C# LINQ

Sorting: 

• OrderBy
• ThenBy
• OrderByDescending
• ThenByDescending
• Reverse

1 public IEnumerable<Customer> SortByName(List<Customer> cumtomerList)
2 {
3     return customerList.OrderBy(c => c.LastName)
4         .ThenBy(c => c.FirstName);
5 }

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

 

Creating:

• Range
• Random　
• Repeat

 1 public IEnumerable<int> BuildIntegerSequence()
 2 {
 3     var integers = Enumerable.Range(0,10)         //0,1,2,3,4,5,6,7,8,9
 4                     .Select(i => 5+(10*i));        //5, 15 ...
 5 
 6     return integers;
 7 }
 8 
 9 public IEnumerable<string> BuildStringSequence()
10 {
11 
12     var strings = Enumerable.Range(0,10)
13                     .Select(i => ((char)('A'+i)).ToString());  //A,B,C,D,E,F,G,H,I
14 
15     Random rand = new Random();
16     var randomStrings = Enumerable.Range(0,10)    
17                     .Select(i => ((char)('A'+rand.Next(0,26))).ToString());
18 
19     return strings;                
20 }

 

Comparing/Combining:

• Intersect
• Except
• Concat
• Distinct
• Union

 1 public IEnumerable<int> CompareSequence()
 2 {
 3     var seq1 = Enumerable.Range(0,10);
 4     var seq2 = Enumerable.Range(0, 10)
 5                 .Select(i => i*i);
 6 
 7     //return seq1.Intersect(seq2); //0,1,4,9
 8     //return seq1.Except(seq2); //2,3,5,6,7,8
 9     //return seq1.Concat(seq2); //0,1,2,3,4,5,6,7,8,9,0,1,4,9,16,25,36,49,64,81
10     //return seq1.Concat(seq2).Distinct(); //Delete duplicates
11     return seq1.Union(seq2); //Also delete duplicates
12 }

 

Projection:

• Select

1 public dynamic GetNamesAndEmail(List<Customer> customerList)
2 {
3     var query = customerList.Select(c => new      //Anonymous Types
4                     {
5                         Name = c.LastName + "," + c.FirstName,
6                         c.EmailAddress
7                     });
8     return query;
9 }

• SelectMany

 1 public IEnumerable<Customer> GetOverdueCustomers(List<Customer> customerList)
 2 {
 3     var query = customerList
 4                 .Select(c => c.InvoiceList                                             //One customer has a sequence of invoices
 5                                 .Where(i => (i.IsPaid ?? false) == false));            //return IEnumeralbe<IEnumeralbe<Inovice>> type
 6 
 7     
 8     var query2 = customerList
 9                 .SelectMany(c => c.InvoiceList                                         //SelectMany flatten resluting sequences into one sequence    
10                                 .Where(i => (i.IsPaid ?? false) == false),
11                                 (c, i) => c);                                        //return IEnumerable<Customer> type
12 
13 }

 

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;
}