The XSD file type is mainly used for XML Schema Definition files, which describe an XML document beyond the basic XML syntax. XSD files define the valid elements of the XML document, including the document's content, attributes and structure. These schema files store information as plain text, meaning they can opened with a basic text editor with little trouble. However, software designed specifically for editing XML and XSD is generally the better option for fine-tuning XSD documents.
XML Schema Editors
If you're looking for maximum control over your XSD document, or a more detailed visual breakdown of the file and its contents for easier editing, XML schema editors are your best bet. These editors feature specialized tools designed to work with XSD documents and maximize your control, while also providing alternatives to the standard text-only view of the document. Some examples of XML and XML schema editors include Microsoft Visual Studio, XMLSpy, Eclipse, Liquid XML Studio, Oxygen XML Editor, Stylus Studio, XMLFox, XML Seed, CAM Editor and the browser-based XMLGrid.
XML Syntax-Friendly Text Editors
As an alternative, you can also open and edit XSD documents in a text editor designed to handle XML syntax. These editors have less features than full XML and XSD editors but feature support for XML syntax. This puts them at the midpoint between full-featured editors and simple text editors. Some examples of these editors include XML Notepad and Notepad++.
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Since XML and XSD documents are plain-text, they can also be opened and edited in virtually any text editor. Generally, plain text editors are the better choice as the formatting of more sophisticated text editors can be cumbersome or intrusive. Notepad is generally the go-to option for plain-text XSD editing, although you can also use Wordpad or most other text editors.
There are also numerous options for Mac Users. Xmplify is a Mac-exclusive XML editor that also supports editing XSD documents. Oxygen XML Editor, CAM XML Editor, XML Seed and XMLGrid are also available on Mac OSX as well as Windows.