Functional side effects occur when functions

1. modify a global variable, or
2. modify one of its parameters

Modifying one of its parameters

Mutations made by the function will persist only if the parameters are passed by reference, which can be achieved using either the keyword ref or out.

For example,

int fun1(ref int num)
{
    num = 100;
    return 10;
}

void main()
{
    int a = 2;
    a = fun1(ref a) + a;
    Console.WriteLine(a);
}

The output will be 110

However, the variable will not be modified if there is no ref or out keyword:

int fun1(int num)
{
    num = 100;
    return 10;
}

void main()
{
    int a = 2;
    a = fun1(a) + a;
    Console.WriteLine(a);
}

The result will be 12

The difference between ref and out is that ref parameter must be initialized before it is passed; while out parameter do not have to be explicitly initialized before they are passed (Docs, 2018).

References

1. Docs, M. (2018, March 6). ref (C# Reference). Retrieved from https://docs.microsoft.com/en-us/dotnet/csharp/language-reference/keywords/ref