Hi folks

I wanted to enforce the Users object (standard sf object) so that we can't have duplicate "Alias" entries since we key off certain reports and views based on the User Alias field.

I was able to write a basic trigger code and a basic public class to execute it. But when I try to write a testmethod for it, I am getting
all sorts of errors about not being able to do this and that with Users.

So, my trigger code is straight forward:

trigger UserAliasDuplicatePreventer on User (before insert, before update) {
EnsureUniqueUserData.checkUniqueUserAlias(Trigger.new);
}

And all it does is call on this apex class EnsureUniqueUserData and the method in it checkUniqueuserAlias.
And the logic works fine when I am just testing it out directly in sandbox by updating/creating new User entries with same Alias value.

public class EnsureUniqueUserData
{

public static void checkUniqueUserAlias (User[] users)
{
for (User u:users)
{
for (User Existusers : [select id,Alias,Name from User where Alias = :u.Alias])
{
// If there is a match
// System.assertEquals(Existusers.name,null);
u.addError('An User with this Alias already exists!');
}
}
}

static testmethod void testUniqueUserAlias()
{

User User1 = new User (Name = 'test user1', Alias = 'testu');
insert User1;
// make sure User is created
System.assert(User1.id != null);

// Now create a 2nd User entry that uses same Alias, it should fail
User User2 = new User (Name = 'test user2', Alias = 'testu');
insert User2;
// make sure User is created
System.assertEquals(User2.id,null);


// Now delete the 1st User entry with the testu alias and try again, it should work now

delete User1;
User User3 = new User (Name = 'test user3', Alias = 'testu');
insert User3;
// make sure User is created
System.assert(User3.id != null);
System.assertEquals(User3.Alias,'testu');

}


The problem is when I try to save the testmethod code, it complained with this:
Error: Compile Error: Field is not writeable: Name at line 20 column 34

I try to change my creation of user entry so that instead of:

User User1 = new User (Name = 'test user1', Alias = 'testu');
insert User1;

I just use:

User User1 = new User (Alias = 'testu');
insert User1;

But now it complains on the insert statement with message:
Error: Compile Error: DML not allowed on User at line 21 column 4


How  can I get this into production if the unit testing doesn't seem to be working properly (i.e. are there restrictions on the
Users object because that's how salesforce is essentially creating sf login)??

thx in advance for your help.