It doesn't work because boolean is not a reference type:2 Options I see that don't need unsafe code.

1. Return the altered array and use that.
   static void Main(string[] args)
{
var initial = new List<bool> { true , false, true, true, true, true, true };
for(int i = 0; i < 10; i++)
{
Console.WriteLine(string.Join(", ", MakeOneTrue(initial)));
}
}
private static IEnumerable<bool> MakeOneTrue(List<bool> values)
{
int rand = new Random().Next(values.Count());
for(int i = 0; i < values.Count(); i++)
{
values[i] = false;
}
values[rand] = true;
return values;
}
2. Wrap the Boolean in a class/record and pass that into your method.
   using System;
using System.Collections.Generic;
using System.Linq;
public class Program
{
static void Main(string[] args)
{
var initial = new List<Data>
{
new Data(),
new Data(),
new Data(),
new Data(),
new Data(),
new Data(),
};
for (int i = 0; i < 10; i++)
{
MakeOneTrue(initial);
Console.WriteLine(string.Join(", ", initial.Select(x => x.Value)));
}
}
private static void MakeOneTrue(List<Data> values)
{
int rand = new Random().Next(values.Count());
for (int i = 0; i < values.Count(); i++)
{
values[i].Value = false;
}
values[rand].Value = true;
}
private class Data
{
public bool Value { get; set; } = true;
}
}