Bài giảng Công nghệ lập trình tiên tiến: Chương 2 trình bày các nội dung kiến thức về truy xuất dữ liệu với LINQ như các thao tác trên máy tính, câu lệnh, cú pháp,... Tham khảo nội dung bài giảng để hiểu rõ hơn về các nội dung trên.
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Nội dung Text: Bài giảng Công nghệ lập trình tiên tiến: Chương 2 - ĐH Công nghệ Đồng Nai
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LINQ
Language
Integrated
Query
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1. Basic concepts & Types LinQ
Ø
Basic concepts
ü
LINQ requirements
ü
Concepts
Ø
Types
ü
LINQ to objects
ü
LINQ to SQL
ü
LINQ to Entity
ü
LINQ to XML
ü
LINQ to Dataset
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2. New features in language:
Ø
Generics
Ø
Implicitly Typed Variables
Ø
Object Initializers
Ø
Anonymous Types
Ø
Extension Methods
Ø
Lambda Expressions
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2.0. Generics
Ø
The creation of various types of collection
Ø
Type safety
Ø
Binary Code Reuse
using
System.Collections.Generic;
List intS = new
List();
intS.Add(113);
intS.Add(114);
int nAt = intS[0];
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2.1. Implicitly Typed Variables
Ø
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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2.1. Implicitly Typed Variables
Ø
Example
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); 6
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2.1. Implicitly Typed Variables
Ø
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
Ø
Constructor
Ø
Allow to assign values to object properties
(fields) when create object
Ø
We do not have to explicitly invoke a
constructor
Ø
Useful in any context
– Especially useful in LINQ expressions
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2.2. Object Initializers
Ø
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+""); 9
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2.3. Anonymous Types
Ø
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
Ø
Also called Projections
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2.3. Anonymous Types
Ø
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
Ø
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
Ø
Special kind of Static method
Ø
Allow the addition of methods to an existing
class
– Without creating a new derived type
– Without re-compiling or modifying the
original type
Ø
Called The Same way regular methods are
called
Ø
Define in static class
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2.4. Extension Methods
Ø
Example
namespace StudyLinQ
{ public static class MyExtensionMethod
{
public static int SumFrom1toN(this int
n)
{ int sum = 0;
for (int i = 1; i
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2.4. Extension Methods
Ø
Example
MessageBox.Show(n.SumFrom1toN()+"");
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2.4. Extension Methods
Ø
Example
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;
}
}
}
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2.4. Extension Methods
Ø
Example
private void btnNoi_Click
(object sender, EventArgs e)
{
string s = "Tý";
MessageBox.Show( s.NoiChuoi("Mập"));
}
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2.4. Extension Methods
using System.Windows.Forms;
Ø
Example
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
Ø
Example
btnColor.ChangeColorToRed();
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2.4.a. Delegate
Ø
Delegate
– refers to method.
– When initialize a delegate, we initialize
it with method.
Ø
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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