变量传递中的ref, out

date

Aug 9, 2022

slug

10025

status

Published

tags

summary

type

Post

By value (this is the default): Think of these as being in-only. By reference as a ref parameter: Think of these as being in-and-out. As an out parameter: Think of these as being out-only.

在一个类文件Person.cs里定义一个方法：

public void PassingParameters(int x, ref int y, out int z)
        {
            // out parameters cannot have a default
            // AND must be initialized inside the method
            z = 99;
            // increment each parameter
            x++;
            y++;
            z++;
        }

在Program.cs的main方法里面调用

int a = 10;
int b = 20;
int c = 30;
WriteLine($"Before: a = {a}, b = {b}, c = {c}");
bob.PassingParameters(a, ref b, out c);
WriteLine($"After: a = {a}, b = {b}, c = {c}");

输出为：

Before: a = 10, b = 20, c = 30 After: a = 10, b = 21, c = 100

看到，默认传递a没有改变，ref传递由于传递了引用所以b的值变了，out传递则直接使用的方法内部的值99.

When passing a variable as a parameter by default, its current value gets passed, not the variable itself. Therefore, x has a copy of the value of the a variable. The a variable retains its original value of 10.

When passing a variable as a ref parameter, a reference to the variable gets passed into the method. Therefore, y is a reference to b. The b variable gets incremented when the y parameter gets incremented.

When passing a variable as an out parameter, a reference to the variable gets passed into the method. Therefore, z is a reference to c. The value of the c variable gets replaced by whatever code executes inside the method. We could simplify the code in the Main method by not assigning the value 30 to the c variable since it will always be replaced anyway.

C#7.0之后，可以直接在调用的时候申明out传递

int d = 10;
int e = 20;
WriteLine($"Before: d = {d}, e = {e}, f doesn't exist yet!");
// simplified C# 7.0 or later syntax for the out parameter
bob.PassingParameters(d, ref e, out int f);
WriteLine($"After: d = {d}, e = {e}, f = {f}");