I think you're not getting a proper context setup before you call the invoke. Here's what I use for something similar. You'll need to set your own variables:
I'm using System.DirectoryServices.AccountManagement
to get the functions.
//Domain related info
string _DCToUse = "myserver.domain.local";
string _ADConDomain = "DC=domain,DC=local";
string _AdDomain = "domain";
string _ADAdminUser = "administrator";
string _ADAdminPass = "password";
//User specific
string _UserName = "jsmith";
string _CurrentPass = "oldPass";
string _NewPass = "newPass";
PrincipalContext principalContext =
new PrincipalContext(ContextType.Domain, _DCToUse,
_ADConDomain, _ADDomain+@"\"+_ADAdminUser, _ADAdminPass);
UserPrincipal user = UserPrincipal.FindByIdentity(principalContext, _UserName);
if (user == null)
{
string ADErrorMsg = "Couldn't find user, check your spelling.";
return Changed;
}
user.ChangePassword(oldPass, newPass);
user.Save();