C ＃ explain out output parameters in detail

        out Parameters and ref The parameters are similar to , All are Let the formal parameter become the alias of the argument , I won't describe it in detail here , You can refer to the following blog ：C# The illustration ref Keyword passes parameters by reference . Let's take a look at the simple code out Function of parameters ：

Use out front ：

class Program
{
    static void Method(int v)
    {
        v = 20;    // Yes v Assignment cannot make a1 become 20
    }
    
    static void Main()
    {
        int a1 = 10;
        Method(a1);
        Console.Write(a1);   // Output 10
    }
}

Use out after ：

class Program
{
    static void Method(out int v)   // add to out The place of 
    {
        v = 20;     // For output parameters v assignment , Here you can change a1 The value of the 
    }
    
    static void Main()
    {
        int a1 = 10;   // Statement out after , The initial value of the argument 10 No method will be passed , So the assignment here is unnecessary 
        Method(out a1);  // add to out The place of 
        Console.Write(a1);  // Output 20
    }
}

        Be careful ： Because the initial value of the argument will not be passed into the method , Then if there is any execution path in the method, the view reads its value before the method assigns a value to the output parameter , The compiler will report an error ：

class Program
{
    static void Method(out int v)
    {
        int temp = v;   // Report errors ！ Because output variables in methods v Not assigned , Therefore, the output variable cannot be read and assigned to temp
    }
    
    static void Main()
    {
        int a1 = 10;   // Statement out after , The initial value of the argument 10 It will not be passed into the method 
        Method(out a1);  
        Console.Write(a1);  
    }
}

        summary ：ref and out There is a difference , namely ref Require variables to be initialized before passing , and out no need , It means to declare out After the embellishment , You have to code in the method out Modify the formal parameters to assign values .

Of course out Parameters have another purpose , By using out Parameters , You can make the method Return multiple values ：

class Program
{
    static void Method(out int v, out string s, out double d)   // Be careful out Use 
    {
        v = 1;
        s = "hello";
        d = 3.1415926;
    }
    
    static void Main()
    {
        int a1;
        string s1;
        double d1;
        Method(out a1, out s1, out d1);    // Be careful out Use 
        Console.WriteLine(a1);  
        Console.WriteLine(s1);
        Console.WriteLine(d1);
    }
}

Output results ：

1
hello
3.1415926