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dbt 
airflow

一、基础50

1.1 new keyword

1. new operator
2. new modifier
3. new constraint

1.2 static class vs. static method

1. static class

1. 只能包含静态成员（字段、方法、属性、事件）
2. 不能实例化（new 不出来）
3. 不能继承或被继承（sealed + abstract 的结合体）
4. 默认是 sealed（防止继承）
5. 在程序第一次使用时，CLR 会初始化一次（静态构造函数）

2. static method

1. 属于类的本身，而不是对象成员
2. 不能访问实例成员

3. static constructor 静态构造函数

1. 没有访问修饰符，没有参数
2. CLR 在 第一次访问类 时调用一次
3. 用于初始化静态字段
4. 不能手动调用

public class Logger
{
    public static string FilePath;

    static Logger()
    {
        FilePath = "log.txt";
        Console.WriteLine("Static constructor called");
    }
}

4. 静态成员的生命周期

静态字段在 应用程序域（AppDomain）级别唯一

1.3 LinQ

1.what is LinQ

1. LINQ is a set of technologies that allow simple and efficient querying over different kinds of data
2. There are two ways of querying data： ①query syntax ；② method syntax

2. List、IEnumerable、IQueryable

1. List → 具体集合类，数据已经在内存。
2. IEnumerable → 内存中逐个枚举，延迟执行，适合集合操作。
3. IQueryable → 针对数据库等远程源，查询会被翻译成 SQL 执行，更高效。

3. 在数据库里用 IEnumerable

• 如果你在数据库中使用 IEnumerable，查询会在内存中执行，导致先把整张表的数据拉到本地再过滤，非常低效；
• 而 IQueryable 会把查询翻译成 SQL 在数据库端执行，性能更好。

1.4 ==和equals

1. ==

• == 默认比较对象引用，值类型比较值

1. 用途：检查 两个对象的引用是否相同（默认情况下）。
2. 可被重载：像 string、int 等类型重载了 ==，会比较值而不是引用。
3. 适合：值类型或经过重载的引用类型。

2. Equals

• Equals 默认也比较引用，但可以重写来比较对象的逻辑内容

class Person
{
    public string Name { get; set; }
    public override bool Equals(object obj)
    {
        return obj is Person p && this.Name == p.Name;
    }
}

1.5 综合

1. Property vs. Field

• A property is a way to encapsulate a field and provide controlled access to it through get and set methods, allowing you to enforce validation, read-only or write-only behavior, and maintain encapsulation.

2. const和readonly

• const is a compile-time constant that’s implicitly static, while readonly is a runtime constant that can be assigned at declaration or in a constructor but not changed afterward.

1.6 Convariance(out) vs. Contravariance(in)

1. out

1. 泛型参数只能作为输出（返回值），可以把子类当父类用
2. 允许你在泛型返回类型上使用更具体的类型。
3. 子类可以当作父类返回。

2. in

1. 泛型参数只能当作输入参数
2. 允许你在泛型输入参数上使用更泛化的类型。
3. 父类可以当作子类输入。
4. 通用处理器就能处理所有继承自父的子事件

二、Meadium 50

2.1 is vs. as

1. is

• is is used to check if an object is of a given type, it returns a boolean type. Both the value type and the reference type can use it

2. as

• as is used to cast a variable to a given type; if the cast fails, the value will be null; as can only be used for casting to reference types or nullable value types

difference between regular cast and as

• Regular casting fails, and an invalidCastException will be thrown.
• If casting with as fails, the result will be null

2.2 using keyword

1. using namespaces

• it is used to import namespaces

2. Dispose Resources

• Ensures that objects implementing IDisposable (like Stream, DbConnection, etc.) are cleaned up automatically, even if an exception occurs.

using (var file = new StreamWriter("test.txt"))
{
    file.WriteLine("Hello world");
} // file.Dispose() is called automatically here

3. Using Alias

• You can give a namespace or class a shorter alias name.

using Project = MyCompany.Project.Very.Long.Namespace;
class Program
{
    static void Main()
    {
        Project.MyClass obj = new Project.MyClass();
    }
}

2.3 throw vs. throw Ex

• throw preservers the stack trace, while throw ex not
• throw helps us to find the original source of the problem

1. what is the stack trace

1. Stack trace is a trace of all methods that have been called and it allows us to locate the exact line in the code

2.4 typeof vs. GetType

1. Both of them return Type object
2. typeof(Person) → tells you what properties exist on the type at compile time, but not their values.
3. tells you the runtime type of p, and with reflection (prop.GetValue§), you can also get property values.

2.5 float vs. double vs. decimal

Feature	float (Single)	double (Double)	decimal (Decimal)
Size	4 bytes (32-bit)	8 bytes (64-bit)	16 bytes (128-bit)
Precision	~7 decimal digits	~15–16 decimal digits	~28–29 decimal digits
Representation	Binary floating-point (base-2)	Binary floating-point (base-2)	Decimal floating-point (base-10)
Performance	Fastest, least memory	Fast, hardware-optimized	Slowest, software-based arithmetic
Default literal	Needs f suffix (3.14f)	Default for floating literals (3.14)	Needs m suffix (3.14m)
Typical Usage	Graphics, games, sensors, 3D math	Science, engineering, general purpose	Finance, money, currency, accounting

2.6 GetHashCode()

1. 什么是GetHashCode()

• 引用类型 → 默认返回的是 对象引用的内部哈希值，和内存地址相关。
• 值类型 → 会基于字段值生成哈希。

2. 和 Equals() 的关系

3. 重写