C ＃ basic knowledge (3)

Write it at the front

Main contents of this paper ：Console class 、Math class 、Random class 、DateTime class 、Regex class 、 String common operations 、 Data conversion

1、C# Several common classes provided

1.1、Console class

Common methods

Method	describe
Write	Output content to the console without wrapping
WriteLine	Output content to the console and wrap
Read	Read a character from the console
ReadLine	Read a line of characters from the console

1.2、Math class

Method	describe
Abs	Take the absolute value
Ceiling	Returns the smallest integer value greater than or equal to the specified double precision floating-point number
Floor	Returns the maximum integer value less than or equal to the specified double precision floating-point number
Equals	Returns whether the specified object instances are equal
Max	Returns the value of the larger of two numbers
Min	Returns the decimal value of two numbers
Sqrt	Returns the square root of a specified number
Round	Returns the rounded value

1.3、Random class

Method	describe
Next()	Each time a different random positive integer is generated
Next(int max Value)	To produce a ratio max Value Small positive integers
Next(int min Value,int max Value)	Produce a minValue~maxValue The positive integer , But does not contain maxValue
NextDouble()	Produce a 0.0~1.0 Floating point number
NextBytes(byte[] buffer)	To fill an array of specified bytes with random numbers

1.4、DateTime class

Method	describe
Date	Get the date part of the instance
Day	Get the date represented by the instance is the day of a month
DayOfWeek	Get the day of the week represented by the instance
DayOfYear	The date represented by this instance is the day of the year
Add(Timespan value)	Add a time interval value to the specified date instance value
AddDays(double value)	Add the specified number of days to the specified date instance value
AddHours(double value)	Adds the specified number of hours to the specified date instance value
AddMinutes(double value)	Adds the specified number of minutes to the specified date instance value
AddSeconds(double value)	Adds the specified number of seconds to the specified date instance value
AddMonths(int value)	Add the specified month to the specified date instance value
AddYears (int value)	Adds the specified year to the specified date instance value

example

class Program
{
    
    static void Main(string[] args)
    {
    
        DateTime dt = DateTime.Now;
        Console.WriteLine(" Current date is ：{0}", dt);
        Console.WriteLine(" The current time is the... Of this month {0} God ", dt.Day);
        Console.WriteLine(" Now it's ：{0}", dt.DayOfWeek);
        Console.WriteLine(" The current year is {0} God ", dt.DayOfYear);
        Console.WriteLine("30  The date in days is {0}", dt.AddDays(30));

        DateTime dt1 = DateTime.Now;
        DateTime dt2 = new DateTime(2019, 6, 1);
        TimeSpan ts = dt2 - dt1;
        Console.WriteLine(" The number of days between is {0} God ", ts.Days);
    }
}

1.5、Regex class （ Regular expressions ）

Metacharacters in regular expressions

character	describe
.	Match all characters except newline
\w	Match the letter 、 Numbers 、 Draw the line down
\s	Match blanks （ Space ）
\d	Match the Numbers
\b	Match the beginning or end of the expression
^	Match the beginning of the expression
$	Match the end of the expression

A character that represents a repetition in a regular expression

word operator	Sketch Statement
*	0 One or more characters
?	0 Time or 1 Sub characters
+	1 One or more characters
{n}	n Sub characters
{n,M}	n To M Sub characters
{n, }	n More than characters

class Program
{
    
    static void Main(string[] args)
    {
    
        Console.WriteLine(" Please enter a mailbox ");
        string email = Console.ReadLine();
        Regex regex = new Regex(@"^(\w)+(\.\w)*@(\w)+((\.\w+)+)$");
        if (regex.IsMatch(email))
        {
    
            Console.WriteLine(" The email format is correct .");
        }
        else
        {
    
            Console.WriteLine(" The email format is incorrect .");
        }
    }
}

2、 String common operations

class Program
{
    
    static void Main(string[] args)
    {
    
    	//  To obtain the length of the 
        string str = " This is a string !";
        Console.WriteLine(" The length of the string is ：" + str.Length);
        Console.WriteLine(" The first character in the string is ：" + str[0]);
        Console.WriteLine(" The last character in the string is ：" + str[str.Length - 1]);

		//  Character position , No return -1
		int i = str.IndexOf("@");
		
		//  Character substitution 
		str = str.Replace("!", ".");
		
		//  Character interception 
		str = str.Substring(0, 2);

		//  Character insertion 
		str = str.Insert(1, ",");
    }
}

3、 Data conversion

3.1、 Implicit conversion

int a = 100;
double d = a;  // take int Type conversion to double type 
float f = 3.14f;
d = f;    // take float Type conversion to double type 

3.2、 Explicit conversion

Method	describe
ToBoolean	If possible , Convert type to Boolean .
ToByte	Convert type to byte type .
ToChar	If possible , Convert the type to a single Unicode Character type .
ToDateTime	Put the type （ Integer or string type ） Convert to date - Time structure .
ToDecimal	Converts a floating point or integer type to a decimal type .
ToDouble	Convert type to double precision floating point type .
ToInt16	Convert type to 16 Bit integer type .
ToInt32	Convert type to 32 Bit integer type .
ToInt64	Convert type to 64 Bit integer type .
ToSbyte	Convert type to signed byte type .
ToSingle	Convert type to small floating-point type .
ToString	Convert type to string type .
ToType	Converts the type to the specified type .
ToUInt16	Convert type to 16 Bit unsigned integer type .
ToUInt32	Convert type to 32 Bit unsigned integer type .
ToUInt64	Convert type to 64 Bit unsigned integer type .

3.3、 Coercive transformation

double dbl_num = 12345678910.456;
int k = (int) dbl_num ;// Cast is used here 

3.4、 Parse Methods and ToString Method

Parse Method is used to convert a string type to any type

int num1 = int.Parse("12348");

ToString Method is used to convert any data type to a string type

int a=100;
string str=a.ToString();

3.5、Convert Method

It can convert the value of any data type into any data type , The premise is not to exceed the range of the specified data type .

Method	explain
Convert.ToInt16()	Convert to integer (short)
Convert.ToInt32()	Convert to integer (int)
Convert.ToInt64()	Convert to integer (long)
Convert.ToChar()	Convert to character (char)
Convert.ToString()	Convert to string (string)
Convert.ToDateTime()	Convert to date type (datetime)
Convert.ToDouble()	Convert to double precision floating point (double)
Conert.ToSingle()	Convert to single precision floating point (float)

class Program
{
    
    static void Main(string[] args)
    {
    
         float num1 = 82.26f;
         int integer;
         string str;
         integer = Convert.ToInt32(num1);
         str = Convert.ToString(num1);
         Console.WriteLine(" Value converted to integer data {0}", integer);
         Console.WriteLine(" Convert to string {0},",str);
    }
}

3.6、 Packing and unpacking （ Value type and reference type ）

int val = 100;
object obj = val;
Console.WriteLine(" The value of the object  = {0}", obj);
// This is a packing process , Is the process of converting a value type to a reference type 


int val = 100;
object obj = val;
int num = (int) obj;
Console.WriteLine("num: {0}", num);
// It's an unpacking process , Is to convert a value type to a reference type , The process of converting a reference type to a value type