The SoapFormatter

The SoapFormatter class handles serialization of object graphs to SOAP format. It produces SOAP messages complete with the envelope, the body and, optionally, application specific header tags. In reverse, the SoapFormatter can turn a SOAP message into a live object that our applications can easily access. Message deserialization also allows access to any custom SOAP headers a message might contain.

Table The SoapFormatter constructors allow some degree of control to override default serialization behavior

Using the SoapFormatter in our applications is very simple. We have to add a reference to the System.Runtime.Serialization.Formatters.Soap library and our applications can instantiate SoapFormatter objects. In the Visual Studio.NET IDE we add references using the References menu in the Solution Explorer window. If we invoke the complier from a shell-script or the command-line we need to add the /r command-line option.

The syntax for serializing and deserializing objects with the SoapFormatter is very similar to the XmlSerializer but there are far fewer requirements on the types that the SoapFormatter can process. The XmlSerializer only processes publicly accessible members of a type and classes must provide a default constructor to instantiate objects the “regular” way. The behavior of the SoapFormatter is very different. It takes an accurate snapshot of the all fields, public and private, of the object being serialized. The generated XML contains the complete object graph, with all references intact. Even cyclical references are properly resolved and reflected in the output document. When the SOAP message is deserialized later on, the entire object graph is reconstructed and all relationships are restored. The .NET remoting architecture leverages this functionality to transmit objects from one process to another, regardless if they are running on the same computer or on a different one across the internet. This process is also referred to as Marshal-By-Value or MBV.

Permissions Required by the SoapFormatter

These powerful capabilities come at a price: Running the SoapFormatter requires a special security permission, the Serialization Formatter permission, because the formatter accesses private data members. The .NET security infrastructure by default only grants this permission to locally installed applications. Applications installed over a network or running from a WebServer are not considered to be trustworthy and cannot access to private data members. Starting an application linked against the SoapFormatter’s assembly even fails with a SecurityException if the Serialization Formatter permission is not granted to the application. Furthermore, the SoapFormatter checks if the active permission set includes the Serialization Formatter permission every time it executes. If this permission is not available it also throws a SecurityException.

Modifying permissions for the SoapFormatter

The.NET security model groups multiple permissions to a permission set. These permission sets can be granted at different levels: to different users running programs on a computer or dependent on whether an application is installed locally or was downloaded across a network, specifically the internet. By default only the permission set for fully trusted applications, i.e. locally installed applications, grants the permission to run the SoapFormatter.

Modifying the active permission set is a little bit tricky and you really need a basic understanding of the .NET code access security model. The .NET Framework supplies a tool to add a modify permission sets and apply them to pre-defined security zones. The tool is implemented as a MMC snap-in called mscorcfg.msc, which is located in the Microsoft.NET folder of your Windows installation directory. You can invoke it through the shortcut in the “Administrative Tools” section of the Windows Control Panel or directly from the command-line.

The tool allows you to create and edit your own permission sets, the system supplied ones are read-only. If you need to alter the default permissions you have to create your own permission set first. You can create one from scratch or by selecting “Duplicate” from the context menu. Once you have an editable permission set you can select “Change Permissions…” from the context menu, double-click on “Security” on the list of available permissions and modify the “Enable serialization formatter” option as shown in figure 13.2. Then click on OK and you the new permission set is ready.

Figure The Permission viewer dialog displays and allows editing a permission set. The Serialization Formatter permission has to be granted for runtime serialization to work. Be default this permission is disabled in untrusted security zones, e.g the Internet Zone.

Finally, you have to assign the new permission set to the Code Group active in your environment. Select the Code Group you would like to modify under the “Runtime Security Policy” node, click on the “Edit Code Group Properties” link on the right hand menu and assign your permission set on the Permission Set tab. This is enough of a trip into .NET security land, let’s go back to serialization.