c# 中的空合并运算符
原文:https://www . geeksforgeeks . org/null-聚结-运算符 in-c-sharp/
在 C# 中？？运算符被称为空值合并运算符。如果左侧操作数不为空，它将返回其值。如果它为空，那么它将计算右边的操作数并返回其结果。或者，如果左侧操作数的计算结果为非空，则不会计算其右侧操作数。
语法:
p ?? q
这里，p 是左操作数，q 是右操作数？？操作员。p 的值可以是可空类型，但 q 的值必须是不可空类型。如果 p 的值为空，则返回 q 的值，否则返回 p 的值。
要点:
- 那个？？运算符用于检查空值，您还可以为值为空(或可空类型)的变量分配默认值。
- 你不准霸王？？操作员。
- 它是右联想的。
- 在吗？？运算符，可以使用 throw 表达式作为？？运算符，这使得您的代码更加简洁。
-
You are allowed to use ?? operator with value types and reference types.
示例:
```cs // C# program to illustrate how to use // ?? operator with value types and // reference types using System;
namespace example {
class Program { static void Main(string[] args) {
// Reference types string item_1 = null; string item_2 = "GeeksforGeeks"; string item_3 = "GFG";
string item_4 = item_1 ?? item_2; item_3 = item_4 ?? item_2;
Console.WriteLine("Value of item_4 is: {0} \n"+ "Value of item_3 is: {1}", item_4, item_3);
// Value types int ? item_5 = null;
Program obj = new Program();
// Using ?? operator assigns // the value of a value type // and also you are allowed // to use method with ?? operator int ? item_6 = item_5 ?? obj.Add(10, 30); Console.WriteLine("Value of item_6 is: {0}", item_6); }
// Method public int Add(int a, int b) { int result = a + b; return result; } } } ```
输出:
```cs Value of item_4 is: GeeksforGeeks Value of item_3 is: GeeksforGeeks Value of item_6 is: 40
```
-
With the help of ?? operator you can prevent InvalidOperationException.
示例:
```cs // C# program to illustrate how ?? // operator prevent the // InvalidOperationException using System;
namespace example {
class GFG {
// Main Method static void Main(string[] args) { // Creating items of nullable types int ? item_1 = null;
/ Here if you use this commented part, then this statement will give you an InvalidOperationException. So to overcome this problem we use ?? operator int? item_2 = item_1.Value; /
// With the help of ?? operator we // assign a default value to the item_2 // And the value of item_1 is null. int ? item_2 = item_1 ?? 100; Console.WriteLine("Value of item_1 is: {0}", item_1); Console.WriteLine("Value of item_2 is: {0}", item_2); } } } ```
输出:
```cs Value of item_1 is: Value of item_2 is: 100
```
-
With the help of ?? operator you can remove many redundant “if-else” conditions and make your code compact and readable.
示例:
```cs // C# program to illustrate how ?? // operator replaces if-else statements using System;
namespace example {
class GFG {
// Main Method static void Main(string[] args) { // Creating items of nullable types int ? item_1 = null;
int ? item_2;
if (item_1.HasValue) { item_2 = item_1; } else { item_2 = 200; } Console.WriteLine("Value of item_1 is: {0}", item_1); Console.WriteLine("Value of item_2 is: {0}", item_2); } } } ```
输出:
```cs Value of item_1 is: Value of item_2 is: 200
```
```cs // C# program to illustrate how ?? // operator replaces if-else statements using System;
namespace example {
class GFG {
// Main Method static void Main(string[] args) { // Creating items of nullable types int ? item_1 = null;
// Using ?? operator int ? item_2 = item_1 ?? 200; Console.WriteLine("Value of item_1 is: {0}", item_1); Console.WriteLine("Value of item_2 is: {0}", item_2); } } } ```
输出:
```cs Value of item_1 is: Value of item_2 is: 200
```
-
?? operator can be nested. It will make your code more readable and also reduce multiple if-else conditions.
示例:
```cs // C# program to illustrate how // we use nested ?? operator using System;
namespace example {
class GFG {
// Main Method static void Main(string[] args) { // Creating items of nullable types int ? item_1 = null; int ? item_2 = null; int ? item_3 = 500;
// Nested ?? operator int ? item_4 = item_1 ?? item_2 ?? item_3;
Console.WriteLine("Value of item_4 is: {0} ", item_4); } } } ```
输出:
cs Value of item_4 is: 500
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