It is not a desktop application, I am developing ArcGIS Server SOE.
For example in desktop application you can access to NetAnalysisExt like this:
pID = new UID();
pID.Value = "esriEditorExt.UtilityNetworkAnalysisExt";
pNetAnalysisExt = (INetworkAnalysisExt)app.FindExtensionByCLSID(pID);
I want to know how I can access to NetworkAnalysisExt of the mxd from the SOE.