Downloading master contract from API server is a time consuming job, hence the GetMasterContract will return immediately and the files are downloaded in the background.
After calling GetMasterContract.
You should wait for the symbol download to finish.
In your code, SubscribeQuotes is called immediately.
You need to check the symbol_download_status property to confirm symbol download, then call all other methods.
upstox.GetMasterContract(); Do Threading.Thread.Sleep(1000); While ((upstox.symbol_download_status == false)); upstox.SubscribeQuotes("NSE", "RELIANCE");