Runat='Server
' Indicates the accessibility of the
control
at Serverside.
Let Me make you more clear about it.
If you puts
runat="server"
inside any of the
control
then you can use that
control
at the
server
side.
e.g
<asp:TextBox id="txt" Runat="Server"></asp:TextBox>
string s = txt.Text;
and you can also access Like
TextBox tBox = (TextBox)Page.FindControl("txt");
If you will not provide then it will be not able to find as server doesn't know who is txt.
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Accept Answer if it Helped.