You can expose the service in two different endpoints.
the SOAP one can use the binding that support SOAP e.g. basicHttpBinding, the RESTful one can use the webHttpBinding. I assume your REST service will be in JSON, in that case, you need to configure the two endpoints with the following behaviour configuration
<endpointBehaviors>
<behavior name="jsonBehavior">
<enableWebScript/>
</behavior>
</endpointBehaviors>
An example of endpoint configuration in your scenario is
<services>
<service name="TestService">
<endpoint address="soap" binding="basicHttpBinding" contract="ITestService"/>
<endpoint address="json" binding="webHttpBinding" behaviorConfiguration="jsonBehavior" contract="ITestService"/>
</service>
</services>
so, the service will be available at
Apply [WebGet] to the operation contract to make it RESTful.
e.g.
public interface ITestService
{
[OperationContract]
[WebGet]
string HelloWorld(string text)
}
Note, if the REST service is not in JSON, parameters of the operations can not contain complex type.
Reply to the post for SOAP and RESTful POX(XML)
For plain old XML as return format, this is an example that would work both for SOAP and XML.
[ServiceContract(Namespace = "http://test")]
public interface ITestService
{
[OperationContract]
[WebGet(UriTemplate = "accounts/{id}")]
Account[] GetAccount(string id);
}
POX behavior for REST Plain Old XML
<behavior name="poxBehavior">
<webHttp/>
</behavior>
Endpoints
<services>
<service name="TestService">
<endpoint address="soap" binding="basicHttpBinding" contract="ITestService"/>
<endpoint address="xml" binding="webHttpBinding" behaviorConfiguration="poxBehavior" contract="ITestService"/>
</service>
</services>
Service will be available at
REST request
try it in browser,
http://www.example.com/xml/accounts/A123
SOAP request
client endpoint configuration for SOAP service after adding the service reference,
<client>
<endpoint address="http://www.example.com/soap" binding="basicHttpBinding"
contract="ITestService" name="BasicHttpBinding_ITestService" />
</client>
in C#
TestServiceClient client = new TestServiceClient();
client.GetAccount("A123");
Another way of doing it is to expose two different service contract and each one with specific configuration. This may generate some duplicates at code level, however at the end of the day, you want to make it working.
Best Answer
If you decouple the two pieces sufficiently, you will also permit the use of clients running something other than .NET. Using the first option, you could only support .NET clients. This may turn out to be important, even if today you are absolutely certain that only .NET will ever be used - tomorrow, your company may be purchased by another which is a Java or PHP shop.
Even if you never need to support a non .NET client, coupling to the assemblies will require you to maintain version compatibility between the client and server. If this is not necessary, then use option #2.