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(0,10)         //0,1,2,3,4,5,6,7,8,9
                    .Select(i => 5+(10*i));        //5, 15 ...

    return integers;
}

public IEnumerable<string> BuildStringSequence()
{

    var strings = Enumerable.Range(0,10)
                    .Select(i => ((char)('A'+i)).ToString());  //A,B,C,D,E,F,G,H,I

    Random rand = new Random();
    var randomStrings = Enumerable.Range(0,10)    
                    .Select(i => ((char)('A'+rand.Next(0,26))).ToString());

    return strings;                
}

Comparing/Combining:

• Intersect
• Except
• Concat
• Distinct
• Union

public IEnumerable<int> CompareSequence()
{
    var seq1 = Enumerable.Range(0,10);
    var seq2 = Enumerable.Range(0, 10)
                .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;
}