Consider the following code in which i add nodes to a treeview control by reading an MS Access database.
Obviously, if I execute in a seperate thread I will get exception about control accessed from a thread other than the one in which it is created.
Now whenever I am adding one node to the tree I could marshel it to the GUI thread like this - void addnode(string textofnode)
-
{
-
}
The problem is that I have got some nodes which are parents of child nodes.
So we create a node and add all children of it and finally add it to treeview.
If i create the children in worker thread and send the parent to GUI thread to add it to the treeview I guess I wont be able to access those children from GUI thread(coz they are cerated in the worker thread)
How would i get around this problem? - TreeNode extensions = new TreeNode("Extensions");
-
treeexchange.Nodes.Add(extensions);
-
-
DataTable dtgroups = new DataTable("Groups"); //get groups
-
OleDbDataAdapter adp = new OleDbDataAdapter("select * from t_extensiongroups", @"Provider=Microsoft.Jet.OLEDB.4.0;Data Source=" + Application.StartupPath + @"\exchangesandtrunks.mdb;Persist Security Info=False");
-
adp.Fill(dtgroups);
-
-
for (int i = 0; i < dtgroups.Rows.Count; i++)//for all the groups
-
{
-
//create a groupnode with the group name in it
-
TreeNode group = new TreeNode(dtgroups.Rows[i][0].ToString());
-
-
//get no of extensions corresposnding to that group
-
adp = new OleDbDataAdapter("select * from t_extensions where group_name='" + dtgroups.Rows[i]["groups"].ToString() + "'", @"Provider=Microsoft.Jet.OLEDB.4.0;Data Source=" + Application.StartupPath + @"\exchangesandtrunks.mdb;Persist Security Info=False");
-
DataTable dtextensions = new DataTable("Extensions");
-
adp.Fill(dtextensions);
-
-
//add all the extension nodes to the groupnode
-
for (int j = 0; j < dtextensions.Rows.Count; j++)
-
{
-
group.Nodes.Add(dtextensions.Rows[j]["name"].ToString() + " (" + dtextensions.Rows[j]["number"].ToString() + ")");
-
}
-
-
//add the group
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extensions.Nodes.Add(group);
-
}
-
-
//create a trunk node
-
TreeNode trunks = new TreeNode("Trunks");
-
-
//get trunks
-
adp = new OleDbDataAdapter("select connected_trunk,name from t_trunk_lines", @"Provider=Microsoft.Jet.OLEDB.4.0;Data Source=" + Application.StartupPath + @"\exchangesandtrunks.mdb;Persist Security Info=False");
-
DataTable dttrunks = new DataTable("Trunks");
-
adp.Fill(dttrunks);
-
-
//add all trunks to the trunk node
-
for (int i = 0; i < dttrunks.Rows.Count; i++)
-
{
-
trunks.Nodes.Add(dttrunks.Rows[i]["name"].ToString() + " (" + dttrunks.Rows[i]["connected_trunk"].ToString() + ")");
-
}
-
-
//add the trunk node
-
treeexchange.Nodes.Add(trunks);
-
-
//add the pbx node
-
treeexchange.Nodes.Add("My PBX");
-
-
treeexchange.Nodes.Add("User");
16  2715 
Your code is unformatted and very difficult to read.
When updating controls on a WinForm from a thread, use a derivative of Control.Invoke to update the control.
A very safe way to update controls on a WinForm is to spawn the thread using a BackgroundWorke r component.
Here is a great article discussing thread updating WinForms: Threading in Windows Forms.
