how to make a part of the textbox as readonly

i have a tagtextbox on my page when a user selects a category that category name goes 开发者_运维问答to tagtextbox

now i want that this category name user cant change but he can append some more tags means first tag becomes readonly and further he can add too in same text box

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This is not possible.

Instead, you can put the first category name in a <span>, and put the textbox after the span.
You can then apply border: none to the textbox and make your own border around the entire thing to make it look like a single textbox.

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CSS :

<style type="text/css">
.div {
   margin:20px; 
   padding:10px; 
   background:#ccc; 
   float:left;
}

.textarea {
   border:0; 
   display:block; 
   font-size:12px; 
   padding:5px 5px 0; 
   outline:none; 
}

.span {
   display:block;
   background:#fff;
   font-size:14px;  
   padding:5px 5px 5px; 
   color:green; 
   outline:none;
}
</style>

HTML :

<div class="div">
<div class="span">
<asp:TextBox runat="server" ID="txtNumber" BorderStyle="None" />
<asp:RegularExpressionValidator runat="server" ID="RegularExpressionValidator1" ValidationExpression="\d+" ControlToValidate="txtNumber" ErrorMessage="Only Numbers are allowed" Display="Dynamic" />
</div>    
</div>

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You can't make part of a textbox editable and another not.

My suggestion would be to change the type of element from a TextBox to a div or span via Javascript as soon as the user begins to enter the next tag.

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If you want to allow append has in "paste" action, then you can't do that. Else and you just want to append via your code, why not use just a div to show what he has selected and add the selections to your post on a hidden textbox?

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Adding to what SLaks said - Here is an example of a CSS & HTML hack where someone has added a span under the textbox. To make it look like it is part of the same textbox there is a border round the enclosing div. It looks quite effective.

textbox with uneditable name at the bottom