I would recommend to define the array as an static member of a class and then initialize it with help of Global.asax, use the Application_Start event handler.
to add Global.asax to you project in Visual Studio:
File -> New -> File -> Global Application Class
Here is a sample C# code for Global.asax.cs:
public class Global : HttpApplication
{
protected void Application_Start(object sender, EventArgs e)
{
// ... Your initialization of the array done here ...
}
protected void Session_Start(object sender, EventArgs e)
{
}
protected void Application_BeginRequest(object sender, EventArgs e)
{
}
protected void Application_AuthenticateRequest(object sender, EventArgs e)
{
}
protected void Application_Error(object sender, EventArgs e)
{
}
protected void Session_End(object sender, EventArgs e)
{
}
protected void Application_End(object sender, EventArgs e)
{
}
}