The standard query operators utilize generic Func delegates.
Func<TSource,bool> matches a TSource=>bool lambda expression: one that accepts a TSource argument and returns a bool value.
Similarly, Func<TSource,TResult> matches a TSource=>TResult lambda expression.
The signature of the Select query operator:
public static IEnumerable<TResult> Select<TSource,TResult>
(this IEnumerable<TSource> source, Func<TSource,TResult> selector)
Func<TSource,TResult> matches a TSource=>TResult lambda expression: one that maps an input element to an output element.
TSource and TResult are different types.
The following query uses Select to transform string type elements to integer type elements:
using System;
using System.Collections;
using System.Collections.Generic;
using System.Linq;
class Program
{
static void Main()
{
string[] names = { "C", "Java", "C#", "Javascript" };
IEnumerable<int> query = names.Select(n => n.Length);
foreach (int length in query)
Console.Write(length + "|");
}
}
The output:
1|4|2|10|
Here is the Where query operator method signature.
public static IEnumerable<TSource> Where<TSource>
(this IEnumerable<TSource> source, Func<TSource,bool> predicate)
using System;
using System.Collections;
using System.Collections.Generic;
using System.Linq;
class Program
{
static void Main()
{
int[] numbers = { 10, 9, 8, 7, 6 };
IEnumerable<int> firstThree = numbers.Take(3);
foreach (int i in firstThree){
Console.WriteLine(i);
}
}
}
The output:
10
9
8
Skip operator ignores the first x elements and outputs the rest:
using System;
using System.Collections;
using System.Collections.Generic;
using System.Linq;
class Program
{
static void Main()
{
int[] numbers = { 10, 9, 8, 7, 6 };
IEnumerable<int> lastTwo = numbers.Skip(3); // { 7, 6 }
foreach (int i in lastTwo){
Console.WriteLine(i);
}
}
}
The output:
7
6Not all query operators return a sequence.
The element operators extract one element from the input sequence.
First, Last, and ElementAt all return a single element
using System;
using System.Collections;
using System.Collections.Generic;
using System.Linq;
class Program
{
static void Main()
{
int[] numbers = { 10, 9, 8, 7, 6 };
int firstNumber = numbers.First(); // 10
int lastNumber = numbers.Last(); // 6
int secondNumber = numbers.ElementAt(1); // 9
int lowestNumber = numbers.OrderBy(n => n).First(); // 6
}
}
The Aggregation operators return a scalar value; usually of numeric type:
using System;
using System.Collections;
using System.Collections.Generic;
using System.Linq;
class Program
{
static void Main()
{
int[] numbers = { 10, 9, 8, 7, 6 };
int count = numbers.Count(); // 5;
Console.WriteLine(count);
int min = numbers.Min(); // 6;
Console.WriteLine(min);
}
}
The output:
5
6The quantifiers return a bool value:
using System;
using System.Collections;
using System.Collections.Generic;
using System.Linq;
class Program
{
static void Main()
{
int[] numbers = { 10, 9, 8, 7, 6 };
bool hasTheNumberNine = numbers.Contains(9); // true bool
bool hasMoreThanZeroElements = numbers.Any(); // true bool
bool hasAnOddElement = numbers.Any(n => n % 2 == 1); // true
}
}
Some query operators accept two input sequences.
Concat appends one sequence to another
Union does the same but with duplicates removed:
using System;
using System.Collections;
using System.Collections.Generic;
using System.Linq;
class Program
{
static void Main()
{
int[] seq1 = { 1, 2, 3 };
int[] seq2 = { 3, 4, 5 };
IEnumerable<int> concat = seq1.Concat(seq2); // { 1, 2, 3, 3, 4, 5 }
IEnumerable<int> union = seq1.Union(seq2); // { 1, 2, 3, 4, 5 }
}
}
In query syntax
using System;
using System.Collections;
using System.Collections.Generic;
using System.Linq;
class Program
{
static void Main()
{
string[] names = { "C", "Java", "C#", "Javascript" };
IEnumerable<string> query =
from n in names
where n.Contains("a") // Filter elements
orderby n.Length // Sort elements
select n.ToUpper(); // Translate each element (project)
foreach (string name in query)
Console.WriteLine(name);
}
}
The output:
JAVA
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