How to populate a textbox from toolstripcombobox?

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I have a custom combobox (www.codeproject.com/Articles/205358/ButtonDropDownMenuControl) control that I have filled with some filenames from a listarray, but now I would like it if I clicked on a filename that the source file loads into the textbox.

This is the code I got so far.

private void buttonDutch_Click(object sender, EventArgs e)
        {
            string[] Filelistarray = Directory.GetFiles(@"C:\Users\a961835\Programming", "*.txt");
            List<string> fileNames = new List<string>();
            foreach (string FileName in Filelistarray)
            {
                string filenameWithoutPath = Path.GetFileName(FileName);
                fileNames.Add(filenameWithoutPath);
            }

            toolStripComboBox1.Items.AddRange(fileNames.ToArray());

            //this.toolStripComboBox1.SelectedIndexChanged += new System.EventHandler(toolStripComboBox1_SelectedIndexChanged);

            
            
        }

        private void ComboBox_SelectedIndexChanged(object sender, EventArgs e)
        {
            string[] Filelistarray = Directory.GetFiles(@"C:\Users\a961835\Programming", "*.txt");
            List<string> files = new List<string>();
            //foreach (string FileName in Filelistarray)

            string selectedItem = (string)toolStripComboBox1.SelectedItem;
            
            
            
            int resultIndex = -1;

            resultIndex = toolStripComboBox1.FindStringExact(selectedItem);

            while (resultIndex!=-1)
            {
                toolStripComboBox1.Items.RemoveAt(resultIndex);
                resultIndex = toolStripComboBox1.FindStringExact(selectedItem, resultIndex);
            }

            textBox1.Text = selectedItem;
        }

The problem is it only shows me the textfilename and not the the contents in that textfile.
Any ideas how to resolve this issue?