Best way in asp.net to force https for an entire site?

The other thing you can do is use HSTS by returning the "Strict-Transport-Security" header to the browser. The browser has to support this (and at present, it's primarily Chrome and Firefox that do), but it means that once set, the browser won't make requests to the site over HTTP and will instead translate them to HTTPS requests before issuing them. Try this in combination with a redirect from HTTP:

protected void Application_BeginRequest(Object sender, EventArgs e)
{
  switch (Request.Url.Scheme)
  {
    case "https":
      Response.AddHeader("Strict-Transport-Security", "max-age=300");
      break;
    case "http":
      var path = "https://" + Request.Url.Host + Request.Url.PathAndQuery;
      Response.Status = "301 Moved Permanently";
      Response.AddHeader("Location", path);
      break;
  }
}

Browsers that aren't HSTS aware will just ignore the header but will still get caught by the switch statement and sent over to HTTPS.

The IIS7 module will let you redirect.

    <rewrite>
        <rules>
            <rule name="Redirect HTTP to HTTPS" stopProcessing="true">
                <match url="(.*)"/>
                <conditions>
                    <add input="{HTTPS}" pattern="^OFF$"/>
                </conditions>
                <action type="Redirect" url="https://{HTTP_HOST}/{R:1}" redirectType="SeeOther"/>
            </rule>
        </rules>
    </rewrite>

Please use HSTS (HTTP Strict Transport Security)

from http://www.hanselman.com/blog/HowToEnableHTTPStrictTransportSecurityHSTSInIIS7.aspx

<?xml version="1.0" encoding="UTF-8"?>
<configuration>
    <system.webServer>
        <rewrite>
            <rules>
                <rule name="HTTP to HTTPS redirect" stopProcessing="true">
                    <match url="(.*)" />
                    <conditions>
                        <add input="{HTTPS}" pattern="off" ignoreCase="true" />
                    </conditions>
                    <action type="Redirect" url="https://{HTTP_HOST}/{R:1}"
                        redirectType="Permanent" />
                </rule>
            </rules>
            <outboundRules>
                <rule name="Add Strict-Transport-Security when HTTPS" enabled="true">
                    <match serverVariable="RESPONSE_Strict_Transport_Security"
                        pattern=".*" />
                    <conditions>
                        <add input="{HTTPS}" pattern="on" ignoreCase="true" />
                    </conditions>
                    <action type="Rewrite" value="max-age=31536000" />
                </rule>
            </outboundRules>
        </rewrite>
    </system.webServer>
</configuration>

Original Answer (replaced with the above on 4 December 2015)

basically

protected void Application_BeginRequest(Object sender, EventArgs e)
{
   if (HttpContext.Current.Request.IsSecureConnection.Equals(false) && HttpContext.Current.Request.IsLocal.Equals(false))
   {
    Response.Redirect("https://" + Request.ServerVariables["HTTP_HOST"]
+   HttpContext.Current.Request.RawUrl);
   }
}

that would go in the global.asax.cs (or global.asax.vb)

i dont know of a way to specify it in the web.config