I have recently studied the interfaces and summarized them.

Interface defined in C:

Public interface sk_userscl
{
Int uid {Get ;}
String username {set; get ;}
String password {set; get ;}
String sex {set; get ;}
String connectionstring {set; get ;}
Int insuserscl ();
Int getdatauserscl ();

}

Classes that implement interfaces:

Public class sk_usersclscl: sk_userscl
{
Private int puid;
Private string pusername;
Private string ppassword;
Private string limit X;
Private string pconnectionstring;
Private SqlConnection conn;

Public int Uid
{
Get {return puid ;}
}
Public string UserName
{
Set {pusername = value ;}
Get {return pusername ;}
}
Public String Password
{
Set {ppassword = value ;}
Get {return ppassword ;}
}
Public String sex
{
Set {struct x = value ;}
Get {return response X ;}
}
Public String connectionstring
{
Set {pconnectionstring = value ;}
Get {return pconnectionstring ;}
}
Public void open ()
{
Conn = new sqlconnection (pconnectionstring );
If (conn. State = system. Data. connectionstate. Closed)
{
Conn. Open ();
}
}
Public void Close ()
{
If (conn. State = System. Data. ConnectionState. Open)
{
Conn. Close ();
}
}
Public int InsusersCL ()
{
Open ();
String strinssql = "insert into users (user_name, pwd, sex) values ('" + UserName + "', '" + PassWord + "', '" + Sex + "') ";
SqlCommand cmd = new SqlCommand (strinssql, conn );
Int intnum = cmd. ExecuteNonQuery ();
Close ();
Return intnum;
}

Public int UpdusersCL ()
{
Open ();
String strupdsql = "update users set pwd = '" + PassWord + "'where user_name ='" + UserName + "'";
SqlCommand cmd = new SqlCommand (strupdsql, conn );
Int rcount = cmd. ExecuteNonQuery ();
Close ();
Return rcount;
}
Create a logic layer:

// Create an interface
Sk_usersCL.Sk_usersCL userCL;
# Region defines attributes,
Private int puid;
Private string pusername;
Private string ppassword;
Private string limit X;
Private string pconnstr;
Public int Uid
{
Get {return puid ;}
Set {puid = value ;}
}
Public string UserName
{
Get {return pusername ;}
Set {pusername = value ;}
}
Public String Password
{
Get {return ppassword ;}
Set {ppassword = value ;}
}
Public String sex
{
Get {return response X ;}
Set {struct x = value ;}
}
Public string Connectionstring
{
Get {return pconnstr ;}
Set {pconnstr = value ;}
}
# Endregion
Public string GetConnstr ()
{
Return "server = Data-11; User id = 11; Pwd = 11; database = 11 ";
}
Public int insusers ()
{
Sk_usersclsCL users = new sk_usersclsCL ();
UserCL = users;
UserCL. UserName = pusername;
UserCL. PassWord = ppassword;
UserCL. Sex = policx;
UserCL. Connectionstring = GetConnstr ();
Return userCL. InsusersCL ();
}

Public int Updusers ()
{
Sk_usersclsCL users = new sk_usersclsCL (); // creates an object at the data layer.
UserCL = users; // assign a value to the interface object. An interface call inherits the class of this interface.
UserCL. UserName = pusername;
UserCL. PassWord = ppassword;
UserCL. Sex = policx;
UserCL. Connectionstring = GetConnstr ();
Return userCL. UpdusersCL ();
}
}

Presentation Layer call logic layer:

Sk_usersBL.Class1 us = new Sk_usersBL.Class1 (); // declares the object of the logic layer.
Us. UserName = TBusername. Text;
Us. PassWord = TBpassword. Text;
If (us. Updusers ()> 0)
{
Response. Write ("<script language = javascript> alert ('modification successful! ') </Script> ");
}
Else
{
Response. Write ("<script language = javascript> alert ('modification failed! ') </Script> ");
}