Differences

1. Difference between count(*) and count(1)

• COUNT(*) returns the number of items in a group. This includes NULL values and duplicates.
• COUNT(1) returns the number of items in a group. This includes NULL values and duplicates.
• COUNT(ALL expression) evaluates expression for each row in a group and returns the number of nonnull values.
• COUNT(DISTINCT expression) evaluates expression 
• 
  

1. Hashtable vs. Dictionary 

• Dictionary:

  • It returns error if we try to find a key which does not exist.
  • It is faster than a Hashtable because there is no boxing and unboxing.
  • Only public static members are thread safe.
  • Dictionary is a generic type which means we can use it with any data type.

  Hashtable:

• It returns null if we try to find a key which does not exist.
• It is slower than dictionary because it requires boxing and unboxing.
• All the members in a Hashtable are thread safe,
• Hashtable is not a generic type.
• Difference Between IS and AS

                          //  IS str1 variable type of string

                        ISkeyword is used to check the varaibles are same type or not
                        object str1=123;
                       if(str1 is string)
                            {
                                    c.w.l("This is of same type");
                             }

                      As keyword is helps to convert object one type to other type

                      string x=str1 as string

• Difference Between Ref and Out
• Difference Between Async and Await

1. async and await are markers,which mark code positions from where control should resume after task(thread) completes

EX: Public staticasync void method()

{

await Task.run(new Action(LongTask)

C.W.L("New Thread");

}

Public staticvoidLongTAsk()

{

Thread.sleep(20000);

}

• Difference Between Constant and readonly
• Difference Between var and Dynamic
• Private constructor
• Difference Between Throw and Throw ex