cs11dotnet7/vscode/Chapter11/LinqWithObjects/Program.cs

// a string array is a sequence that implements IEnumerable<string>
string[] names = new[] { "Michael", "Pam", "Jim", "Dwight",
  "Angela", "Kevin", "Toby", "Creed" };

SectionTitle("Deferred execution");

// Question: Which names end with an M?
// (written using a LINQ extension method)
var query1 = names.Where(name => name.EndsWith("m"));

// Question: Which names end with an M?
// (written using LINQ query comprehension syntax)
var query2 = from name in names where name.EndsWith("m") select name;

// Answer returned as an array of strings containing Pam and Jim
string[] result1 = query1.ToArray();

// Answer returned as a list of strings containing Pam and Jim
List<string> result2 = query2.ToList();

// Answer returned as we enumerate over the results
foreach (string name in query1)
{
  WriteLine(name);
}

SectionTitle("Writing queries");

// Where requires a Func<T, T> delegate instance...
// var query = names.Where(
//   new Func<string, bool>(NameLongerThanFour));

// ...but the compiler can create it for us...
// var query = names.Where(NameLongerThanFour);

// ...or we can use a lambda expression.
IOrderedEnumerable<string> query = names
  .Where(name => name.Length > 4)
  .OrderBy(name => name.Length)
  .ThenBy(name => name);

foreach (string item in query)
{
  WriteLine(item);
}

SectionTitle("Filtering by type");

List<Exception> exceptions = new()
{
  new ArgumentException(),
  new SystemException(),
  new IndexOutOfRangeException(),
  new InvalidOperationException(),
  new NullReferenceException(),
  new InvalidCastException(),
  new OverflowException(),
  new DivideByZeroException(),
  new ApplicationException()
};

IEnumerable<ArithmeticException> arithmeticExceptionsQuery =
  exceptions.OfType<ArithmeticException>();

foreach (ArithmeticException exception in arithmeticExceptionsQuery)
{
  WriteLine(exception);
}

string[] cohort1 = new[]
  { "Rachel", "Gareth", "Jonathan", "George" };

string[] cohort2 = new[]
  { "Jack", "Stephen", "Daniel", "Jack", "Jared" };

string[] cohort3 = new[]
  { "Declan", "Jack", "Jack", "Jasmine", "Conor" };

SectionTitle("The cohorts");

Output(cohort1, "Cohort 1");
Output(cohort2, "Cohort 2");
Output(cohort3, "Cohort 3");

SectionTitle("Set operations");

Output(cohort2.Distinct(), "cohort2.Distinct()");
Output(cohort2.DistinctBy(name => name.Substring(0, 2)),
  "cohort2.DistinctBy(name => name.Substring(0, 2)):");
Output(cohort2.Union(cohort3), "cohort2.Union(cohort3)");
Output(cohort2.Concat(cohort3), "cohort2.Concat(cohort3)");
Output(cohort2.Intersect(cohort3), "cohort2.Intersect(cohort3)");
Output(cohort2.Except(cohort3), "cohort2.Except(cohort3)");
Output(cohort1.Zip(cohort2, (c1, c2) => $"{c1} matched with {c2}"),
  "cohort1.Zip(cohort2)");