Read a Registry Key

I have a web application which is importing DLLs from the bin folder.

const string dllpath = "Utility.dll";

    [DllImport(dllpath)]

Now what I want to do is first import the DLLs from a folder not in the current project but at some different location.

The path of that folder is stored in a registry key.

How should I do this?

Edit:

Why can't I work this out???

public partial class Reports1 : System.Web.UI.Page
{

    RegistryKey registryKey = Registry.CurrentUser.OpenSubKey(@"Software\xyz");
    string pathName = (string)registryKey.GetValue("BinDir");

    const string dllpath = pathName;
    [DllImport(dllpath)]
    public static extern bool GetErrorString(uint lookupCode, [MarshalAs(UnmanagedType.LPWStr)] StringBuilder buf, uint bufSize);

    protected void Page_Load(object sender, EventArgs e)
    {

string pathName = (string)registryKey.GetValue("BinDir"); is not working here, but is working in the pageload event...

But if I do this DLL import won't work... How can I fix this?