XML document validation

Applies to: yesVisual Studio noVisual Studio for Mac noVisual Studio Code

The XML editor checks XML 1.0 syntax and also performs data validation as you type. The editor can validate using a document type definition (DTD) or a schema. Red wavy underlines highlight any XML 1.0 well-formed errors. Blue wavy underlines show semantic errors based on DTD or schema validation. Each error has an associated entry in the error list. You can also view the error message by pausing the mouse over the wavy underline.

Schemas used in validation are found by matching the targetNamespace of a compiled schema with the element's xmlns declaration. Compiled schemas are loaded from one of the following locations, listed in order of priority:

  • From the file name specified in the Schemas field of the document Properties window.

  • An inline schema or DTD.

  • An external DTD or an xsd:schemaLocation and xsd:noNamespaceSchemaLocation attribute

  • An "x-schema" XDR schema namespace URI.

Schemas can also be found in the following additional locations when the schema has a non-empty target namespace:

  • Another editor window that contains the schema.

  • A schema in the current solution.

  • A schema from the schema cache directory.

XSLT files

When editing an XSLT file, the xslt.xsd file located in the schema cache is used for validation. Validation errors are shown as blue wavy underlines. Errors from the XSLT compiler are shown as red wavy underlines.

XML schema (XSD) files

When editing an XML Schema file, the xsdschema.xsd file located in the schema cache is used for validation. Validation errors are shown as blue wavy underlines. Any compilation errors are also shown with red wavy underlines.

Entity Reference Limit

DTD processing limits the number of entity references by default to 10,000 references and can accomodate most XML schemas. The error message in Visual Studio may read "Exceeded entity references limit for filename".

If you encounter this limitation in processing an XML document and wish to extend the validator to a larger schema, this can be changed with the MaxNumberOfDtdEntityReferences Visual Studio registry key. See editing the registry for a Visual Studio instance for more information on making this change. Please note that this applies to all XML documents opened by the user on that machine.

See also