How Do I Prevent A User From Entering A Same Text Twice In A Textbox In Vb.Net?

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I wanted to fix this problem that i encountered. Basically what i want is to let a user enter certain string in a textbox and then check if the string entered by the user has been entered before by the user. If he entered it before then it will give an error message like ("Input has been previously entered") else it does what is supposed to be done. I am stuck and i want to fix this part. Can anyone suggest me an idea or something that i can use?
Thanks

Posted 13-May-15 14:10pm

Surajit Das

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Sergey Alexandrovich Kryukov 13-May-15 20:22pm

What have you tried so far?
—SA

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Sergey Alexandrovich Kryukov · Answer 1 · 2015-05-13T14:21:00

For example, you can keep all entered texts in the instance of the System.Collections.Generic.HashSet<string>:
https://msdn.microsoft.com/en-us/library/bb359438%28v=vs.110%29.aspx[^].

You need to check of the string is already in set and, if it is not, add a new one. This collection types provides time complexity of O(1).

If you want to persist the set of string, you can save it in file and restore back on application start.
It could be as simple as plain text file (for strings without end-of-line characters) or some XML, something like that.

—SA