How to detect if machine is joined to domain?

Don't fool with pinvoke if you don't have to.

Reference System.DirectoryServices, then call:

System.DirectoryServices.ActiveDirectory.Domain.GetComputerDomain()

Throws an ActiveDirectoryObjectNotFoundException if the machine is not domain-joined. The Domain object that's returned contains the Name property you're looking for.

Can also be called by using system.net

string domain = System.Net.NetworkInformation.IPGlobalProperties.GetIPGlobalProperties().DomainName

If the domain string is empty the machine isn't bound.

Documentation on the property returned https://docs.microsoft.com/en-us/dotnet/api/system.net.networkinformation.ipglobalproperties.domainname?view=netframework-4.7.2#System_Net_NetworkInformation_IPGlobalProperties_DomainName

You can PInvoke to Win32 API's such as NetGetDcName which will return a null/empty string for a non domain-joined machine.

Even better is NetGetJoinInformation which will tell you explicitly if a machine is unjoined, in a workgroup or in a domain.

Using NetGetJoinInformation I put together this, which worked for me:

public class Test
{
    public static bool IsInDomain()
    {
        Win32.NetJoinStatus status = Win32.NetJoinStatus.NetSetupUnknownStatus;
        IntPtr pDomain = IntPtr.Zero;
        int result = Win32.NetGetJoinInformation(null, out pDomain, out status);
        if (pDomain != IntPtr.Zero)
        {
            Win32.NetApiBufferFree(pDomain);
        }
        if (result == Win32.ErrorSuccess)
        {
            return status == Win32.NetJoinStatus.NetSetupDomainName;
        }
        else
        {
            throw new Exception("Domain Info Get Failed", new Win32Exception());
        }
    }
}

internal class Win32
{
    public const int ErrorSuccess = 0;

    [DllImport("Netapi32.dll", CharSet=CharSet.Unicode, SetLastError=true)]
    public static extern int NetGetJoinInformation(string server, out IntPtr domain, out NetJoinStatus status);

    [DllImport("Netapi32.dll")]
    public static extern int NetApiBufferFree(IntPtr Buffer);

    public enum NetJoinStatus
    {
        NetSetupUnknownStatus = 0,
        NetSetupUnjoined,
        NetSetupWorkgroupName,
        NetSetupDomainName
    }

}