ASPX file extension is used to store ASP.NET server-side code. ASPX files are stored on the server and executed when web application requests them. ASPX file contain instructions which are interpreted by ASP.NET framework. ASP.NET framework was created by Microsoft to facilitate development of web server applications based on .NET framework. ASP.NET framework allows developers to write software code in any language supported by .NET framework. These include C#, Visual Basic.NET, C++/CLI, F# and many more.
Typically, users of web applications are not supposed to open ASPX files. Even when ASPX file is visible in the URL of the web applications it is not supposed to be downloaded by the browser. Instead it is executed on the web server and HTML code is delivered to user inside the browser. If due to some reason ASPX file gets downloaded instead it typically means that server malfunctioned or there is a configuration error on the server.
ASPX file is a plain text code file. To open ASPX file and view its contents you can use any text editor. You can also use development environment such as Microsoft Visual Studio to open it. But unless you want to make changes to the ASPX file and debug it opening it in Visual Studio may be an overkill.
Typically, ASPX file is accompanied by codebehind file with functions used by ASPX code. File extension of codebehind file depends on the language which was used to write ASP.NET server application. If C# was used, then file will have ASPX.CS file extension. If Visual Basic .NET was used, then ASPX.VB file extension will be used.
Typically, ASPX files could not be executed outside of the server environment. That is why accidentally downloaded ASPX file doesn’t pose danger to end user. Still, it is possible for hackers to use ASPX files with malicious intent by uploading such file to a target server and making it execute compromised code.