is a Dictionary<string, List<Data>> with

class Data
{
     public string xValue { get; set; }
     public string yValue { get; set; }
}

To do so I tried:

Dictionary<string, List<Data>> productsPerPeriodPerCategory = new Dictionary<string, List<Data>>();
productsPerPeriodPerCategory = groupedProducts
                               .ToDictionary(p => p.categoryId, 
                                             p => new List<Data>().AddRange(
                                                  groupedProducts.Where(g => g.categoryId == p.categoryId)
                                                                 .Select(x => new Data()
                                                                    {
                                                                       xValue = x.prodDate,
                                                                       yValue = x.amount.ToString()
                                                                    }).ToList()));

But it gives me the following error:

Cannot convert lambda expression to type 'System.Collections.Generic.IEqualityComparer<string>' because it is not a delegate type

Answer 1

Try this:

productsPerPeriodPerCategory =
        groupedProducts
            .GroupBy(p => p.categoryId)
            .ToDictionary(
                g => g.Key,
                g =>
                g.Select(
                    r =>
                    new Data {xValue = r.prodDate, yValue = r.amount.ToString()}));