FileInfo..ctor(string fileName) throwing exception: bug in SL 4.0 or .NET 4.0?

The following test case passes in .NET 4.0:

var fiT = new FileInfo("myhappyfilename");
Assert.IsNotNull(fiT);

... but fails in Silverlight 4.0 with the following error:

System.ArgumentNullException: Value cannot be null.
Parameter name: format
   at System.String.Format(IFormatProvider provider, String format, Object[] args)
   at System.Environment.GetResourceString(String key, Object[] values)
   at System.IO.FileSecurityState.EnsureState()
   at System.IO.FileInfo.Init(String fileName, Boolean checkHost)
   at System.IO.FileInfo..ctor(String fileName)

Either the failure is a bug in SL 4.0, or the non-failure is a bug in .NET 4.0. Anyone know which it is?

(For the record, I'm running SL 4.0 on VS 2010 RC, which may be contributing to the problem).

See the MSDN documentation for FileInfo for Siverlight 4:

When it is called by an elevated-trust application, provides instance methods for the creation, copying, deletion, moving, and opening of files, and aids in the creation of FileStream objects. This class cannot be inherited.

Chances are your application isn't running with elevated trust. If you want to access those restricted methods, it'll need to be.

As to why it's returning a null - that may well be a bug, possibly an improperly propagated SecurityException. Then again, it may be as designed - the docs are also still pre-release. EDIT: gabe's answer is most likely correct on this point.

Since you generally can't access the filesystem from Silverlight (you need a fully-trusted OOB app), it looks like SL4 is trying to throw an exception, but is failing because the text for that exception isn't available in the SL4 beta. Presumably you would get the correct exception once SL4 is released.