How to get the root URL of my project?

Hi,
In my webform project of C# code, I need to use default root URL which contains port number, instead of hardcode the port number, I need to C# code to the default root URL. and then I can append the rest of specific sites.
For example: my URL is: http://localhost:3346/WebSite1/test1.aspx or http://localhost:3346/WebSite1/test2.aspx
I need the part of http://localhost:3346/WebSite1/ only, then I will append the specific site to it. Because the port number may change sometime, so I do not want to hard code this part. Anybody knows how to do it in c# code? thanks a lot,
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