string camId="300,50,65,52";
long[] campaignId=new long[]{300,50,65,52};

string camId = "300,50,65,52";
                long[] campaignId = camId.Split(new char[] { ',','，'}).Select(a=>long.Parse(a)).ToArray();
                /*
                         [0]    300    long
                        [1]    50    long
                        [2]    65    long
                        [3]    52    long

                 */

------解决方案--------------------
string camId="300,50,65,52";
long[] campaignId=camId.ToCharArray();  


------解决方案--------------------
这是赋给string[]数组的方法
C# code

string camId = "300,50,65,52";
string[] campaignId = null;
campaignId = camId.Split(',');