.NET IWebProxy: Avoid Casting System Proxies to WebProxy

IWebProxy policy = WebRequest.GetSystemWebProxy();
request.Proxy = policy;

Keep the returned object as IWebProxy. This fragment applies to existing WebRequest-based code, not a recommendation to adopt that legacy API for a new application.

WebProxy implements the interface, but not every object implementing the interface inherits from WebProxy. A system wrapper can therefore reject the cast before any network request occurs. A private .NET Framework class name such as WebProxyWrapperOpaque is an implementation detail, not a proxy endpoint or account problem.

For a route decision, call IsBypassed(destination) and GetProxy(destination) with the actual destination URI. A bypass may return that destination itself, and a PAC-backed policy can select another route for another host.

For authentication, IWebProxy already exposes Credentials. Use the supported account and method without casting to reach the property.

Extracting one URI to construct a replacement WebProxy discards the original policy's per-destination behavior. Do that only when a fixed endpoint is the intended application configuration, rather than as a type-error workaround.