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Resolving External Resources

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The XmlResolver property of the XmlDocument is used by the XmlDocument class to locate resources that are not inline in the XML data, such as external document type definitions (DTDs), entities, and schemas. These items can be located on a network or on a local drive, and are identifiable by a Uniform Resource Identifier (URI). This allows the XmlDocument to resolve EntityReference nodes that are present in the document and validate the document according to the external DTD or schema.

Fully-Trusted XmlDocument

The XmlResolver property affects the functionality of the XmlDocument.Load method. The table below shows how the XmlDocument.XmlResolver property works when the XmlDocument object is fully trusted. The following table shows the XmlDocument.Load methods when the input to the Load is a TextReader, String, Stream, or URI. This table does not apply to the Load method if the XmlDocument is loaded from an XmlReader.

XmlResolver property Function Notes
The property is set to an XmlResolver class that has been previously created and has properties already set on it by the user. The XmlDocument uses the XmlResolver that is given to resolve file names, to resolve references to external resources such as DTDs, entities, and schemas.

The XmlResolver is also used when resolving external resources that are needed when adding or editing nodes in the XmlDocument.		 The XmlResolver given to the XmlDocument is the resolver that is used whenever external resources need to be located and resolved.
The property is set to null (Nothing in Microsoft Visual Basic .NET). Features that require an external resource are not supported, such as locating an external schema, or DTD. Nor will external entities be resolved, and performing editing functions, such as inserting entity nodes that require resolution, are not supported. The XmlDocument loads files as anonymous and does not attempt to resolve any other resources.
The property is not set, but left in its default state. An XmlUrlResolver with NULL credentials will be instantiated and used by the XmlDocument when resolving file names, locating external DTDs, entities, and schemas, and null credentials are used when editing nodes.

The following table shows the XmlDocument.Load method when the input to the Load is an XmlReader and the XmlDocument is fully trusted.

XmlResolver property Function Notes
The XmlResolver class used by the XmlDocument is the same class being used by the XmlReader. The XmlDocument uses the XmlResolver that was assigned to the XmlReader.

The XmlDocument.Resolver property cannot be set, regardless of the XmlDocument trust level, because it is getting an XmlResolver from the XmlReader. You cannot attempt to override the settings of the XmlReaders' XmlResolver by setting the XmlResolver property of the XmlDocument.		 The XmlReader can be the XmlTextReader, XmlValidatingReader, or a custom-written reader. If the reader used supports entity resolution, external entities are resolved. If the reader passed does not support entity references, then entity references are not resolved.

Semi-Trusted XmlDocument

The following table shows how the XmlDocument.XmlResolver property works when the object is semi-trusted. This table applies to the XmlDocument.Load methods when the input to the Load is a TextReader, String, Stream, or URI. This table does not apply to the Load method if the XmlDocument is loaded from an XmlReader.

XmlResolver property Function Notes
In the semi-trusted scenario, the XmlResolver property cannot be set to anything other than null. An XmlUrlResolver with null credentials will be instantiated and used by the XmlDocument when resolving file names, locating external DTDs, entities, and schemas, and null credentials are used when editing nodes. This behavior is identical to the behavior when the XmlResolver property is not set, but left in its default state.

The XmlDocument uses anonymous permissions for all actions.
The property is set to null (Nothing in Microsoft Visual Basic .NET). No features that require an external resource are supported, such as locating an external schema or DTD. Nor will external entities be resolved, and performing editing functions such as inserting entity nodes that require resolution, are not supported. When the property is null, the behavior is the same regardless if the XmlDocument is fully trusted or semi-trusted.
The property is not set, but left in its default state. An XmlUrlResolver with null credentials will be instantiated and used by the XmlDocument when resolving file names, locating external DTDs, entities, and schemas, and null credentials are used when editing nodes. The XmlDocument uses anonymous permissions for all actions.

This table applies to the XmlDocument.Load method when the input to the Load is an XmlReader, and the XmlDocument is semi-trusted.

XmlResolver property Function Notes
The XmlResolver class used by the XmlDocument is the same one being used by the XmlReader. The XmlDocument uses the XmlResolver that was assigned to the XmlReader.

The XmlDocument.Resolver property cannot be set, regardless of the XmlDocument trust level, because it is getting an XmlResolver from the XmlReader. You cannot attempt to override the settings of the XmlReaders XmlResolver by setting the XmlResolver property of the XmlDocument.		 The XmlReader can be the XmlTextReader, validating XmlReader, or a custom-written reader. If the reader used supports entity resolution, external entities are resolved. If the reader passed in does not support entity references, then entity references are not resolved.

Setting the XmlResolver to contain the correct credentials allows access to external resources.

Note

There is no way to retrieve the XmlResolver property. This helps to prevent a user from reusing an XmlResolver on which credentials have been set. Also, if an XmlTextReader or validating XmlReader is used to load the XmlDocument and the XmlDocument has a resolver that has been set, the resolvers from these readers are not cached by the XmlDocument after the Load phase, since this also presents a security risk.

For more information, see the Remarks section of the XmlResolver reference page.

See also