Different approaches for resolving URLs in ASP.NET

There are various ways that are available in ASP.NET that we can use to resolve relative paths to a resource on the server-side and making it available on the client-side. I know of 4 ways -

1) Request.ApplicationPath
2) System.Web.VirtualPathUtility
3) Page.ResolveUrl
4) Page.ResolveClientUrl

To show the difference, let's take an example. Let's say our application has 2 folders, with the following files -

/Forms1/forml.aspx
/Forms2/form2.aspx

form1.aspx redirects to form2.aspx and needs to get the path to that file.

We will consider 2 case -
a.) When application is hosted in a virtual directory

http://www.server.com/virtual-directory/Forms2/form2.aspx

b.) When application is not in a sub-directory

http://www.server.com/Forms2/form2.aspx

1) Request.ApplicationPath
The most common way people use this method is by concatenating strings to create the Url to a resource.

sUrl = Request.ApplicationPath + "/Forms2/form2.aspx";

In case a), Request.ApplicationPath will return "/virtual-directory". Notice there is no slash at the end, and the above sUrl gets a value

/vitural-directory/Forms2/form2.aspx

In case b). Request.ApplicationPath will return "/". Notice there is a now a slash at the end, and sUrl gets a value

//Forms2/form2.aspx

In this case, rather than requesting "http://www.server.com/Forms2/form2.aspx", the browser requests "http://Forms2/form2.aspx".

So, don't use Request.ApplicationPath to get relative paths to files.


2) Page.ResolveClientUrl

sUrl = Page.ResolveClientUrl("~/Forms2/form2.aspx"); 

This will result in - 
"../Forms2/form2.aspx"

Notice that the path returned is relative to the calling page.


3) Page.ResolveUrl

sUrl = Page.ResolveUrl("~/Forms2/form2.aspx"); 

This will result in - 
"/Forms2/form2.aspx"

Notice that the path returned in this case is relative to the site root, ie, from the top of the virtual directory.

So which one is better between ResolveClientUrl and ResolveUrl? I don't think one is better than the other, but most people use Page.ResolveUrl.


4) System.Web.VirtualPathUtility

sUrl = VirtualPathUtility.ToAbsolute("~/Forms2/form2.aspx");

This will resolve, same as Page.ResolveUrl, to 
"/Forms2/form2.aspx"

Whats the diffirence between VirtualPathUtility and ResolveUrl? Well, there are a few -

• ResolveUrl can only be called in the context of a Page. But VirtualPathUtility can be used from anywhere.


• VirtualPathUtility will throw an error if there are Query String parameters in the URL -
  

  // throws error
  sUrl = VirtualPathUtility.ToAbsolute("~/Forms2/form2.aspx?q=123");
  
  // works as expected
  sUrl = Page.ResolveUrl("~/Forms2/form2.aspx?q=123"); 
  

To conclude, I think Page.ResolveUrl should be used anytime you are in the context of the Page. Everywhere else, use VirtualPathUtility.