Узнать имя пользователя

string userName = System.Security.Principal.WindowsIdentity.GetCurrent().Name;

ManagementObjectSearcher searcher = new ManagementObjectSearcher("SELECT UserName FROM Win32_ComputerSystem");
ManagementObjectCollection collection = searcher.Get();
string username = (string)collection.Cast<ManagementBaseObject>().First()["UserName"];

public static string GetSessionUserName()
{
    int sessionId = Process.GetCurrentProcess().SessionId;
    IntPtr ptrBuffer;
    uint bytesReturned;
    if (WTSQuerySessionInformation(IntPtr.Zero, sessionId, WTSInfoClass.WTSUserName, out ptrBuffer, out bytesReturned) && ptrBuffer != IntPtr.Zero)
    {
        string username = Marshal.PtrToStringAnsi(ptrBuffer);
        WTSFreeMemory(ptrBuffer);
        if (WTSQuerySessionInformation(IntPtr.Zero, sessionId, WTSInfoClass.WTSDomainName, out ptrBuffer, out bytesReturned) && ptrBuffer != IntPtr.Zero)
        {
            username = Marshal.PtrToStringAnsi(ptrBuffer)+"\\"+username;
            WTSFreeMemory(ptrBuffer);
        }
        return username;
    }
 
    return null;
}
 
[DllImport("Wtsapi32.dll")]
static extern bool WTSQuerySessionInformation(IntPtr hServer, int sessionId, WTSInfoClass wtsInfoClass, out IntPtr ppBuffer, out uint bytesReturned);
 
[DllImport("wtsapi32.dll", ExactSpelling=true, SetLastError=false)]
static extern void WTSFreeMemory( IntPtr memory );
 
enum WTSInfoClass
{
    /// <summary>
    /// A null-terminated string that contains the name of the user associated with the session.
    /// </summary>
    WTSUserName = 5,
 
    /// <summary>
    /// A null-terminated string that contains the name of the domain to which the logged-on user belongs.
    /// </summary>
    WTSDomainName = 7
}