how to convert string DateTime to Date time object c#

Question

i have date and time in format ("MM/dd/yyyy , HH:mm:ss:fff")... so i want to get only time from this sting and also want to change it into DateTime Object... so how can i do this plz give me any right  solution,, i have tried lot of solution but every time i got this Error...

"String was not recognized as a valid DateTime"  plz give me the best and final solution