Why the last line in this code do not work?

Question

static void Sqr(ref int x)
{
x = x * x;
}
static void Main()
{
int a = 3;
Sqr(ref a);
Console.WriteLine(a); // Outputs 9
}
.......................................................................
I have tried to write the last line as:
Console.WriteLine(Sqr(a));
But I received this error:
Argument 1: cannot convert from 'void' to 'bool'
why?

Answer 1

Your function didn't return anything so has no value to display. This would work:
static int Sqr(ref int x)
{
x = x * x;
return x;
}
static void Main()
{
int a = 3;
Console.WriteLine(Sqr(ref a));
}