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Can I call Web API from a client configured to call an .asmx service?

I'm looking at converting an existing web service into a Web API. I've only worked with a WS a little bit and it was a long time ago. What I do remember is that in my project I would make reference to a service location and then use that reference to call whatever method I needed.
EX: I would reference http://mydomain/webservicename/mobile.asmx and then would call objWS.MethodName() what was coded within the mobile.asmx file.
If I convert over to using a Web API I would basically call the HTTP by going to something like http://mydomain/controllername/myMethod.
As of right now I don't have access to the client code to be able to change the way that it calls the service. That being said am I stuck with using a traditional web service vs web api?
This is an app on a handheld scanner that I believe is running Windows CE. We are having some connectivity issues/database deadlocks and I was asked to look at it and see if I can help out. The current WS code is overly complicated IMO since it's only doing either an insert or an update to a database. I would also think that going with a Web API would make it a faster app since it's depending on cellular access for it's communication. JSON should be a smaller payload than XML.
So, I would like to just re-write it using Web API 2 and Entity Framework. However, I'm afraid I'm stuck to using WS since I don't have access to the client code.
Any suggestions?

It's a fairly broad architectural suggestion, but what you're proposing certainly sounds possible and even quite reasonable.
If I understand correctly, you currently have this:
Client -> ASMX Service
And you can't change the Client, only the ASMX Service. The first thing you're going to want to do this ensure that server-side business logic is de-coupled from the platform technology:
Client -> ASMX Service -> Business Logic
The idea here is that any application host should be able to reasonably invoke the same business logic, even if that logic is nothing more than direct database access. The application host itself should be little more than a pass-through set of operations to be invoked.
At that point, you can create a second application host alongside the first one:
Client -> ASMX Service ----|
|-> Business Logic
WebAPI Service --|
So now you have two different services which expose the same business logic, using two different web service technologies. Each of them should be very thin, as application host technologies should always be easily replaceable.
At this point, assuming there are no significant gaps in the operations available between the two services, you can publish the new service's specifications to clients and begin plans to deprecate the old service. When you can deprecate it is more of a contractual issue than a technical issue. However long you've committed to maintaining it, that's how long clients will have a reasonable expectation to still use it.
If you really want to, you can even have the ASMX Service be a pass-through to the WebAPI Service, but in my personal experience that adds unnecessary layering to the whole setup and artificially complicates the abstraction of the business logic. Either way, the interface exposed by the ASMX Service wouldn't change.
The main thing here is the logical abstraction of the operations being exposed and the analysis of any gaps between what the ASMX Service can do and what the WebAPI Service can do. If that gets complex, then that's an indication that the business logic (and indeed the whole solution domain) is tightly coupled to the application technology being used, namely ASMX web services. That is the problem to be solved. Once solved, creating different application hosts and exposing different services which invoke the same underlying business operations becomes almost trivial.

You are right; you are stuck if you can't change the client and you want to change service protocols. Your client currently has a specific .asmx endpoint it is configured to point to and until you can update that endpoint and have the client stop using the proxy generated from the service, you can't change to Web API.
I'd still rewrite the service to use EF, though.

Confusion about WCF Class library and WCF application hosting

I have a web site and would like to expose certain functionalities using WCF.
Before deciding which type of WCF project I need to use I wanted to compare the differences between WCF Class library and WCF application. I know this question has been asked many times and answered many times and the answers are usually about different hosting options each one offers but I wanted to try and see the differences, so this is what I tried:
Step 1 - In a same solution, I have created a WCF Service library project and created Client console app project, set the console app as a startup project, referenced the service library project from the client project
and in the client console app I could instantiate the service and can consume the service methods. I didn't even add a service reference to the client project.
Step 2 - In a same solution, I have created a WCF Service application project and created Client console app project, set the console app as a startup project, referenced the service application project from the client project
and in the client console app I could instantiate the service and can consume the service methods. I didn't even add a service reference to the client project.
On both steps after compiling the solutions I was able to copy the client app's exe and the service dll's to a different location and still be able to run the clients.
Based on this little excercie I am confused about the hosting part. It seems wether I use WCF Class library or WCF application type I get the same result.
This is just like using multiple projects in a solution, you reference one from another and use the methods, there must be something I am missing which highlights the differences between the two and highlights the benefits of using WCF, also in the past I remember I had to add a service reference to the clients apps in order to consume the service, why is this not the case here?
Thanks

1) Running a wcf service application allows you to provide communication into a single application, where you have a single instance of a thing you want to provide access to. Maybe this is a game, or a chat room without an external state engine or datastore. This is useful for providing diagnostic information about an application you might have written for example. I used this to provide external control for an industrial robot that I wanted to provide remote control access for.
That is to say, that you write an application, it has a function. You want to expose part of that functionality to remote applications. You do this by adding a WCF endpoint to your existing application, so your application itself is controlling the WCF hosting elements, lifecycle of the endpoint etc.
2) Running a WCF Service is for when you've got an external data store, or your service is stateless. A translation service, lookup service and web page requests fall into this category.
With a service class, you're saying here is this service, this thing that provides a function. It isn't tied to the lifecycle of another application or process and is typically hosted by IIS. IIS manages when the class is loaded and run based on the requests that come into it. These services have no internal persistence and rely on an external datastore, or are, by nature, stateless (think of a postcode lookup, or a calculator service)
It sounds like you're actually adding the projects as references, rather than connecting to them as services. That is to say, that the consuming application is actually loading the service as an assembly (in the same application/ memory space) rather than as a separate application/ service that your application then uses WCF to communicate with.

