[DllImport("OpenNetStream.dll")] 
public static extern int OpenSDK_Data_GetDevList(IntPtr szAccessToken, int iPageStart, int iPageSize,
                                                         ref IntPtr pBuf, ref int iLength);

//其中pBuf参数，为返回的一个byte[]数据，现在要将其转换为byte[],然后在转换为string

int size = 1024;
byte[] managedArray = new byte[size];
Marshal.Copy(pBuf, managedArray, 0, size);

//获得string类型
string json = System.Text.Encoding.Default.GetString(managedArray);