Treeview node: "Object reference not set to an instance of an object"

Question

My TreeView control is displaying the structure of some selected hard disk drive. In my addToParentNode, I make a call from after the tree view is expanded. But when I pass the node from one method to another, an "Object reference not set to an instance of an object" exception is thrown.

void addToParentNode(TreeNode childNodes)
{
    DirectoryInfo getDir = new DirectoryInfo(childNodes.Tag.ToString());
    DirectoryInfo[] dirList = getDir.GetDirectories();
    foreach (DirectoryInfo dir in dirList)
    {
        TreeNode parentNode = new TreeNode();
        parentNode.Text = dir.Name;
        parentNode.Tag = dir.FullName;
        childNodes.Nodes.Add(parentNode);
    }
}

private void tv_fileExplore_AfterExpand(object sender, TreeViewEventArgs e)
{
    foreach (TreeNode item in e.Node.Nodes)
    {
        addToParentNode(item);
    }
}

Can someone point me in the right direction?

Please use the debugger to find out which variable or member is null at the time your application crashes. Also, a catch (Exception) is rather unwise; if there's an exception, that's because something is wrong and you shouldn't just generally swallow (and thereby hide) all exceptions that might occur. — O. R. Mapper
– O. R. Mapper, Commented Aug 26, 2012 at 12:20
So the DirectoryInfo getDir = new DirectoryInfo(childNodes.Tag.ToString()); the childNodes.Tag comes through as null. How should I grab that information? — jpavlov
– jpavlov, Commented Aug 26, 2012 at 12:41
Ok, so you know what's null. I see you're setting the Tag property to some non-null value in your addToParentNode method, but somewhere, you must be creating a root node. Are you sure you're setting Tag for that node, too? — O. R. Mapper
– O. R. Mapper, Commented Aug 26, 2012 at 12:56
you solved my problem!!! thanks a million. I fat fingered my root node and typed Text instead of tag. That is incredible that you picked up on that without seeing the whole program. Thanks so much — jpavlov
– jpavlov, Commented Aug 26, 2012 at 13:16

O. R. Mapper · Accepted Answer · 2012-08-26 13:18:59Z

3

According to the comments on your question, the Tag property of a tree node is null.

You are assigning a non-null value to every tree node in your addToParentNode method, but somewhere, there must be a start and you must be creating a root node. Therefore, that root node apparently has its Tag property still set to null.

answered Aug 26, 2012 at 13:18

O. R. Mapper

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Comments

rene · Accepted Answer · 2012-08-26 12:46:05Z

There is not enough context but you might add some safeguards, and handle the exceptions (only catch them if you want to handle them. You could for example add a tooltip to the TreeNode to inform the user what is wrong for this node,

 void addToParentNode(TreeNode childNodes) 
    { 
        if ((childNodes != null) && (childNodes.Tag != null))
        {
            DirectoryInfo getDir = null;
            try {
               getDir = new DirectoryInfo(childNodes.Tag.ToString()); 
            }
            catch(SecurityException) {
                 childNodes.ToolTipText = "no access";
            }
            catch(PathTooLongException) {
                childNodes.ToolTipText = "path more then 254 chars";
            }
            catch(ArgumentException)
            {
               childNodes.ToolTipText = "huh?";
            }
            if (getDir!=null) && (!getDir.Exists) return;
            DirectoryInfo[] dirList = null;
            try {
               dirList = getDir.GetDirectories(); 
            }
            catch(UnauthorizedException) {
                childNodes.ToolTipText = "no access";
            }
            catch(SecurityException)
            {
                 childNodes.ToolTipText = "no access";
            } 
            if (dirList == null) return;
            foreach (DirectoryInfo dir in dirList) 
            { 
                TreeNode parentNode = new TreeNode(); 
                parentNode.Text = dir.Name; 
                parentNode.Tag = dir.FullName; 
                childNodes.Nodes.Add(parentNode); 
            } 
       }
    }

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