Here is the formatted code : - treeexchange.Nodes.Clear();
-
TreeNode extensions = new TreeNode("Extensions");
-
treeexchange.Nodes.Add(extensions);
-
-
DataTable dtgroups = new DataTable("Groups"); //get groups
-
OleDbDataAdapter adp = new OleDbDataAdapter("select * from t_extensiongroups", @"Provider=Microsoft.Jet.OLEDB.4.0;Data Source=" + Application.StartupPath + @"\exchangesandtrunks.mdb;Persist Security Info=False");
-
adp.Fill(dtgroups);
-
-
for (int i = 0; i < dtgroups.Rows.Count; i++)//for all the groups
-
{
-
//create a groupnode with the group name in it
-
TreeNode group = new TreeNode(dtgroups.Rows[i][0].ToString());
-
-
//get no of extensions corresposnding to that group
-
adp = new OleDbDataAdapter("select * from t_extensions where group_name='" + dtgroups.Rows[i]["groups"].ToString() + "'", @"Provider=Microsoft.Jet.OLEDB.4.0;Data Source=" + Application.StartupPath + @"\exchangesandtrunks.mdb;Persist Security Info=False");
-
DataTable dtextensions = new DataTable("Extensions");
-
adp.Fill(dtextensions);
-
-
//add all the extension nodes to the groupnode
-
for (int j = 0; j < dtextensions.Rows.Count; j++)
-
{
-
group.Nodes.Add(dtextensions.Rows[j]["name"].ToString() + " (" + dtextensions.Rows[j]["number"].ToString() + ")");
-
}
-
-
//add the group
-
extensions.Nodes.Add(group);
-
}
-
-
//create a trunk node
-
TreeNode trunks = new TreeNode("Trunks");
-
-
//get trunks
-
adp = new OleDbDataAdapter("select connected_trunk,name from t_trunk_lines", @"Provider=Microsoft.Jet.OLEDB.4.0;Data Source=" + Application.StartupPath + @"\exchangesandtrunks.mdb;Persist Security Info=False");
-
DataTable dttrunks = new DataTable("Trunks");
-
adp.Fill(dttrunks);
-
-
//add all trunks to the trunk node
-
for (int i = 0; i < dttrunks.Rows.Count; i++)
-
{
-
trunks.Nodes.Add(dttrunks.Rows[i]["name"].ToString() + " (" + dttrunks.Rows[i]["connected_trunk"].ToString() + ")");
-
}
-
-
//add the trunk node
-
treeexchange.Nodes.Add(trunks);
-
-
//add the pbx node
-
treeexchange.Nodes.Add("My PBX");
-
-
treeexchange.Nodes.Add("User");
I will make two suggestions, but I have not written an example to prove they will work correctly. Let me know if they do not.
Suggestion #1: - Instantiate the initial TreeView in the GUI.
- Pass it as a parameter to a BackgroundWorke r thread (also created in the GUI).
- Have the thread fill in the nodes (your posted code).
- Handle the RunWorkerComple ted event to update the actual display of the filled-in TreeView.
- How To: Run an Operation in the Background.
Suggestion #2:
If you still receive a thread access violation after implementing the above, then add a helper method to your GUI which will clone the nodes created in the thread to the nodes in your GUI TreeView. This can be a simple recusion loop and should run quickly, since you have already performed the data access part in your thread.
However, I truly believe the second step will not be necessary. If you spawn a BackgroundWorke r thread from the GUI, it is guaranteed to execute on the same thread as the GUI itself. It is provided in the Component namespace specifically to eliminate the cross-threading issues associated with WinForms.
@mldisibio
Actually the task you specify to DoWork is run on a separate thread. That's why the interface remains responsive. The key thing is to make sure that you don't do any GUI updates from DoWork. If intermediate GUI updates are required as in this case then the OnProgressChang ed method should be used which raises the ProgressChanged event. This allows you to queue the interface updates to the UI interface thread.
I'm guessing you want something alone the lines of : -
delegate void AddNodeDelegate(string nodeName);
-
-
void AddNode(string nodeName)
-
{
-
if(treeexchange.InvokeRequired)
-
{
-
AddNodeDelegate addDelegate = new AddNodeDelegate(AddNode);
-
this.Invoke(addDelegate, new object[] { nodeName });
-
}
-
else
-
{
-
this.treeexchange.Nodes.Add(nodeName);
-
}
-
}
-
Your alternative (which I don't like)... is to not do the work in a background thread at all. And put the following in appropriate places to refresh your GUI.
.... And yes, as you say, interface updates must be done via the RunWorkerComple ted event, and optionally the ProgressChanged event if desired.
@mldisibio
If I add nodes in the BackgroundWorke r thread then will I be able to access them from the main GUI thread ?
In some code i am reading the Text of the nodes.
.... And yes, as you say, interface updates must be done via the RunWorkerComple ted event, and optionally the ProgressChanged event if desired.
Not precisely. The RunWorkerComple ted is used to update the interface when the background task has completed while the ProgressChanged is used for updating the interface while the background task is still running. It's a way for providing intermediate results of the ask to the interface.
EDIT:mldisibio, I'm terribly sorry that I have messed up your response above. I accidentally clicked the edit button when I wanted to click the quote button so in the end I ended up deleting the first and last parts of your post. Only the part that I quoted now remains!
Please feel free to post it again if you wish.
No problem. For the record I simply corrected my original post, agreeing with your response: the DoWork thread is indeed a separate thread, not the same as the UI thread, but spawing the BackgroundWorke r from the UI guarantees that the worker thread will be correctly marshalled back to the UI thread.
A great link on this subject from Alhahari: Threading in C# - Part 3 - Using Threads Sign in to post your reply or Sign up for a free account.
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