Set your Locale and Culture to Sri Lankra - either in the system configuration or in your app:
CultureInfo.CurrentCulture Property (System.Globalization) | Microsoft Docs[
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But ... generally speaking it's a poor idea: you should be using the user's preferred culture rather than forcing one upon him, and changing the Culture can affect number display formats, how dates are shown, and so forth as well as the currency.
It is possible to do this on a "string by string" basis, but it's a fair amount of work:
c# - How to get specific culture currency pattern - Stack Overflow[
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