C# pass element of value type array by reference

Yes, that's absolutely possible, in exactly the same way as you pass any other variable by reference:

using System;

class Test
{
    static void Main(string[] args)
    {
        int[] values = new int[10];
        Foo(ref values[0]);
        Console.WriteLine(values[0]); // 10
    }

    static void Foo(ref int x)
    {
        x = 10;
    }
}

This works because arrays are treated as "collections of variables" so values[0] is classified as a variable - you wouldn't be able to do a List<int>, where list[0] would be classified as a value.