The WCF Service Application template can be used to create WCF services with a hosting website created within the project
The WCF Service Library template can be used to create WCF services that are hosted by the WCF Service Host, and these can be tested using the WCF service Test Client.
the biggest advantage of using a standalone library (apart from decoupling the logic) is that you can easily migrate your service, i.e. host it in another application or another type of application. E.g., let's say you're hosting your service using IIS - you can easily move your service to a standalone application, etc.

How can I write tests for web services (in C#) outside of the Solution they exist in?

I have been recently added to a project where I will need to be doing functional and load testing for some existing Web Services (SOAP).
I was wondering if it is possible to call a web service from a console application for testing purposes? I do not have permissions to code in the existing Project so I need to create a new one, which is what my real issue is, I do not know how to call the web services (or the web service code) from outside of their Solution.
I know this will make things more difficult, especially for recording test results, but I have no choice as I am a contractor and can not get the required permissions. I can pull code and see the source code, I just don't want to code in the existing project as my local changes will get blown away every time there is new code to pull down.
I have done some basic testing using SoapUI, but the open Source version does not meet all the requirements I have and would rather just write my own tests then try and find a free 3rd party solution (as I also have no budget).
Thanks,
cwlovell13

You can write a simple WCF client to consume existing Web Services (SOAP).
Maybe you can reference this .
Accessing Services Using a WCF Client

Accessing an SQL database from a Silverlight client (no web project)

I want to access a remote SQL database for my Silverlight client. I know that there are various ways to do this (Entity Framework model, Linq to SQL), but all of those rely on something in the web project (or, at least, all of the tutorials I've read do). What's unusual for me is that there is no web project, which of course prevents me from using various assemblies.
The only thing I do have which might be useful is a WCF service running on IIS, which will have less limitations (and I can add that as a service reference in my SL project). But I'm not even sure if this is a feasible option for this problem.
Any solutions or suggestions would be great.
Thanks.
Edit: Thanks for the suggestions guys, but here's a little more: I am making an individual XAP that is loaded into something else (someone else's project is importing and loading the XAP files), so I don't have access to the web component. The loader itself will be hosted within a website (at least, I assume it will be).
I already have a WCF service set up which is currently hosted in IIS, so I can potentially create something here. How? Well, it seems I can generate the ORM classes using SQLMetal.exe, and then import that code into the WCF service. This will allow the service to make calls to the remote database and have that data returned in C# classes. The perhaps I'll make those data classes as ServiceContracts and pass the data that way. What's the problem here? Well, to be honest, I'm not really sure how it would work. I mean, I call the WCF Service from the SL application, which does its thing. It then must contact the SL application (I have no idea how to do that part) and send along a load of data. It would be great if someone had has experience of this and would offer some suggestions. I know it's not really how you're supposed to do it, but I've drawn the short straw so I'm stuck with it.

Ok, so if I understand it correctly, you use a Silverlight application (XAP) which is started locally from a network share or something? Because why would you not have a web project when using Silverlight? (is there no server available to host it?)
But ok, when that is the case, you can access a WCF service from Silverlight. However it has some limititations. For instance you can only use asynchronous calls to the service, and you can only use WCF basichttpbinding as binding for the WCF service.
See: http://msdn.microsoft.com/en-us/library/cc197959%28v=vs.95%29.aspx

First, I would recommend you use WPF instead of Silverlight for this project. If you're not using this as a web client, then WPF is a million times easier / better.
But if that's not a possibility:
You can write a self-hosted WCF service and run it somewhere accessible. Self-hosted will allow for WCF connections to connect without the IIS necessary. In your self-hosted program you need:
Front end WCF defined
Back end SQL database
Depending on how smart this client needs to be, a BusinessLogic layer to transform the data from WCF to SQL.

Silverlight is sandboxed, so it can only access its own Web application. Therefore, your best bet is to include a WCF or ASMX web service in your web application that handles the DB access.
If you don't want to run a sandboxed UI on a Web application, you cannot use Silverlight but should use WebForms or Windows Presentation Foundation (WPF) instead.

How to structure WCF web service solution so it would be easy to develop and host in different ways?

I'm about to start building WCF web service, and i would like to structure solution in such way that is would be:
Easy to develop - i would like so that developers on my team could just get sources, build, and start service, probably best would be to have possibility to start it as console app?
Easy to host this service later in IIS, WAS or windows service.
I was thinking about having this projects:
Shared (for web service interface)
WebService (for actual implementation of web service and svc file)
ConsoleHost (for hosting web service in console app)
Will there be any problems with such approach?
What could be things that i should consider in advance?
Maybe there is better structure?
I would appreciate any insights and links to resources that could help me chose right structure.
p.s. web service itself is simple, but this approach would be reused for more services.

What I tend to do is to place all my interface definitions in one assembly/project, all the service implementations in another assembly project, and the hosting environment, whether it be console, IIS (ie the .svc files) or windows service or whatever, in a separate assembly.

With this information, I say this structure looks fine.
You might also consider creating a separate ServiceModel assembly to store all your custom behavior and WCF. They could then be reused for other projects