Language Integrated Query potx

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LINQ
Language
Integrated
Query
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
Basic concepts

LINQ requirements

Concepts

Types

LINQ to objects

LINQ to SQL

LINQ to Entity

LINQ to XML
1. Basic concepts & Types LinQ
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2. New features in language:

Generics

Implicitly Typed Variables

Object Initializers


Anonymous Types

Extension Methods

Lambda Expressions
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2.0. Generics

The creation of various types of collection

Type safety

Binary Code Reuse
using System.Collections.Generic;
List<int> intS = new List<int>();
intS.Add(113);
intS.Add(114);
int nAt = intS[0];
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2.1. Implicitly Typed Variables

Implicitly Typed Variables
– Declare variables without specifying their type
– Strongly type, not variant, not object
– Visual Studio will determine type
• Predict what the compiler will choose
• Intelligence support
– Type inference -> most general
• “3/10/2010” -> string, not date

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
Example
2.1. Implicitly Typed Variables
private void button2_Click(object sender, EventArgs e)
{ var x = 113;
var y = "1/1/2012";
var z = 1.7;
var k = new DateTime(2012, 1, 1);
string msg = "x type="+x.GetType() + "\n"+
"y type = "+y.GetType() + "\n" +
"z type ="+z.GetType() + "\n" +
"k type = " + k.GetType();
MessageBox.Show(msg);
}
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2.1. Implicitly Typed Variables

Note
– Always declare the type if we know
– Implicitly Typed Variables are suited for LINQ and anonymous type
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2.2. Object Initializers

Constructor

Allow to assign values to object properties (fields) when create object

We do not have to explicitly invoke a constructor


Useful in any context
– Especially useful in LINQ expressions
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2.2. Object Initializers

Example
class Test
{
public int a, b;
public int a2 { set;get;}
public int b2 { get; set; }
}
private void button3_Click(object sender, EventArgs e)
{
var x = new Test() {a=113,b=114,a2=115,b2=116};
MessageBox.Show(x.a2+"");
}
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2.3. Anonymous Types

Implicitly type functionality for objects
– Set property values to object without writing class definition
– The resulting class has no usable name
– Class name is generated by compiler, inherits from Object
– The result: an anonymous type that is not available at source code level

Also called Projections
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2.3. Anonymous Types


When to user anonymous types
– Need a temporary object to hold related data
– Don’t need method
– If we need a different set of properties for each declaration
– If we need to change the order of properties for each declaration
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2.3. Anonymous Types

When not to user anonymous types
– Need to define methods
– Need to define another variable
– Need to shared data across methods
private void btn4_Click(object sender, EventArgs e)
{
var teo = new { ID=1234,Name="Tèo Hả Tèo"};
MessageBox.Show(teo.ID +"-"+teo.Name);
}
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2.4. Extension Methods

Special kind of Static method

Allow the addition of methods to an existing class
– Without creating a new derived type
– Without re-compiling or modifying the original type

Called The Same way regular methods are called

Define in static class
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2.4. Extension Methods

Example
namespace StudyLinQ
{ public static class MyExtensionMethod
{
public static int SumFrom1toN(this int n)
{ int sum = 0;
for (int i = 1; i <= n; i++)
sum += i;
return sum;
}
}
}
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2.4. Extension Methods

Example
MessageBox.Show(n.SumFrom1toN()+"");
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namespace StudyLinQ
{
public static class MyExtensionMethod
{
public static int SumFrom1toN(this int n){…}
public static string NoiChuoi(this string s1, string s2)
{
return s1 + "-" + s2;
}
}

}
2.4. Extension Methods

Example
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private void btnNoi_Click
(object sender, EventArgs e)
{
string s = "Tý";
MessageBox.Show( s.NoiChuoi("Mập"));
}
2.4. Extension Methods

Example
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2.4. Extension Methods

Example
using System.Windows.Forms;
using System.Drawing;
namespace StudyLinQ
{
public static class MyExtensionMethod
{
public static int SumFrom1toN(this int n) {… }
public static string NoiChuoi(this string s1, string s2){…}
public static void ChangeColorToRed(this Button btn)
{
btn.BackColor = Color.Red;
}

}
}
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2.4. Extension Methods

Example
btnColor.ChangeColorToRed();
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2.4.a. Delegate

Delegate
– refers to method.
– When initialize a delegate, we initialize it with method.

Example
//Defines a delegate
public delegate int ChangeInt(int x);
public int Tang2(int x){return x + 2;}
public int Giam2(int x) {return x - 2;}
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2.4.a. Delegate

Example
private void btndlg_Click
(object sender, EventArgs e)
{
ChangeInt d = new ChangeInt(Tang2);
MessageBox.Show(d(2)+"");
d = new ChangeInt(Giam2);
MessageBox.Show(d(2)+"");

}
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2.4.a. delegate: why?
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private void btnEven_Click(object sender, EventArgs e)
{
listNumber.SelectedIndex = -1;
for (int i = 0; i < listNumber.Items.Count; i++)
{
if (Convert.ToInt32(listNumber.Items[i]) % 2 == 0)
listNumber.SelectedIndex = i;
}
}
private void btnOdd_Click(object sender, EventArgs e)
{
listNumber.SelectedIndex = -1;
for (int i = 0; i < listNumber.Items.Count; i++)
{
if (Convert.ToInt32(listNumber.Items[i]) % 2 != 0)
listNumber.SelectedIndex = i;
}
}
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private bool isPrime(int x)
{
if (x < 2)
return false;
for (int i = 2; i <= Math.Sqrt(x); i++)
if (x % i == 0)
return false;

return true;
}
private void btnPrime_Click(object sender, EventArgs e)
{
listNumber.SelectedIndex = -1;
for (int i = 0; i < listNumber.Items.Count; i++)
{
int x=Convert.ToInt32(listNumber.Items[i]);
if ( isPrime(x))
listNumber.SelectedIndex = i;
}
}
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2.4.a. delegate: why?
public static class MyExtensionMethod
{
public delegate bool HandleFunction(int x);
public static void SelectItemInListBox(this ListBox lst,
HandleFunction fn)
{
lst.SelectedIndex = -1;
for (int i = 0; i < lst.Items.Count; i++)
{
int x=Convert.ToInt32(lst.Items[i]);
if (fn(x))
lst.SelectedIndex = i;
}
}
}