What does AspCompat=”true” mean and when should I use it?

AspCompat is an aid in migrating ASP pages to ASPX pages. It defaults to false but should be set to true in any ASPX file that creates apartment-threaded COM objects–that is, COM objects registered ThreadingModel=Apartment. That includes all COM objects written with Visual Basic 6.0. AspCompat should also be set to true (regardless of threading model) if the page creates COM objects that access intrinsic ASP objects such as Request and Response. The following directive sets AspCompat to true:
<%@ Page AspCompat="true" %>

Setting AspCompat to true does two things. First, it makes intrinsic ASP objects available to the COM components by placing unmanaged wrappers around the equivalent ASP.NET objects. Second, it improves the performance of calls that the page places to apartment- threaded COM objects by ensuring that the page (actually, the thread that processes the request for the page) and the COM objects it creates share an apartment. AspCompat=”true” forces ASP.NET request threads into single-threaded apartments (STAs). If those threads create COM objects marked ThreadingModel=Apartment, then the objects are created in the same STAs as the threads that created them. Without AspCompat=”true,” request threads run in a multithreaded apartment (MTA) and each call to an STA-based COM object incurs a performance hit when it’s marshaled across apartment boundaries.
Do not set AspCompat to true if your page uses no COM objects or if it uses COM objects that don’t access ASP intrinsic objects and that are registered ThreadingModel=Free or ThreadingModel=Both.

Can two different programming languages be mixed in a single ASMX file?
No.

What namespaces are imported by default in ASMX files?

The following namespaces are imported by default. Other namespaces must be imported manually.• System, System.Collections,System.ComponentModel,System.Data, System.Diagnostics,System.Web,System.Web.Services

How do I provide information to the Web Service when the information is required as a SOAP Header?

The key here is the Web Service proxy you created using wsdl.exe or through Visual Studio .NET’s Add Web Reference menu option. If you happen to download a WSDL file for a Web Service that requires a SOAP header, .NET will create a SoapHeader class in the proxy source file. Using the previous example:
public class Service1 : System.Web.Services.Protocols.SoapHttpClientProtocol
{
public AuthToken AuthTokenValue;

[System.Xml.Serialization.XmlRootAttribute(Namespace="http://tempuri.org/", IsNullable=false)]
public class AuthToken : SoapHeader { public string Token; }}

In this case, when you create an instance of the proxy in your main application file, you’ll also create an instance of the AuthToken class and assign the string:
Service1 objSvc = new Service1();
processingobjSvc.AuthTokenValue = new AuthToken();
objSvc.AuthTokenValue.Token = ;
Web Servicestring strResult = objSvc.MyBillableWebMethod();

What is WSDL?

WSDL is the Web Service Description Language, and it is implemented as a specific XML vocabulary. While it’s very much more complex than what can be described here, there are two important aspects to WSDL with which you should be aware. First, WSDL provides instructions to consumers of Web Services to describe the layout and contents of the SOAP packets the Web Service intends to issue. It’s an interface description document, of sorts. And second, it isn’t intended that you read and interpret the WSDL. Rather, WSDL should be processed by machine, typically to generate proxy source code (.NET) or create dynamic proxies on the fly (the SOAP Toolkit or Web Service Behavior).

What is a Windows Service and how does its lifecycle differ from a “standard” EXE?

Windows service is a application that runs in the background. It is equivalent to a NT service.
The executable created is not a Windows application, and hence you can’t just click and run it . it needs to be installed as a service, VB.Net has a facility where we can add an installer to our program and then use a utility to install the service. Where as this is not the case with standard exe

How can a win service developed in .NET be installed or used in Win98?

Windows service cannot be installed on Win9x machines even though the .NET framework runs on machine.