here is the working code:
<form id="form1" runat="server">
<div>
<asp:TextBox runat="server" ID="txt1" TextMode="MultiLine" />
<asp:HiddenField runat="server" ID="hidSelectedText" />
<asp:Button runat="server" ID="showselected" Text="Show selected" OnClientClick="doselection();" OnClick="showselected_Click" />
<script type="text/javascript">
function doselection()
{
if (window.getSelection)
{
txt = window.getSelection();
}
else if (document.getSelection)
{
txt = document.getSelection();
}
else if (document.selection)
{
txt = document.selection.createRange().text;
}
else
{
alert(
'selection not supported');
}
document.getElementById(
'<%=hidSelectedText.ClientID %>').value = txt;
}
</script>
</div>
</form>
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protected void showselected_Click(object sender, EventArgs e)
{
Response.Write(hidSelectedText.Value);
}