在C＃中有没有类似于PHP isset（）的东西？ [英] Is there anything similar to PHP isset() in C#?

问题描述

您好，



我来自PHP背景，想知道是否有办法检查变量是否存在，如果没有创建它。



与下面的PHP代码中看到的类似。

Hi There,

I come from a PHP background and would like to know if there's a way to check if a variable exists, and if not create it.

Something similar as seen in the the PHP code below.

if(isset($variable)){
    echo 'variable is set';
}else{
    echo 'variable is not set';
    $variable = 'This variable is now set';
}

目前我收到错误在声明之前不能使用局部变量'pricelist'。请记住，这是一个C＃控制台应用程序，而不是基于Web的应用程序。



问候，



我尝试了什么：



在线研究，但遗憾的是找不到任何东西。

Currently I am getting the error "Cannot use local variable 'pricelist' before it is declared". Keep in mind this is a C# console application, not a web based app.

Regards,

What I have tried:

Online research but could not find anything unfortunately.

推荐答案

variable））{
echo'variable is set';
}其他{
echo'变量未设置';

variable)){ echo 'variable is set'; }else{ echo 'variable is not set';

variable ='此变量现已设置';
}

variable = 'This variable is now set'; }

目前我收到错误在声明之前不能使用局部变量'pricelist'。请记住，这是一个C＃控制台应用程序，而不是基于Web的应用程序。



问候，



我尝试了什么：



在线研究，但遗憾的是找不到任何东西。

Currently I am getting the error "Cannot use local variable 'pricelist' before it is declared". Keep in mind this is a C# console application, not a web based app.

Regards,

What I have tried:

Online research but could not find anything unfortunately.

在C＃中所有变量必须在使用前声明：

In C# all variables must be declared before they are used:

string variable = null; // <- declaration
if(variable != null)
{
    Console.WriteLine("variable is set");
}
else
{
    Console.WriteLine("variable is not set");
    variable = "This variable is now set";
}

但前面的例子有点傻。我们假设您有一个方法，并且您想测试一个参数：

But the previous example is a bit silly. Let's assume you have a method and you want to test an argument:

public void Run(string variable)
{
    if(variable != null)
    {
        Console.WriteLine("variable is set");
    }
    else
    {
        Console.WriteLine("variable is not set");
    }
}

无论如何，变量必须在您第一次使用之前存在。 C＃强制您显式初始化所有局部变量。另一方面，类的成员字段获得默认值，因此这将起作用：

In any case the variable must exist before you first use it. And C# forces you to initialize all local variables explicitly. On the other hand member fields of a class get default values assigned so this will work:

class Test
{
    private string variable;
    
    public void Run()
    {
        if(variable != null)
        {
            Console.WriteLine("variable is set");
        }
        else
        {
            Console.WriteLine("variable is not set");
            variable = "This variable is now set";
        }
    }
}

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