C#范围内随机取值，不重复且全部取完

static void Main(string[] args)
{
List<string> arr = new List<string>();
arr.Add("red");
arr.Add("green");
arr.Add("blue");
arr.Add("orange");
arr.Add("white");
List<string> a = new List<string>();

foreach (var ar in arr)
{
bool flag = true;
while (flag)
{
string s = GetRandom(arr);
if (a.Contains(s))
{
}
else
{
a.Add(s);
flag = false;
}
}
}
}
public static string GetRandom(List<string> arr)
{
Random ran = new Random();
int n = ran.Next(arr.Count);

return arr
;
}