Question

For example I want to register all classes with Service attribute using StructureMap

[Service]
public class A {}

Answer 1

As far as I know you are required to create a custom IRegistrationConvention found in the StructureMap.Graph Namespace.

Implement the convention

public class ServiceDiscoveryConvention
                                : StructureMap.Graph.IRegistrationConvention
{
    public void Process(Type type, Registry registry)
    {
        // check if type has attribute
        // add to registry using the registry variable
    }
}

Add the convention to the scanner

Scan(cfg =>
       {
            cfg.TheCallingAssembly(); // or whatever assemblies you are scanning
            cfg.Convention<ServiceAttributeConvention>();
        });

Remarks

If you are free on your decision you may want to use interfaces instead of attributes. Using an Interface you are able to provide a common contract for all classes and they are way easier to work with when the project grows.

Attributes have a tendency to spread all over your code, refactoring them can be a real pain. Interfaces have much better tooling support when it comes to refactoring.

I am using interfaces for a similar task (plugin system) a convention like

public class TypeScanner<T> : IRegistrationConvention
{
    private static readonly Type PluginInterface = typeof(T);

    public void Process(Type type, Registry registry)
    {
        if (type.IsAbstract || type.BaseType == null) return;
        if (PluginInterface.IsAssignableFrom(type) == false) return;

        registry.For(PluginInterface).Singleton().Add(instance);
    }
}

usage would be similar:

Scan(cfg =>
       {
            cfg.TheCallingAssembly(); // or whatever assemblies you are scanning
            cfg.Convention<TypeScanner<IYourService>>();
        });

If your convention takes constructor arguments you can use With:

Scan(cfg =>
       {
            cfg.TheCallingAssembly(); // or whatever assemblies you are scanning
            var convention = new SomeConvention(x,y,z);
            cfg.With(convention);
        });