What is an ASMX file?
A file with .asmx extensions is an ASP.NET Web Service file that provides communication between two objects over the internet using the Simple Object Access Protocol (SOAP). It is deployed as a service on the Windows-based Web Server to process incoming request and return the response. Unlike ASPX files which contain the code for visual display of ASP.NET webpages, ASMX files run at the server in background and perform different tasks such as connecting to database, retrieving data, and returning it in a format in which the request was made. These are used specifically for XML webs services.
ASMX File Format
ASMX files are in plain text format and can be opened or edited in applications such as Microsoft Visual Studio or text editors. It is a proprietary file format of Microsoft and has a well-defined syntax for creation of web services. A response by an ASMX file in the form of SOAP XML has the following elements.
Envelop- A root element that identifies the XML document as a SOAP message.Header- An optional element that contains application-specific information such as authentication data. If the Header element is present it must be the first child element of the Envelope element.Body- Contains the SOAP message intended for the recipient.Fault- An optional element that’s used to indicate error messages. If the Fault element is present, it must be a child element of the Body element.
ASMX files can be written in .NET languages such as C#, Visual Basic or JScript.
How is ASMX different than ASPX and ASCX?
ASMX files are different than ASPX and ASCX files.
- ASPX, Active Server Pages, files are programming files that are generated using Microsoft ASP.NET framework running on web servers. These are rendered in client’s web browser when user requests to access such a page.
- ASCX, Active Server User Control, defines user controls that are used to define reusable controls in ASP.NET web pages or entire website.
Key Characteristics of the ASMX File Format
| Characteristic | Description |
|---|---|
| File-Based Service Definition | Each ASMX file typically corresponds to a single web service class, though code-behind models separate declaration from implementation. |
| IIS Hosting Requirement | ASMX services must be hosted within Internet Information Services (IIS), limiting deployment flexibility. |
| Heavy XML Overhead | All communication uses verbose XML formatting, resulting in larger message sizes compared to JSON-based alternatives. |
| Automatic Serialization | The framework handles XML serialization/deserialization of .NET objects automatically through XmlSerializer. |
| Code-Behind Model | Supports separation of service declaration (.asmx) from implementation (.asmx.cs or .asmx.vb). |
| ASP.NET Pipeline Integration | Fully participates in the ASP.NET HTTP pipeline with access to HttpContext and application events. |
| Strong Typing | Service methods use strongly-typed parameters and return values with automatic type mapping. |
FAQ
Q1: What does ASMX stand for in web development?
A: ASMX stands for Active Server Methods Extended, representing ASP.NET’s original framework for creating XML-based web services.
Q2: Are ASMX web services still supported by Microsoft?
A: While they still function in .NET Framework, Microsoft deprecated ASMX in 2007 in favor of WCF and newer technologies.
Q3: Can ASMX services return JSON instead of XML?
A: Yes, through configuration changes and ScriptService attributes, but they remain primarily XML/SOAP-focused.
Q4: What is the main alternative to ASMX in modern .NET development?
A: ASP.NET Web API (for .NET Framework) and ASP.NET Core Web API (for cross-platform) are the primary modern alternatives.
Q5: How do ASMX services differ from WCF services?
A: WCF offers more transport protocols, better security, and hosting flexibility, while ASMX is simpler but limited to HTTP.
