SQLCLR File System Access

I have just begun to delve into the SQLCLR objects in SQL Server 2005. I have created a simple VB function as a SQLFunction. Here's the code:

Imports System

Imports Microsoft.SqlServer.Server

Partial Public Class UserDefinedFunctions

<Microsoft.SqlServer.Server.SqlFunction()> _

Public Shared Function FileDate(ByVal FilePath As String) As Date

Return FileDateTime(FilePath)

End Function

End Class

The function is run with this T-SQL:

declare @FileDate datetime

select @FileDate = dbo.FileDate('C:setup.bat)

select @FileDate

This works fine, but I had to set the Permission Level of the Visual Studio project to "Unsafe" rather than "External" in order to avoid the following error message:

A .NET Framework error occurred during execution of user defined routine or aggregate 'FileDate':

System.Security.SecurityException: Request for the permission of type 'System.Security.Permissions.SecurityPermission, mscorlib, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089' failed.

Is there any way to avoid having to set the permission level to "Unsafe"? If not, what does "unsafe" mean in terms of overall system security? Is it acceptable to use on an internal LAN?

Thanks,

Ron