C ＃ fine sorting knowledge points 12 exception handling (recommended Collection)

1. The concept of abnormality

An exception is actually an event in which an error in a program interrupts the normal instruction flow .( You can think of an exception as any error condition that interrupts the normal program flow ).

Conditions that produce exceptions ：

​ 1： The file you want to open does not exist

​ 2： Lost network connection

​ 3： Call empty reference

​ 4： except 0 abnormal

....

because C# It's object-oriented , All errors are encapsulated in exception objects

Once an error occurs , Will receive a specific exception object .

2. Exception class

.NET Framework All exceptions in the class library are derived from Exception class , Exceptions include system exceptions and application exceptions .

By default, all system exceptions are derived from System.SystemException, All application exceptions derive from System.ApplicationException.

System exception Generally unpredictable , For example, memory stack overflow , Empty object reference , permissions , Hardware read error, etc

Application exception Generally predictable , For example, the file object cannot be found , The value is out of range , Inconsistent data types, etc , Design , Processing logic can judge .

Common system exception classes

Exception class	explain
System.OutOfMemoryException	use new Failed to allocate memory
System.StackOverflowException	Too many recursions 、 Too deep
System.NullReferenceException	The object is empty
Syetem.IndexOutOfRangeException	An array
System.ArithmaticException	The base class of arithmetic operation exception
System.DivideByZeroException	Divide by zero error

3. exception handling

stay C# Exceptions and exceptions in language Processing statements namely try … catch… finally.

Use the meaning of keywords ：

• **try：** Used to check for exceptions that occur , And help send any possible exceptions .

• **catch：** Dealing with errors in a more controlling way , There can be multiple catch Clause .

• **finally：** Whether or not an exception was thrown ,finally All code blocks will be executed .

example ：

class Program
{
    static void Main(string[] args)
    {
        while(true)
        {
            try
            {
                // Try the following statement 
                Console.WriteLine(" Please enter divisor ( Input q sign out )：");
                string str = Console.ReadLine(); // Exception may be thrown 
                if(str.Contains("q"))
                {
                    break;
                }
                
                int num1 = int.Parse(str);// Exception may be thrown 
                Console.WriteLine(" Please enter the dividend ：");
                int num2 = int.Parse(Console.ReadLine());
                Console.WriteLine(" The result of the calculation is :{0}", num1 / num2); // Exception may be thrown 
            }
            catch(Exception e)
            {
                //  Capture exception , Then process 
                Console.WriteLine(" Something goes wrong ：{0}", e.Message);
            }
            finally
            {
                 // Finish processing 
            }
        }
    }
}

4. Custom exception

Although in C# Many exception handling classes have been provided in the language , But in actual programming, we will still encounter some exception handling that are not involved .

For example, you want to put the validation of data into exception handling , That is, the entered age must be 18〜45, At this time, you need to customize the exception class to realize .

Custom exception classes It has to be Inherit Exception class

sentence ：

class  Exception class name :Exception
{
    
}

Throw your own exception , Use keywords throw .

Such as ：

throw new  Custom exception classes ( parameter list );

give an example ：

Limit the range of numbers entered by the user to 1-100.

 //InputException.cs 
/// <summary> 
///  Custom exception classes  
///  Thrown when the input value does not meet the requirements  
/// </summary>
class InputException:Exception
{
    /// <summary> 
    ///  Construction method 
    /// </summary> 
    /// <param name="inputNum"></param>
    public InputException(int inputNum):base(string.Format(" abnormal : The input range is 1‐ 100, The value is {0}",inputNum))
    {
        
    }
}

class Program
{
    static void Main(string[] args){
        int input = 0;
        while(true)
        {
            try
            {
                input = GetUserInput();
                break;
            }
            catch(InputException ex)
            {
                Console.WriteLine(ex.Message);
            }
            catch(FormatException)
            {
                Console.WriteLine(" The input is not a number ")
            }
            catch(Exception ex)
            {
                Console.WriteLine(ex.Message);
                Console.WriteLine(" Something goes wrong , Please try again ！");
            }
        }
        Console.WriteLine("Input Value :{0}", input);
    }
    
    static int GetUserInput()
    {
        Console.WriteLine(" Please enter a number ：");
        string str = Console.ReadLine();
        int input = int.Parse(str)
            if(input<1||input>100)
            {
                throw new InputException(input);
            }
        return input;
    }
}