String

char a collection of string name = "John Doe"; Double quotes, char is a single quote
string is immutable, once it is initialized, it cannot be changed. Every time you use the existing stringAfter the operation, a new one is actually createdstring, and then assign the new value to this variable, the old value It was cleaned up by the framework.

thisNew assignment and clean up old The process of value has been repeated 10,000 times, which is not smart enough. It is recommended to use StringBuilder

Append can be used if StringBuilder is used Method to expand string, this efficiency should be higher than that

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string operator
+ Connect two or more strings (you can add other types to ToString first and then +)
Format is the same format as python, regardless of the original data type, {} will do.

Length　　Get string length
SubString　　Get String
IndexOf　　Find the first index of a certain character
Replace　　Replace
Contains　　Whether it contains a certain character or string
Startswith　　What starts with
Endswith　　 ends with

< p style="margin: 0px 0px 1.2em !important;"> The meaning of \ escape character is not mentioned, the universe is universal
@　　 If you think \ is used too much and it is not intuitive, you can add it in front of string as a whole @ Symbol, so what is in the string is nothing, it will not be escaped, see the picture below

Data & Time

The default is: date+time
When formatting, m means minutes and M means month

< p style="margin: 0px 0px 1.2em !important;">The time display caused by cultural differences in different regions can be set by setting System.Globalization.CultureInfo to solve

Parse　　 and TryParse
The latter will not report an error when parsing, and it is more secure

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empty type null

Normal data types cannot be assigned to null的，　　int a = null ;This is the compiler will report an error
The method is to add a question mark after the data type ,? 　　So this amount can be assigned to null (It can be assigned to null, instead of having to give him a value of null)

After assigning the value to null, of course you can assign it a new value Value
nullable = 123;

and the above if< /code>Compared with two judgments, there is a value and it is 42, and the output It's 42!
The following is GetValueOrDefault< /code> method, return the value if there is a value, it is null will return the default value of the basic type (int’s default value is 0) It’s more concise than the above to use

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var keyword

We can put a var to implicitly define its data type

The above int is an explicit definition< br>name is an implicit definition, not explicitly stated as string The compiler sees what you give name is assigned string< /code>, he puts name is defined as string
This is as fast as the display definition, don’t worry about the efficiency difference

var can only be used in method Can not be used in class level

Sometimes we use var, or directly use the return value of the function as the variable type (I don’t care what type is returned, I var result = someMethod() will do)

Above, one var Let us reduce the number of keyboard letters, you have to say that I can copy and paste the previous line and modify it. . . .

In most cases, we can put var to a fixed type, but in anonymous type (anonymous types) must be used, I'll talk about this later.

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