Creating a Cross-Process EventWaitHandle

Question

I have two windows application, one is a windows service which create EventWaitHandle and wait for it. Second application is a windows gui which open it by calling EventWaitHandle.OpenExisting() and try to Set the event. But I am getting an exception in OpenExisting. The Exception is "Access to the path is denied".

windows Service code

EventWaitHandle wh = new EventWaitHandle(false, EventResetMode.AutoReset, "MyEventName");
wh.WaitOne();

Windows GUI code

try
{
    EventWaitHandle wh = EventWaitHandle.OpenExisting("MyEventName");
    wh.Set();
}
catch (Exception ex)
{
    MessageBox.Show(ex.Message);
}

I tried the same code with two sample console application, it was working fine.

Solution

You need to use the version of the EventWaitHandle constructor that takes an EventWaitHandleSecurity instance. For example, the following code should work (it's not tested, but hopefully will get you started):

// create a rule that allows anybody in the "Users" group to synchronise with us
var users = new SecurityIdentifier(WellKnownSidType.BuiltinUsersSid, null);
var rule = new EventWaitHandleAccessRule(users, EventWaitHandleRights.Synchronize | EventWaitHandleRights.Modify,
                          AccessControlType.Allow);
var security = new EventWaitHandleSecurity();
security.AddAccessRule(rule);

bool created;
var wh = new EventWaitHandle(false, EventResetMode.AutoReset, "MyEventName", out created, security);
...

Also, if you're running on Vista or later, you need to create the event in the global namespace (that is, prefix the name with "Global\"). You'd also have to do this on Windows XP if you use the "Fast User Switching" feature.

Comments

[Navaneeth]

i logged in as administrator only and i am using windows XP

[Josh]

Good thinking, but I don't think Run as Administrator will help since the other process is running in an isolated session. For example, I don't believe Mutexes can be shared between a service in "session 0" and a logged on interactive session. I am no expert in this regard however.

[Josh]

Just found the following document which describes the session 0 impact in Vista. As Codeka mentions, you can share events this way if you explicitly use the Global\ scope. Here's the URL: microsoft.com/whdc/system/sysinternals/Session0Changes.mspx

[dthorpe]

Navaneeth: Ok, XP disqualifies process elevation. That only exists in Vista and Win7. (In Vista and Win7, you can be logged in as an admin user but your GUI apps still will not run with full admin privilege unless you go through the elevation confirmation security question)

[Ignacio Soler Garcia]

ahhh ... the Global prefix. I always forget it.

[c00000fd]

The access right must EventWaitHandleRights.FullControl when you create it from a service.