C# Overwrite string and string[] in another Function

Question

I overwrite the variables 'arr[1]' and 'test' in setValues() function.

arr[1] is changed to 'BBB'

but test doesn't change to '222'

Output: BBB111

but it should be BBB222

Why string test doesn't get updated?

public class Program
    {
        static void Main(string[] args)
        {
            string[] arr = new string[10];

            arr[1] = "AAA";
            string test = "111";

            setValues(arr, test);

            int exit = -1;
            while (exit < 0)
            {

                for (int i = 0; i < arr.Length; i++)
                {
                    if (!String.IsNullOrEmpty(arr[i]))
                    {
                        Console.WriteLine(arr[i] + test);
                    }
                }
            }
        }

        private static void setValues(string[] arr, string test)
        {
            arr[1] = "BBB";
            test = "222";
        }
    }

Answer 1

You need to pass that string by reference to be able to modify it in a method, you can do this by adding the ref keyword:

public class Program
    {
        static void Main(string[] args)
        {
            string[] arr = new string[10];

            arr[1] = "AAA";
            string test = "111";

            setValues(arr, ref test);

            int exit = -1;
            while (exit < 0)
            {

                for (int i = 0; i < arr.Length; i++)
                {
                    if (!String.IsNullOrEmpty(arr[i]))
                    {
                        Console.WriteLine(arr[i] + test);
                    }
                }
            }
        }

        private static void setValues(string[] arr, ref string test)
        {
            arr[1] = "BBB";
            test = "222";
        }
    }

Answer 2

You are only altering the local reference to test in your setValues function. You would need to pass this variable by reference ( ref )

private static void setValues(string[] arr, ref string test)
{
    arr[1] = "BBB";
    test = "222";
}

then call it like this:

setValues(arr, ref test);

Answer 3

这是因为setValues方法中的test未标记为ref因此仅在方法内部进行更改，并且值不会超出范围。

Answer 4

Because the array as object is passed by reference while the string is passed by value. So, the function setValues() update the local copy of test, invisible to the caller, while update the ONLY instance of string[] arr, visible either to the caller than to the callee.

Paolo

Answer 5

The reason is variable scope. The variables referenced in your SetValues() method aren't the same variables referenced in Main. There are 2 ways to resolve this, based on the need of the class:

1. As stated in other answers, pass the values by reference in the SetValues method (this preserves your code as-is):

 private static void setValues(string[] arr, ref string test) { arr[1] = "BBB"; test = "222"; } 

2. Move the variable declaration outside of the body of Main (This changes the scope of the variables to Class-level variables, which scopes them to the entire class, and possibly allows accessing them from other classes):

 public class Program { static string[] arr = new string[10]; static string test = "111"; static void Main(string[] args) { arr[1] = "AAA"; 

and then call it by reference as well:

SetValues(arr, ref test);