c# 获取非托管指针长度

public List<string> GetPDFValues()
        {
            List<string> strs = new List<string>();
            unsafe
            {
                var len = 0;
                //调用c++ 返回char**
                var a = GaussianDistributionServiceProxy.GetPDFValues();

                

                var cx = 0;
                //获取char**指针长度 start
                while (true)
                {
                    var result = Marshal.PtrToStringAnsi((IntPtr)(*(a + cx)));
                    if (result == null)
                    {
                        len = cx - 1;
                        cx = 0;
                        break;
                    }
                    cx++;

                }

         
       //获取char**指针长度 end

//char** 转换为List<string>
for (int i = 0; i < len; i++)
 {
   string s = Marshal.PtrToStringAnsi((IntPtr)a[i]);
   strs.Add(s); }
 }
 return strs; 
}