尝试一下，

Try this,

<asp:TextBox ID="Txt_First" runat="server"></asp:TextBox>
<asp:TextBox ID="Txt_Second" runat="server" Width="280px" TextMode="MultiLine"></asp:TextBox>
<asp:Button ID="Button5" runat="server" onclick="Button5_Click" Text="Button" />

protected void Button5_Click(object sender, EventArgs e)
   {
       string text = (Txt_Second.Text) + "\r\n" + Txt_First.Text;
       Txt_First.Text = "";
       Txt_Second.Text = text;
   }

从第二个文本框中提取文本并将新数据附加到其中.

Extract the text from the second text box and append the new data to it.

亲爱的Shrivathsava，

将文本框多行布尔属性设置为True，将解决您的问题

为了更好地了解，请检查此链接

http://www.dotnetperls.com/textbox [

Dear Shrivathsava,

Set textbox multiline boolean property to True, It will solve your problem

for better understanding check this link

http://www.dotnetperls.com/textbox[^]

Thanks