The only time a textbox has a BorderColor property is when it's a web-based control, and that means you are doing the validation in the wrong place: you should do this in Javascript at the Client, not in C# at eth Server - otherwise every time the user types in the TextBox, you have to do a round trip to the server!
That's slow, and can really slug your UI! It also can load your server considerably if you have a lot of users.
Always do all primary validation at the client, not the server.
For non web-based controls, you have to get more creative:
c# - Change the borderColor of the TextBox - Stack Overflow[
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