how can fix text in textbox and these text don't delete

i want to fix a numeric value like 00.00 in textbox and at run time this value can't delete from textbox
i am using this code:
<asp:TextBox ID="TextBox1" runat="server" Width="143px" ontextchanged="txt_parts_TextChanged" AutoPostBack="True" Text="00.00"></asp:TextBox>
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