how to split the text by comma.
You don't actually want to -
From your spec that seems to demand the comma be in the list box too, and your statement that you will enter "Monday, Tuesday, Wednesday"
in the textbox:
listboxListItems.Items.AddRange(
txtboxUserInput.Text.Split()
);
Split() will split on the spaces.. AddRange takes the array Split returns
与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…