Design Patterns C#

C# > Design patterns

A design pattern provides a framework for building an application. Design patterns makes the design process cleaner and more efficient. You can use existing design patterns to make templates for your objects and allowing you to build software faster.

• Singleton
• Factory
• Dependency injection

Singleton pattern C# Example

 C# > Design Patterns > Singleton

Singleton is used to restrict instantiation of a class to one object. In this way we ensure that the class has only one instance.

When to use?

Usually singletons are used for centralized management  and provide a global point of access to themselves.

• logging
• configuration
• resources

Example

using System;

using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;

namespace WindowsFormsApplication1

{
    class Singleton
    {
        private static Singleton instance;
        protected Singleton()
        {
          // protected constructor
        }

 

             public static Singleton GetInstance()

        {

            if (instance == null)

            {

                lock (typeof(Singleton))

                {

                    instance = new Singleton();

                }

            }

            return instance;

        }

    }

    public partial class Form1 : Form

    {
        public Form1()
        {
            InitializeComponent();
        }
        private void Form1_Load(object sender, EventArgs e)
        {
            // Singleton st = new Singleton(); Error  'WindowsFormsApplication1.Singleton.Singleton()' is inaccessible due to its protection level
            Singleton st1 = Singleton.GetInstance();
            Singleton st2 = Singleton.GetInstance(); // st2=st1
        }
    }
}

 

LINQ to DataSet - Filtering with DataView C#

C# > System.Data   > DataSet > Linq to DataSet

DataView can be created from a LINQ to DataSet query.

Example:

Create a DataView from a LINQ to DataSet query with a Where clause

using System;

using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.IO;
using System.Linq;
using System.Runtime.InteropServices;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;

namespace WindowsFormsApplication1

{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }
        private void Form1_Load(object sender, EventArgs e)
        {
            DataSet ds = new DataSet();
            ds.Tables.Add("Product");

            DataTable dt = ds.Tables[0];
            dt.Columns.Add("id", typeof(Int32));
            dt.Columns.Add("name", typeof(string));
            dt.Columns.Add("quantity", typeof(Int32));

            fillDataTable(dt);

            EnumerableRowCollection<DataRow> query = from p in ds.Tables[0].AsEnumerable()

                                                     where p.Field<Int32>("quantity") > 50 && p.Field<Int32>("quantity") < 70
                                                     select p;
            DataView view = query.AsDataView();
            BindingSource bs = new BindingSource();
            bs.DataSource = view;
        }

        void fillDataTable(DataTable dt)
        {
            for (int i = 0; i < 100; i++)
            {
                DataRow dr = dt.NewRow();
                dr[0] = i;
                dr[1] = "Product ";
                dr[2] = i;
                dt.Rows.Add(dr);
            }
        }
    }
}

What is Oracle PL/SQL?

Oracle PL/SQL

PL/SQL is an language that was designed for processing of SQL commands. 
PL/SQL stands for Procedural Language extension of SQL.
Server-side PL/SQL is stored and compiled in Oracle Database and runs within the Oracle executable.

PL/SQL is a block structured language, which means program is in logical blocks of code.

Structure

DECLARE          <declarations section> BEGIN          <executable command(s)> EXCEPTION          <exception handling> END;

Example

DECLARE    msg varchar2(50):= 'PL/SQL Example'; BEGIN    dbms_output.put_line(msg); END ; /

Links

• System tables

String functions SQL Server

SQL Server > Built-in Functions > String

String functions perform an operation on a string input value and return a string or numeric value.

• SUBSTRING

Operators SQL Server

SQL Server > Operators

An operator is a symbol specifying an action that is performed on expressions

Arithmetic

• Modulo
• Add
• Subtract
• Multiply
• divide

Relational

• PIVOT and UNPIVOT

Set

• Union
• Except and Intersect

How to get SQL Server authentication logins

SQL Server > System Views > Catalog > sys.sql_logins

sys.sql_logins returns information about SQL Server authentication logins.

Example:

SELECT m.name, m.principal_id, m.type_desc, m.password_hash FROM master.sys.sql_logins m

Result:

name principal_id type_desc password_hash
sa 1 SQL_LOGIN 0x0200D225062FC1F5301DB7D08706A19C5CC900AB979B59D33B8ACB61F5
D5FCCF9BA0CE660B47435B9A5BD1F06D63C56A4DC6FB69EA429978866AEE29FA440F4E3D
4AE336D3D2