Following example add the contents of a TextBox control to a ListBox control when the Button control click. this example requires a windows form with Listbox control named ListBox1, one textbox control named TextBox1 and one Button control named button1 with Button1_Click event handler.
Example
[Vb.Net]
Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click ListBox1.Items.Add(TextBox1.Text.Trim() End Sub
[C#]
private void Button1_Click(System.Object sender, System.EventArgs e) { ListBox1.Items.Add(TextBox1.Text.Trim()); }
if you want to avoid duplicate items in the listbox, we can search for a match before adding the contents of the TextBox to the Listbox with the help of items.Contains(val as object) method.
Example
[VB.Net]
Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click If Not ListBox1.Items.Contains(TextBox1.Text.Trim()) Then ListBox1.Items.Add(TextBox1.Text.Trim()) Else MessageBox.Show("text is already present in listbox") End If End Sub
[C#]
private void Button1_Click(System.Object sender, System.EventArgs e) { if (!listBox1.Items.Contains(TextBox1.Text.Trim())) { listBox1.Items.Add(TextBox1.Text.Trim()); } else { MessageBox.Show("text is already present in listbox"); } }