I came to this page while trying to figure out how to detect the framework versions installed on a server without access to remote desktop or registry, so Danny V's answer worked for me.
string path = System.Environment.SystemDirectory;
path = path.Substring( 0, path.LastIndexOf('\\') );
path = Path.Combine( path, "Microsoft.NET" );
// C:\WINDOWS\Microsoft.NET\
string[] versions = new string[]{
"Framework\\v1.0.3705",
"Framework64\\v1.0.3705",
"Framework\\v1.1.4322",
"Framework64\\v1.1.4322",
"Framework\\v2.0.50727",
"Framework64\\v2.0.50727",
"Framework\\v3.0",
"Framework64\\v3.0",
"Framework\\v3.5",
"Framework64\\v3.5",
"Framework\\v3.5\\Microsoft .NET Framework 3.5 SP1",
"Framework64\\v3.5\\Microsoft .NET Framework 3.5 SP1",
"Framework\\v4.0",
"Framework64\\v4.0"
};
foreach( string version in versions )
{
string versionPath = Path.Combine( path, version );
DirectoryInfo dir = new DirectoryInfo( versionPath );
if( dir.Exists )
{
Response.Output.Write( "{0}<br/>", version );
}
}