stay c# in , Arrays are continuous in memory , Indexing is fast , Assignment and modification are also very simple .string[] s=new string[2]; // assignment s[0]="a";
s[1]="b"; // modify s[1]="a1";

But there are some disadvantages in arrays , Inserting data between two pieces of data is cumbersome , And the length of the array must be specified when declaring the array , Too long array length will cause memory waste , Too short will cause memory overflow . If we don't know the length of an array when we declare it , It's going to be a lot of trouble ,

For these disadvantages of array ,c# First of all ArrayList Object to overcome these shortcomings .

ArrayList Is namespace System.Collections Next part , Must be referenced when using this class , At the same time inherited IList Interface , Provides data storage and retrieval .ArrayList The size of an object is dynamically expanded and shrunk according to the data stored in it . therefore , In the statement ArrayList Object without specifying its length .

ArrayList list1 = new ArrayList(); // New data list1.Add("cde"); list1.Add(5678);
// Modify data list[2] = 34; // Remove data list.RemoveAt(0); // insert data list.Insert(0, "qwe");

From the above example ,ArrayList Solves all array shortcomings , So why did it happen list What about ?

In the above example ,ArrayList Not only are strings inserted code, And the numbers are inserted 5678, It's like this ArrayList Inserting different types of data into is allowed . because ArrayList All data inserted into it will be treated as object Type to handle , In our use ArrayList When processing data , Type mismatch is likely , that is ArrayList Not type safe . When storing or retrieving types , The operation of packing and unpacking usually leads to great performance loss .

Concept of packing and unpacking :

In a nutshell :

Packing : It is to package the data of value type into the instance of reference type

For example string Value of type abc Assign to object object obj

object obj=(object)i;
The process of packing and unpacking is very destructive .

generic paradigm List

because ArrayList Disadvantages of unsafe type and unpacking , therefore
The concept of generics has emerged .List Isochronous ArrayList Generic equivalent class of class . Most of its users ArrayList be similar , because List Class also inherits IList Interface . The key difference is , In the statement List When assembling , Our colleagues need to make a statement List Object type of data in the collection :

such as :

// New data list.Add(“abc”); // Modify data list[0] = “def”; // Remove data list.RemoveAt(0);
In the above example , If we go List Insert in collection int array 123,IDE It's a mistake , And it can't be compiled . In this way, the type safety problems mentioned above and the performance problems of packing and unpacking operation are avoided .

summary :

The capacity of an array is fixed , You can only get or set the value of one element at a time , and ArrayList or List<T> The capacity of can be automatically expanded as needed , modify , Delete or insert data .

Arrays can have multiple dimensions , and ArrayList or List<T>
Always has only one dimension . however , You can easily create an array list or a list of lists . Specific types (object except ) The performance of array of is better than ArrayList Performance of . that is because ArrayList Element of Object type ; Therefore, boxing and unboxing operations usually occur when storing or retrieving value types . however , When reallocation is not required ( That is, the initial capacity is very close to the maximum capacity of the list ),List<T>
The performance of is very close to that of arrays of the same type .

In the decision to use List<T>
Still use ArrayList class ( Both have similar functions ) Time , remember List<T> Classes perform better in most cases and are type safe . If yes List<T> Classes perform better and are type safe in most cases . If yes List<T> Type of class T Use reference type , The behavior of the two classes is exactly the same . however , If for type T Use value type , Then we need to consider the implementation and packing problems .

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