C#

namespace file_i_o
{
   public partial class Form1 : Form
   {
      public Form1()
      {
         InitializeComponent();
      }
 
      private void button1_Click(object sender, EventArgs e)
      {
         string path = @"c:\testc.txt";
         try
         {
            StreamWriter textOut = new StreamWriter(new FileStream(path, FileMode.Create, FileAccess.Write));
         }
         catch (IOException ioe)
         {
            MessageBox.Show(ioe.Message);
         }
 
         StreamWriter textOut = new StreamWriter(new FileStream(path, FileMode.Create, FileAccess.Write));
         textOut.Write("test"); //*****THIS IS WHERE MY ERROR IS*****
         textOut.Close();
      }
   }
}

namespace file_i_o
{
   public partial class Form1 : Form
   {
      public Form1()
      {
         InitializeComponent();
      }
 
      private void button1_Click(object sender, EventArgs e)
      {
         string path = @"c:\testc.txt";
         StreamWriter textOut = null;
         try
         {
            textOut = new StreamWriter(new FileStream(path, FileMode.Create, FileAccess.Write));
         }
         catch (IOException ioe)
         {
            MessageBox.Show(ioe.Message);
         }
 
         textOut = new StreamWriter(new FileStream(path, FileMode.Create, FileAccess.Write));
         textOut.Write("test"); //*****THIS IS WHERE MY ERROR IS*****
         textOut.Close();
      }
   }
}

Pete O`Hanlon wrote:

Good answer. It's surprising how many people aren't aware of the performance benefits of AddRange.

class Tree
        {
            public static TreeNode Build()
            {
                //In the real function this parses an xml file and takes less than 0.1s
                TreeNode MainNode = new TreeNode("MainNode");
                for (int i = 0; i <= 5000; i++)
                {
                    TreeNode ChildNode = new TreeNode("ChildNode" + i.ToString());
                    TreeNode SubNode = new TreeNode("SubNode");
                    ChildNode.Nodes.Add(SubNode);
                    MainNode.Nodes.Add(ChildNode);
                }
                return MainNode;
            }
        }

        private void Form1_Load(object sender, EventArgs e)
        {
            //Quick :-)
            TreeNode MainNode = Tree.Build();
            //Slow! This is the line that's taking the time
            treeView1.Nodes.Add(MainNode);
        }

public static void Build(ref TreeView treeView)
{
   TreeNode mainNode = new TreeNode("MainNode");
   List<TreeNode> childNodes = new List<TreeNode>();
   for (int i = 0; i <= 5000; i++)
   {
       TreeNode childNode = new TreeNode("ChildNode" + i.ToString());
       TreeNode subNode = new TreeNode("SubNode");
       childNode.Nodes.Add(subNode);
       childNodes.Add(childNode);
   }
   treeView.Nodes.AddRange(childNodes.ToArray());
}

public static TreeNode Build()
        {
            TreeNode mainNode = new TreeNode("MainNode");
            for (int i = 0; i <= 5000; i++)
            {
                TreeNode childNode = new TreeNode("ChildNode" + i.ToString());
                TreeNode subNode = new TreeNode("SubNode");
                childNode.Nodes.Add(subNode);
                mainNode.Nodes.Add(childNode);
            }
            return mainNode;
        }

        public static void Build(ref TreeView treeView)
        {
            TreeNode mainNode = new TreeNode("MainNode");
            List<TreeNode> childNodes = new List<TreeNode>();
            for (int i = 0; i <= 5000; i++)
            {
                TreeNode childNode = new TreeNode("ChildNode" + i.ToString());
                TreeNode subNode = new TreeNode("SubNode");
                childNode.Nodes.Add(subNode);
                childNodes.Add(childNode);
            }
            treeView.Nodes.Add(mainNode);
            treeView.Nodes[0].Nodes.AddRange(childNodes.ToArray());
            //The suggested code used the following line which missed out the mainNode but no performance difference the above
            //treeView.Nodes.AddRange(childNodes.ToArray());
        }

        private void button1_Click(object sender, EventArgs e)
        {
            treeView1.Nodes.Clear();
            treeView2.Nodes.Clear();
            listBox1.Items.Clear();
            DateTime dtBuild1Start;
            DateTime dtBuild1End;
            TimeSpan tsBuild1;
            DateTime dtBuild2Start;
            DateTime dtBuild2End;
            TimeSpan tsBuild2;
            Application.DoEvents();

            //Takes around 2100 ms
            dtBuild1Start = DateTime.Now;
            TreeNode MainNode = Build();
            //Commenting out the following line reduces to around 15 ms
            treeView1.Nodes.Add(MainNode);
            dtBuild1End = DateTime.Now;
            tsBuild1 = dtBuild1End - dtBuild1Start;
            listBox1.Items.Add("Build1 in " + tsBuild1.TotalMilliseconds.ToString() + " milliseconds");

            //Takes around 2700 ms
            dtBuild2Start = DateTime.Now;
            Build(ref treeView2);
            dtBuild2End = DateTime.Now;
            tsBuild2 = dtBuild2End - dtBuild2Start;
            listBox1.Items.Add("Build2 in " + tsBuild2.TotalMilliseconds.ToString() + " milliseconds");
        }

1. The times are very different. Granted, that is probably due to hardware differences, but I wouldn't have thought it would be that significant.
   
2. The use of AddRange should still be faster.

treeView.Nodes.Add(mainNode);
treeView.Nodes[0].Nodes.AddRange(childNodes.ToArray());

mainNode.Nodes.AddRange(childNodes.ToArray());
treeView.Nodes.Add(mainNode);