5.20 C # Verification Code practice

usingSystem;usingSystem.Collections.Generic;usingSystem.Linq;usingSystem.Text;usingSystem.Threading.Tasks;namespaceconsoleapplication10{classProgram {Static voidMain (string[] args) {Random ran=NewRandom ();  for (; ; ) {                stringAA ="", BB ="";  for(intL =1; L <=4; l++)                {                    intb = ran. Next ( $); string[] C =New string[ $]                 {"A","B","C","D","E","F","G","H","I","J","K","L","M","N","O","P","Q",                    "R","S","T","U","V","W","X","Y","Z","1","2","3","4","5","6","7"                ,"8","9","0"}; AA=C[b]; BB+=AA; } Console.WriteLine ("The verification code is:"+BB); Console.Write ("Please enter a verification code"); stringMa =Console.ReadLine (); Ma=Ma.                    ToUpper (); if(MA = =BB) {Console.WriteLine ("verification code entered correctly");  Break; }                    Else{Console.WriteLine ("input Error Please re-enter");                } console.readline (); }            }        }    }

5.20 C # Verification Code practice