C# LINQ

Sorting:

• OrderBy
• ThenBy
• OrderByDescending
• ThenByDescending
• Reverse

 public IEnumerable<Customer> SortByName(List<Customer> cumtomerList)
 {
     return customerList.OrderBy(c => c.LastName)
         .ThenBy(c => c.FirstName);
 }

OrderBy a property that can have null value. null value will be sorted at the first.

Creating:

• Range
• Random　
• Repeat

 public IEnumerable<int> BuildIntegerSequence()
 {
     var integers = Enumerable.Range(,)         //0,1,2,3,4,5,6,7,8,9
                     .Select(i => +(*i));        //5, 15 ...

     return integers;
 }

 public IEnumerable<string> BuildStringSequence()
 {

     var strings = Enumerable.Range(,)
                     .Select(i => ((char)('A'+i)).ToString());  //A,B,C,D,E,F,G,H,I

     Random rand = new Random();
     var randomStrings = Enumerable.Range(,)
                     .Select(i => ((char)('A'+rand.Next(,))).ToString());

     return strings;
 }

Comparing/Combining:

• Intersect
• Except
• Concat
• Distinct
• Union

 public IEnumerable<int> CompareSequence()
 {
     var seq1 = Enumerable.Range(,);
     var seq2 = Enumerable.Range(, )
                 .Select(i => i*i);

     //return seq1.Intersect(seq2); //0,1,4,9
     //return seq1.Except(seq2); //2,3,5,6,7,8
     //return seq1.Concat(seq2); //0,1,2,3,4,5,6,7,8,9,0,1,4,9,16,25,36,49,64,81
     //return seq1.Concat(seq2).Distinct(); //Delete duplicates
     return seq1.Union(seq2); //Also delete duplicates
 }

Projection:

• Select

 public dynamic GetNamesAndEmail(List<Customer> customerList)
 {
     var query = customerList.Select(c => new      //Anonymous Types
                     {
                         Name = c.LastName + "," + c.FirstName,
                         c.EmailAddress
                     });
     return query;
 }

• SelectMany

 public IEnumerable<Customer> GetOverdueCustomers(List<Customer> customerList)
 {
     var query = customerList
                 .Select(c => c.InvoiceList                                             //One customer has a sequence of invoices
                                 .Where(i => (i.IsPaid ?? false) == false));            //return IEnumeralbe<IEnumeralbe<Inovice>> type

     var query2 = customerList
                 .SelectMany(c => c.InvoiceList                                         //SelectMany flatten resluting sequences into one sequence
                                 .Where(i => (i.IsPaid ?? false) == false),
                                 (c, i) => c);                                        //return IEnumerable<Customer> type

 }

Totaling:

• Sum

Grouping and Summing:

• GroupBy

public dynamic GetInvoiceTotalPaid(List<Invoice> invoiceList)
{
    var query = invoiceList.GroupBy( inv => inv.IsPaid ?? false,       //null as false
                                    inv => inv.TotalAmount,
                                    (groupKey, invTotals) => new    //returned results of GroupBy
                                    {
                                        Key = groupKey,
                                        InvoiceAmount = invTotals.Sum()
                                    });
    foreach(var item in query)
    {
        Console.WriteLine(item.Key + ": " + item.InvoiceAmount);
    }

    return query;
}