重视Linq技术_6

Filtering

//1、Simple Local Filter

string[] names = { "Tom", "Dick", "Harry", "Mary", "Jay" };
IEnumerable<string> query =
    names.Where (name => name.EndsWith ("y"));
query.Dump ("In lambda syntax");

query =
    from n in names
    where n.EndsWith ("y")
    select n;
query.Dump ("In query comprehension syntax");

//2、Multiple Where Clauses

var names = new[] { "Tom", "Dick", "Harry", "Mary", "Jay" }.AsQueryable();
var query =
    from n in names
    where n.Length > 3
    let u = n.ToUpper()
    where u.EndsWith ("Y")
    select u;
query.Dump();

//3、Indexed Filtering

string[] names = { "Tom", "Dick", "Harry", "Mary", "Jay" };
names.Where ((n, i) => i % 2 == 0).Dump ("Skipping every second element");

//4、Contains and LIKE with LINQ to SQL

Customers.Where (c => c.Name.Contains ("a")) 
    .Dump ("Notice the SQL translation uses LIKE");

Customers.Where (c => c.Name.StartsWith ("J")) 
    .Dump ("StartsWith and EndsWith also translate to LIKE");

Customers.Where (c => SqlMethods.Like (c.Name, "_ar%y"))
    .Dump ("A more complex use of LIKE");
// The following string functions all have translations to SQL:
// StartsWith, EndsWith, Contains, Length, IndexOf,
// Trim, Substring, Insert, Remove, Replace,
// ToUpper, ToLower

 

//5、IN and NOT IN with LINQ to SQL

string[] chosenOnes = { "Tom", "Jay" };
Customers.Where (c => chosenOnes.Contains (c.Name))
    .Dump ("This translates to SQL WHERE ... IN");
Customers.Where (c => !chosenOnes.Contains (c.Name))
    .Dump ("This translates to SQL WHERE NOT ... IN");

//6、Extra - Where-based Subqueries

 

 

var names = new[] { "Tom", "Dick", "Harry", "Mary", "Jay" }.AsQueryable();
(
    from n in names
    where n.Length == names.Min (n2 => n2.Length)
    select n
)
.Dump ("Basic subquery");

// The same principle works well in LINQ to SQL:
(
    from c in Customers
    where c.Name.Length == Customers.Min (c2 => c2.Name.Length)
    select c
)
.Dump ("Basic subquery, LINQ to SQL");

// We can construct similar subqueries across association properties:
(
    from c in Customers
    where c.Purchases.Any (p => p.Price > 1000)
    select c
)
.Dump ("Customers who have purchased at least one item > $1000");

//6、Extra - Where-based Subqueries with let

// The let keyword in comprehension queries comes in useful with subqueries: it lets
// you re-use the subquery in the projection:

from c in Customers
let highValuePurchases = c.Purchases.Where (p => p.Price > 1000)
where highValuePurchases.Any()
select new
{
    c.Name,
    highValuePurchases
}

//7、Take and Skip

// reflection (using LINQ, of course!) The following query extracts all type
// names in the mscorlib assembly:

string[] typeNames =
    (from t in typeof (int).Assembly.GetTypes() select t.Name).ToArray();
typeNames
    .Where (t => t.Contains ("Exception"))
    .OrderBy (t => t)
    .Take (20)
    .Dump ("The first 20 matches");

typeNames
    .Where   (t => t.Contains ("Exception"))
    .OrderBy (t => t)
    .Skip (20)
    .Take (20)
    .Dump ("Matches 21 through 40");

 

//8、TakeWhile and SkipWhile

int[] numbers = { 3, 5, 2, 234, 4, 1 };
numbers.TakeWhile (n => n < 100).Dump ("TakeWhile");
numbers.SkipWhile (n => n < 100).Dump ("SkipWhile");

//9、Distinct

"HelloWorld".Distinct()

Purchases.Select (p => p.Description).Distinct()