Converting string to double with specified precision value

I have  a textbox wherein user is allowed to enter the value of type (4,4) but that will be in string and while sending it to database I need to convert this value to double format. I have searched alot but alas no help .Can somebody provide some help in this..
 
for example my string is 12.244 and i need to convert it into double value 12.2440. 
 
Thanks   
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