Supporting user-specified file extensions in custo

I'm working on a custom Visual Studio language service, and have several questions regarding the way file extensions are bound to a particular language service.

Source files for the language "Example Language" has two primary file extensions: .e1 and .e2. My extension has a class ExampleLanguagePackage which extends Package.

When you use the File → Open command and select a C# file (for example), the "Open" button has a dropdown arrow which allows you to select "Open With...". When you click that button, you are presented with options to open the file in the "CSharp Editor (Default)", "CSharp Editor with Encoding", or any of several other options. How can I provide a similar feature for my language, offering "Example Language (Default)" and "Example Language with Encoding" options?
When you open Tools → Options... → Text Editor → File Extension, you have the ability to bind (for example) the .foo extension to "Microsoft Visual C#" or any of several other options. How can I extend this page to allow user-defined file extensions to be associated with the "Example Language"?
What else should I watch out for when registering these items?

Most of these items are addressed by adding a custom implementation of IVsEditorFactory for your language and using a combination of registration attributes to register it. The actual implementation of this interface is beyond the scope of this question, but the documentation for the interface itself (and linked to that page), along with an example DjangoEditorFactory implementation in the Python Tools for Visual Studio project helped me with my initial implementation.

To support the Example language, I will make the following assumptions.

You've implemented an abstract class ExampleEditorFactory which provides the core implementation of IVsEditorFactory. The class should have a protected constructor with a bool argument to specify whether the factory should prompt the user for an encoding (similar to one of the constructors of the DjangoEditorFactory).
- You have a class ExampleEditorFactoryWithoutEncoding which extends ExampleEditorFactory and constructs the base class specifying false for the promptForEncoding argument. This class should be marked with the [Guid] attribute.
- You have a class ExampleEditorFactoryWithEncoding which extends ExampleEditorFactory and constructs the base class specifying true for the promptForEncoding argument. This class should be marked with the [Guid] attribute.
You have added the following entries to your VSPackage.resx resources file. The constants can be changed, but be aware that I have used the constant values 101 and 102 below.
- 101 = Example Language
- 102 = Example Language with Encoding

Registering the editor factories

The first thing to do is register your editor factories. This is done in two parts.

First, use the ProvideEditorFactoryAttribute. This attribute associates a resource identifier for the display name of the factory with the factory type itself.

[ProvideEditorFactory(typeof(ExampleEditorFactoryWithoutEncoding), 101)]
[ProvideEditorFactory(typeof(ExampleEditorFactoryWithEncoding), 102)]

Next, in the Initialize method of ExampleLanguagePackage, add calls to RegisterEditorFactory after you call base.Initialize().

protected override void Initialize()
{
    base.Initialize();

    RegisterEditorFactory(new ExampleEditorFactoryWithoutEncoding(this));
    RegisterEditorFactory(new ExampleEditorFactoryWithEncoding(this));
}

Associate a logical view with the editor factories

I haven't found all the information I wanted about the use cases for the ProvideEditorLogicalViewAttribute attribute, but it's important to include at least the following. Make sure to register the logical view(s) with both factories you created.

[ProvideEditorLogicalView(typeof(ExampleEditorFactoryWithoutEncoding), VSConstants.LOGVIEWID.TextView_string)]
[ProvideEditorLogicalView(typeof(ExampleEditorFactoryWithEncoding), VSConstants.LOGVIEWID.TextView_string)]

If this step is not done, the feature where double clicking in the output window can take you to a line of code will not work as expected. For example, suppose the output window contained a line like the following.

c:\dev\file.e1(14,3): unexpected expression

Associating the TextView logical view allows the IDE to use your factory when you double click on this output line to take you to line 14, column 3 of the file c:\dev\file.e1. Otherwise it will use a different factory to open a new copy of your document, and the new window will likely be missing many features.

Associate the standard file extensions `.e1` and `.e2` with the editor factories

This step provides the "Open With..." support for .e1 and .e2 files described in the original question 1. This step is accomplished with the ProvideEditorExtensionAttribute attribute.

The default priority for the primary factory appears to be 50. The factory with explicit encoding should have a priority less than this, and 49 appears to be a good choice. Note that there is no need to specify the NameResourceID named parameter because it was already specified by the ProvideEditorFactoryAttribute usage above (the generated registry keys are identical).

[ProvideEditorExtension(typeof(ExampleEditorFactoryWithoutEncoding), ".e1", 50)]
[ProvideEditorExtension(typeof(ExampleEditorFactoryWithoutEncoding), ".e2", 50)]
[ProvideEditorExtension(typeof(ExampleEditorFactoryWithEncoding), ".e1", 49)]
[ProvideEditorExtension(typeof(ExampleEditorFactoryWithEncoding), ".e2", 49)]

Associate the `.*` extension with the editor factories

This step provides the "Open With..." support for all other files, and adds support for the File Extension options described in the original question 2. This step also uses the ProvideEditorExtensionAttribute attribute, but uses a much lower priority value to ensure the default editors for other file types are not overridden by the setting. Like in the previous step, the factory with explicit encoding is given a lower priority.

[ProvideEditorExtension(typeof(ExampleEditorFactoryWithoutEncoding), ".*", 2)]
[ProvideEditorExtension(typeof(ExampleEditorFactoryWithEncoding), ".*", 1)]

Final notes

This answer does not cover several details.

(High importance for Visual Studio 2012) If you are planning to support Visual Studio 2012, please see the topic Cannot get custom editor to use the provisional tab for details on the new (and otherwise poorly documented) property ProvideEditorFactoryAttribute.CommonPhysicalViewAttributes.
(Unknown importance) The ProvideEditorFactoryAttribute does not support specifying the LinkedEditorGUID value (search down the page). This value would normally be added to the registration of ExampleEditorFactoryWithEncoding, and the value would be the GUID of ExampleEditorFactoryWithoutEncoding.