你调用的对象是空的。 （C＃）

Question

If the logic within this method is run from an event handler such as Button_Click it works perfectly, but, when running this from a method such as below I get the error:

hostView.SelectedNode.Nodes.Add(newNode);

Object reference not set to an instance of an object.

Here is my code:

private void SetupHostTree()
{
    // Set internal host names
    using (var reader = File.OpenText("Configuration.ini"))
    {
        List<string> hostnames = ParseInternalHosts(reader).ToList();
        foreach (string s in hostnames)
        {
            TreeNode newNode = new TreeNode(s);
            hostView.SelectedNode.Nodes.Add(newNode);

            string title = s;
            TabPage myTabPage = new TabPage(title);
            myTabPage.Name = s;
            tabControl1.TabPages.Add(myTabPage);
        }
    }
}

Answer 1

也许没有选定的节点:)

Answer 2

Probably because no node is currently selected in the hostView TreeView.

The documentation says that the TreeView.SelectedNode property will return null when no node is currently selected. And since you've combined it into an expression, the entire expression is failing because there is no Nodes collection on a null object!

Try this code:

private void SetupHostTree()
{
    // Set internal host names
    using (var reader = File.OpenText("Configuration.ini"))
    {
        List<string> hostnames = ParseInternalHosts(reader).ToList();
        foreach (string s in hostnames)
        {
            // Ensure that a node is currently selected
            TreeNode selectedNode = hostView.SelectedNode;
            if (selectedNode != null)
            {
                TreeNode newNode = new TreeNode(s);
                selectedNode.Nodes.Add(newNode);
            }
            else
            {
                // maybe do nothing, or maybe add the new node to the root
            }

            string title = s;
            TabPage myTabPage = new TabPage(title);
            myTabPage.Name = s;
            tabControl1.TabPages.Add(myTabPage);
        }
    }
}