Dynamically loading classes (with custom behavior)

问题:

We are building an app for few customers, each has its own requirements along with the similar ones. We also want to keep all the code in the same app, not branch it, and the IFs is not good choice since it will be all over places.

I plan having the base classes for all. Then each customer will have its own class in which the override methods will do special logic.

How can we load the assemblies when compiling instead of doing this

public class BaseClass {
    public string getEventId()
}

public class ClassForJohn:BaseClass {
    [override]
    public string getEventId()
}

public class ClassForAdam:BaseClass {
    [override]
    public string getEventId()
}

void UglyBranchingLogicSomewhere() {
   BaseClass  eventOject;
   if("John"==ConfigurationManager.AppSettings["CustomerName"]){
        eventOject = new ClassForJohn();


   }else if("Adam"==ConfigurationManager.AppSettings["CustomerName"]){
        eventOject = new ClassForAdam();


   }else{
        eventOject = new BaseClass ();

   }
  eventId = eventOject.getEventId();
}

回答1:

This is how I load plug-ins (add-ins) into one of my projects:

const string PluginTypeName = "MyCompany.MyProject.Contracts.IMyPlugin";

/// <summary>Loads all plugins from a DLL file.</summary>
/// <param name="fileName">The filename of a DLL, e.g. "C:\Prog\MyApp\MyPlugIn.dll"</param>
/// <returns>A list of plugin objects.</returns>
/// <remarks>One DLL can contain several types which implement `IMyPlugin`.</remarks>
public List<IMyPlugin> LoadPluginsFromFile(string fileName)
{
    Assembly asm;
    IMyPlugin plugin;
    List<IMyPlugin> plugins;
    Type tInterface;

    plugins = new List<IMyPlugin>();
    asm = Assembly.LoadFrom(fileName);
    foreach (Type t in asm.GetExportedTypes()) {
        tInterface = t.GetInterface(PluginTypeName);
        if (tInterface != null && (t.Attributes & TypeAttributes.Abstract) !=
            TypeAttributes.Abstract) {

            plugin = (IMyPlugin)Activator.CreateInstance(t);
            plugins.Add(plugin);
        }
    }
    return plugins;
}

I assume that each plug-in implements IMyPlugin. You can define this interface any way you want. If you loop through all DLLs contained in a plug-ins folder and call this method, you can automatically load all the available plug-ins.

Usually you would have at least three assemblies: One containing the interface definition, the main assembly referencing this interface assembly and at least one assembly implementing (and of course referencing) this interface.

回答2:

Does each customer get their own exe and config file, and there is a shared dll? Or is there a shared exe, and each customer has their own dll?

You can put the full type name in the configuration like this:

Shared.exe.config:

<appSettings>
  <add key="CustomerType" value="NamespaceForJohn.ClassForJohn, AssemblyForJohn"/>
</appSettings>

And put AssemblyForJohn.dll in the same folder as your Shared.exe.

Then you can load it dynamically in code like this:

Shared.exe:

var typeString = ConfigurationManager.AppSettings["CustomerType"];
var parts = typeString.Split(',');
var typeName = parts[0];
var assemblyName = parts[1];
var instance = (BaseClass)Activator.CreateInstance(assemblyName, typeName).Unwrap();

回答3:

maybe this example will be helpful

public MyInterface GetNewType() { 
       Type type = Type.GetType( "MyClass", true ); 
       object newInstance = Activator.CreateInstance( type ); 
       return newInstance as MyInterface; 
    } 

回答4:

Here's one way to handle it with DI using Unity.

IUnityContainer container = new UnityContainer();
string customerNamespace = ConfigurationManager.AppSettings["CustomerNamespace"];
container.RegisterType(typeof(ISomeInterface), 
                       Type.GetType(customerNamespace+".SomeImplementation"));


// ...

ISomeInterface instance = conainer.Resolve<ISomeInterface>();

Where each customer has their own implementation of ISomeInterface in a customer specific namespace.

回答5:

You can create an instance of an external type from an assembly this way:

object obj = Activator.CreateInstance( 
    "External.Assembly.Name", "External.Assembly.Name.TypeName");
BaseClass b = (BaseClass) obj;
b.getEventId();

You'd store the name of the assembly and type in your configuration file or some other appropriate place.

回答6:

I would use Unity, but as a Simple Factory.

Unity Framework: How to Instantiate two classes from the same Interface?

You could store your

I am using Unity.2.1.505.2 (just in case that makes a difference).

  <configSections>
    <section name="unity" type="Microsoft.Practices.Unity.Configuration.UnityConfigurationSection, Microsoft.Practices.Unity.Configuration"/>
  </configSections>


      <unity>
        <container>
          <register type="IVehicle" mapTo="Car" name="myCarKey" />
          <register type="IVehicle" mapTo="Truck" name="myTruckKey" />
        </container>
      </unity>

Here is the DotNet code.

UnityContainer container = new UnityContainer();

UnityConfigurationSection section = (UnityConfigurationSection)ConfigurationManager.GetSection("unity");
                section.Configure(container);

string myKey = "John";  /* read from config file */ /* with this example, the value should be "myCarKey" or "myTruckKey"  */

IVehicle v1 = container.Resolve<IVehicle>(myKey); 

See:

http://msdn.microsoft.com/en-us/library/ff664762(v=pandp.50).aspx

and

http://www.sharpfellows.com/post/Unity-IoC-Container-.aspx