State Management

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ASMX services have access to the HttpContext class, which provides access to state managers with a different scope, such as application scope and session scope. ASP.NET also provides control over how state data is managed. Consequently, you should minimize the use of state in a service because of the effect it has on the scalability of an application.

WCF provides extensible objects that can be used for state management. Extensible objects implement the System.ServiceModel.IExtensibleObject<T> interface. The two main classes that implement the IExtensibleObject interface are ServiceHostBased and InstanceContext. The ServiceHostBased class provides the ability for all service instances in the same host to access the same state data. On the other hand, the InstanceContext class only allows access to state data within the same service instance.

To implement state management in WCF, you need to define a class that implements the IExtension interface, which is used to hold the state data. Instances of this class can then be added to one of the IExtensibleObject classes using the Extensions property. The following code example shows how to implement state using the InstanceContext.

internal class StateData: IExtension<InstanceContext>
{
    public string Identifier = string.Empty;
    public string Data = string.Empty;

    public StateData( string data )
    {
        Identifier = Guid.NewGuid().ToString();
        Data = data;
    }
}

public string AddStateData( string data )
{
    StateData state = new StateData( data );
    OperationContext.Current.InstanceContext.Extensions.Add(state);
    return state.Identifier;
}

public string GetStateData( string identifier )
{
    Collection<StateData> dataCollection = 
        OperationContext.Current.InstanceContext.Extensions.FindAll<StateData>();

    string result = string.Empty;
    foreach( StateData data in dataCollection )
    {
        if( data.Identifier == identifier )
        {
            Result = data.Data;
            break;
        }
    }
    return result;
}

Recommendation

The ability to control where state is maintained in WCF is much more limited than in ASP.NET. As a result, state management is commonly used as the primary reason for enabling the ASP.NET compatibility mode, which provides access to ASP.NET components; this means you have much more control over where state is maintained. For example, when a service is configured for ASP.NET compatibility, you have access to HTTP context classes that provide the same functionality as the ASMX implementation.

When developing new ASMX services, you should avoid the use of state in your services; all services should be stateless.