how to call a variable in code behind to aspx page

The field must be declared public for proper visibility from the ASPX markup. In any case, you could declare a property:

private string clients;
public string Clients { get { return clients; } }

UPDATE: It can also be declared as protected, as stated in the comments below.

Then, to call it on the ASPX side:

<%=Clients%>

Note that this won't work if you place it on a server tag attribute. For example:

<asp:Label runat="server" Text="<%=Clients%>" />

This isn't valid. This is:

<div><%=Clients%></div>

1392/10/11       00000             دسترسی به یک متغیر از code in line در codebehind       ASP.NET