Page class contains two methods,
ResolveClientUrl which at first glance look the same. They both take the same relative URL and return browser friendly URl. So why 2 functions to do the same job? Though their functionality is the same, but their way of returning client browser relative URL makes the huge difference.
ResolveClientUrl returns a path relative to the current page which user is currently browsing, and
ResolveUrl returns a path relative to the site root that is configured as root in IIS.
Let's consider this structure:
Now from above example, root of the IIS directory is ROOT, and all other directories are under this root. Now from MyAccount->default.aspx tries to access Images->mypic.png.
ResolveUrl will do top to bottom parsing to find the resource, on the other hand
ResolveClientUrl will do bottom up parsing to find the same resource.
The difference between these two is very visible if you host your web app under a shared hosting under a subdomain. For example, you host your MY_SITE app under www.example.com subdomain, and your physical file path is example.com\www\MY_SITE under IIS directory. So under this, you placed your file structure given above. Now for any link, if you set:
Watch out for the URL in browser status bar, you will see www.example.comwww/MY_SITE/MyAccount/default.aspx, because
ResolveUrl will parse from top root domain for this resource, so it took full path from root for that specific resource. On the other hand,
ResolveClientUrl can solve this problem. This will parse from browsers' present location up towards the root, so it finds the specific resource path.