namespace support in unity

Hi, when I tried to use "using System.Windows;" & "using System.Windows.Forms;" in a C# script file, an error occurred says like "Windows does not exist in System" "are you missing an assembly or directive", but it is OK when I use "using System.IO;" for example.

Why?

Although System.Windows.Forms is a name space available in .NET 1.1

What .NET version Unity supports ? and is it a full support?

the issue is that I want to use OpenFileDialog. I tried filebrower by AngryAnt but with errors like "DirectoryInfo in unknown identifier"

Thanks

Edit (clarifications):

Thanks I mentioned that I want to use OpenFileDialog (which belongs to System.Windows.Forms according to MSDN website ),so that this (at runtime) gives the user the ability to browse,for example, his harddisk and choose a file such as an mp3 and play it during the game. I knew that Unity is using Mono framework, However I meant in the question the .net functionality support. I searched in C:\Program Files\Unity\Editor\Data\Frameworks\Mono.framework,found System.Windows.Forms.dll and did not found System.IO.dll.However,no error occurred when using System.IO,but an error occurred when using System.Windows.Forms.dll .