Use below code to encode the query string using AntiXSS library for it you need to refer the antixss library in your code.
What is Antixsslibrary ?
It helps you to protect your code from cross site scripting. It helps to encode the text.
Cross site scripting (XSS) occurs mostly on due following .
1. When Dynamically data changing.
2. Passing the data through query string.
3. Displaying data to UI which is allowing to insert from user (they can insert the script aswell)
4. Showing any messages to UI.