C # type conversion is and as and performance traps

1. Before C #2.0, as can only be used for reference types. After C #2.0, it can also be used as an empty type. The result is a null value or a meaningful value.
Example:

 1   static void Main(string[] args) 2         { 3             PrintValueInt32(5); 4             PrintValueInt32("some thing!"); 5     } 6      7       static void PrintValueInt32(object o) 8         { 9             int? nullable = o as int?;10             Console.WriteLine(nullable.HasValue ? nullable.Value.ToString() : "null");11         }

Output:

 

In this way, you can safely convert any reference to a value in one step. In C #1.0, you can only use the is operator first, and then lightweight conversion. This makes the CLR perform two type checks, which is obviously not elegant enough.

 

2. I always thought that a single check would be faster than two. Is that true? Let's test it.

 

1 static void main (string [] ARGs) 2 {3 list <Object> listobj = new list <Object> (); 4 List <Int?> List = new list <Int?> (); 5 6 for (INT I = 0; I <100000; I ++) 7 {8 listobj. add (I); 9} 10 11 stopwatch watch = new stopwatch (); // test time Class 12 watch. start (); 13 foreach (VAR item in listobj) 14 {15 if (item is int) // is and strong type conversion 16 {17 list. add (INT) item); 18} 19} 20 watch. stop (); 21 console. writeline ("is check and strong type conversion time: {0}", watch. elapsed); 22 23 watch. reset (); 24 watch. start (); 25 foreach (VAR item in listobj) 26 {27 Int? Temp = item as Int ?; // As conversion and null value determination, 28 If (temp. hasvalue) // temp is also available here! = NULL to judge 29 {// because temp is a void type, hasvalue, value, and so on can be used to operate on the void type 30 List. add (temp); 31} 32} 33 watch. stop (); 34 console. writeline ("as conversion time: {0}", watch. elapsed); 35 console. readkey (); 36}

Output:

The output results show that the use of the is check and strong type conversion time is much less than the as conversion time. Exercise caution when converting between referenced types and value types frequently to avoid falling into the performance trap.

3. Use of null merge operators.

The null merge operator is introduced in C #2.0. First ?? Second:

(1) Evaluate first.

(2) If first is not null, the value of the entire expression is the value of first.

(3) Otherwise, calculate the value of second as the value of the entire expression.

Sample Code 1: when the first parameter is not null, the value of the entire expression is the first parameter no matter whether the second parameter is null or not:

 

            int? first = 1;            int? second = 4;            int? temp = first ?? second;            Console.WriteLine("{0}", (temp.HasValue ? temp.Value.ToString() : "null"));

Output: (even if second is not empty, the first value is output because first is not empty)

Code Example 2: If the first parameter is null, the value of second is the value of the entire expression.

 

1             int? first = null;2             int? second = 4;3             int? temp = first ?? second;4             Console.WriteLine("{0}", (temp.HasValue ? temp.Value.ToString() : "null"));

Output:

For more information, see Jon steek.

C # type conversion is and as and performance traps