Ways to synchronize interface and implementation comments in C#

You can do this quite easily using the Microsoft Sandcastle (or NDoc) inheritdoc tag. It's not officially supported by the specification, but custom tags are perfectly acceptable, and indeed Microsoft chose to copy this (and one or two other tags) from NDoc when they created Sandcastle.

/// <inheritdoc/>
/// <remarks>
/// You can still specify all the normal XML tags here, and they will
/// overwrite inherited ones accordingly.
/// </remarks>
public void MethodImplementingInterfaceMethod(string foo, int bar)
{
    //
}

Here is the help page from the Sandcastle Help File Builder GUI, which describes its usage in full.

(Of course, this isn't specifically "synchronisation", as your question mentions, but it would seem to be exactly what you're looking for nonetheless.)

As a note, this sounds like a perfectly fair idea to me, though I've observed that some people think you should always respecify comments in derived and implemented classes. (I've actually done it myself in documenting one of my libraries and I haven't see any problems whatsoever.) There's almost always no reason for the comments to differ at all, so why not just inherit and do it the easy way?

Edit: Regarding your update, Sandcastle can also take care of that for you. Sandcastle can output a modified version of the actual XML file it uses for input, which means you can distribute this modified version along with your library DLL instead of the one built directly by Visual Studio, which means you have the comments in intellisense as well as the documentation file (CHM, whatever).

If you're not using it already, I strongly recommend a free Visual Studio addon called GhostDoc. It eases the documentation process. Have a look at my comment on a somewhat related question.

Although GhostDoc won't make the synchronization automatically, it can help you with the following scenario:

You have a documented interface method. Implement this interface in a class, press the GhostDoc shortcut key, Ctrl-Shift-D, and the XML comment from the interface will be added to the implemented method.

Go to the Options -> Keyboard settings, and assign a key to GhostDoc.AddIn.RebuildDocumentation (I used Ctrl-Shift-Alt-D).

Now, if you change the XML comment on the interface, just press this shortcut key on the implemented method, and the documentation will be updated. Unfortunately, this doesn't work vice-versa.

I usually write comments like this:

/// <summary>
/// Implements <see cref="IMyInterface.Foo(string, int)"/>
/// </summary>
/// <returns></returns>

The methods are used only by the interface, so this comment is not even shown in tooltips when coding.

Edit:

If you want to see docs when you call the class directly and not using the interface, you need to write it twice or use a tool like GhostDoc.