What is the use of static constructor in c# ?

A static constructor is used to initialize any static data, or to perform a particular action that needs to be performed once only. It is called automatically before the first instance is created or any static members are referenced.
It is basically used to initialise the static fields and properties of the class. It can be used mainly where, in our program, we want to do something before any reference for the class is created, like changing the background of the application.



Example:-

class SimpleClass

{

    // Static variable that must be initialized at run time.

    static readonly long baseline;

    // Static constructor is called at most one time, before any

    // instance constructor is invoked or member is accessed.

    static SimpleClass()

    {

        baseline = DateTime.Now.Ticks;

    }

}

Static constructors have the following properties:

• A static constructor does not take access modifiers or have parameters.
• A static constructor is called automatically to initialize the class before the first instance is created or any static members are referenced.
• A static constructor cannot be called directly.
• The user has no control on when the static constructor is executed in the program.
• A typical use of static constructors is when the class is using a log file and the constructor is used to write entries to this file.
• Static constructors are also useful when creating wrapper classes for unmanaged code, when the constructor can call the LoadLibrarymethod.
• If a static constructor throws an exception, the runtime will not invoke it a second time, and the type will remain uninitialized for the lifetime of the application domain in which your program is running.

public class Bus

 {

     // Static variable used by all Bus instances.

     // Represents the time the first bus of the day starts its route.

     protected static readonly DateTime globalStartTime;

     // Property for the number of each bus.

     protected int RouteNumber { get; set; }

     // Static constructor to initialize the static variable.

     // It is invoked before the first instance constructor is run.

     static Bus()

     {

         globalStartTime = DateTime.Now;

         // The following statement produces the first line of output, 

         // and the line occurs only once.

         Console.WriteLine("Static constructor sets global start time to {0}",

             globalStartTime.ToLongTimeString());

     }

     // Instance constructor.

     public Bus(int routeNum)

     {

         RouteNumber = routeNum;

         Console.WriteLine("Bus #{0} is created.", RouteNumber);

     }

     // Instance method.

     public void Drive()

     {

         TimeSpan elapsedTime = DateTime.Now - globalStartTime;

         // For demonstration purposes we treat milliseconds as minutes to simulate

         // actual bus times. Do not do this in your actual bus schedule program!

         Console.WriteLine("{0} is starting its route {1:N2} minutes after global start time {2}.",

                                 this.RouteNumber,

                                 elapsedTime.TotalMilliseconds,

                                 globalStartTime.ToShortTimeString());

     }

 }

 class TestBus

 {

     static void Main()

     {

         // The creation of this instance activates the static constructor.

         Bus bus1 = new Bus(71);

         // Create a second bus.

         Bus bus2 = new Bus(72);

         // Send bus1 on its way.

         bus1.Drive();

         // Wait for bus2 to warm up.

         System.Threading.Thread.Sleep(25);

         // Send bus2 on its way.

         bus2.Drive();

         // Keep the console window open in debug mode.

         System.Console.WriteLine("Press any key to exit.");

         System.Console.ReadKey();

     }

 }

 /* Sample output:

     Static constructor sets global start time to 3:57:08 PM.

     Bus #71 is created.

     Bus #72 is created.

     71 is starting its route 6.00 minutes after global start time 3:57 PM.

     72 is starting its route 31.00 minutes after global start time 3:57 PM.      

*/

How to use Merge Statement is Sql Server ?

Here are a few facts that you must know before starting to use Merge Statement:

1. Atomic statement combining INSERT, UPDATE and DELETE operations based on conditional logic

2. Done as a set-based operation; more efficient than multiple separate operations

3. MERGE is defined by ANSI SQL; you will find it in other database platforms as well

4. Useful in both OLTP and Data Warehouse environments
OLTP: merging recent information from external source
DW: incremental updates of fact, slowly changing dimensions.

-- -- Update existing, add missing

MERGE INTO dbo.tbl_Customers AS C

USING dbo.tbl_CustomersTemp AS CT

        ON C.CustID = CT.CustID

WHEN MATCHED THEN

    UPDATE SET

      C.CompanyName = CT.CompanyName,

      C.Phone = CT.Phone

WHEN NOT MATCHED THEN

      INSERT (CustID, CompanyName, Phone)

      VALUES (CT.CustID, CT.CompanyName, CT.Phone)

CREATE TABLE dbo.tbl_Source (id INT, name NVARCHAR(100), qty INT);

CREATE TABLE dbo.tbl_Target (id INT, name NVARCHAR(100), qty INT);

--Synchronize source data with target

MERGE INTO dbo.tbl_Target AS t

    USING dbo.tbl_Source AS s   

        ON t.id = s.id

    WHEN MATCHED AND (t.name != s.name OR t.qty!= s.qty) THEN

        --Row exists and data is different

        UPDATE SET t.name = s.name, t.qty = s.qty

    WHEN NOT MATCHED THEN

        --Row exists in source but not in target

        INSERT VALUES (s.id, s.name, s.qty)

    WHEN SOURCE NOT MATCHED THEN

        --Row exists in target but not in source

        DELETE

In this above post I explained hoiw to use  Merge Statement in Sql server. I hpe you enjoyed it so please send your feedback and queries. Thnak You.