How to Open, Edit, & Convert ASMX Files

Screenshot of several ASMX files in Windows 10 that open with Microsoft Visual Studio


An abbreviation for Active Server Method File, a file with the ASMX file extension is an ASP.NET Web Service Source file.

Unlike ASP.NET web pages that use the .ASPX file extension, ASMX files function as a service that doesn't have a graphical user interface and instead is used to move data and perform other actions behind the scenes.

Be careful to not confuse ASMX and ASCX files. While they might appear similar, they're used for different reasons.

How to Open an ASMX File

ASMX files are files used with ASP.NET programming and can be opened with any program that codes in ASP.NET (like Microsoft's Visual Studio and Visual Web Developer).

You might also be able to use Windows Notepad or another free text editor to open the ASMX file for editing as a text file.

ASMX files are not intended to be viewed or opened by the browser. If you've downloaded an ASMX file and expected it to contain information (like a document or other saved data), it's likely that something is wrong with the website and instead of generating usable information, it provided this server-side file instead. Try renaming the file to the correct extension as a short term fix.

For example, if when trying to download a document in the PDF format, you instead get one with the .ASMX file extension, just delete the four letters after the period and replace them with .PDF.

If you find that an application on your PC does try to open the ASMX file but it's the wrong application or if you would rather have another installed program open ASMX files, see our article How to Change the Default Program for a Specific File Extension guide for making that change in Windows.

How to Convert an ASMX File

You might be able to use one of the Microsoft programs we mentioned above to convert an ASMX file to another format.

Here's some information on migrating ASP.NET Web Services to the Windows Communication Foundation (WCF) platform. This is useful if you need to use .NET 2.0 services under .NET 3.0.

You can learn how to create a Web Services Description Language (WSDL) file from an ASMX file with this WebReference guide.