inject jquery into every ASP.Net page

Using Master Page is easy way to do that. But if you already constructed your site, implementing this with master page may be mass of work. Instead of that you can create BasePage class which is inherited System.Web.UI.Page as below:

Check this out

public class BasePage:System.Web.UI.Page
    {

        protected override void OnInit(EventArgs e)
        {
            // Define the name, type and url of the client script on the page.
            String csname = "ButtonClickScript";
            String csurl = "~/script_include.js";
            Type cstype = this.GetType();

            // Get a ClientScriptManager reference from the Page class.
            ClientScriptManager cs = Page.ClientScript;

            // Check to see if the include script exists already.
            if (!cs.IsClientScriptIncludeRegistered(cstype, csname))
            {
                cs.RegisterClientScriptInclude(cstype, csname, ResolveClientUrl(csurl));
            }

            base.OnInit(e);
        }
    }

Every page which derived from BasePage includes that script source dynamically. (see below)

public partial class Default : BasePage
    {
        protected void Page_Load(object sender, EventArgs e)
        {
            //Some code ...
        }
    }

But, if you didn't create your site yet, Master Page is the easiest way to do that.