I'd like to instantiate a class by name (a string) without specifying a namespace or assembly. Like this (Unity syntax):
var processor = container.Resolve<IProcessor>("SpecialProcessor");
would instantiate the first IProcessor it finds called SpecialProcessor. Maybe
MyNamespace.SpecialProcessor
I'd like to avoid having to create an entry in a config every time somebody adds a new processor. I'm fine with having an entry for all candidate assemblies though.
Can I use an IoC container for something like this or should I roll my own?
Here's a function that does something very similar to what you want. You can modify it to filter based on a specific class name pretty easily.
These functions have references to a few utilities we use for logging and exception handling. You'll need to replace them with whatever you normally do in these situations.
public static T FindAndCreate<T>(bool localOnly, bool exportedOnly)
{
Type[] types = FindAssignableClasses(typeof(T), localOnly, exportedOnly, false);
if (types.Length == 0)
{
return default(T);
}
if (types.Length != 1)
{
Log.Warn(typeof(ReflectionUtil),
"FindAndCreate found {0} instances of {1} whereas only 1 was expected. Using {2}. {3}",
types.Length,
typeof(T).FullName,
types[0].FullName,
String.Join("\r\n ", Array.ConvertAll<Type, String>(types, GetFullName)));
}
try
{
return (T)Activator.CreateInstance(types[0]);
}
catch (Exception ex)
{
throw ExceptionUtil.Rethrow(ex,
"Unable to create instance of {0} found for interface {1}.",
types[0].FullName,
typeof(T).FullName);
}
}
public static Type[] FindAssignableClasses(Type assignable, bool localOnly, bool exportedOnly, bool loadDll)
{
var list = new List<Type>();
string localDirectoryName = Path.GetDirectoryName(typeof(ReflectionUtil).Assembly.CodeBase);
if (loadDll && !_loadedAllDlls)
{
foreach (string dllPath in Directory.GetFiles(localDirectoryName.Substring(6), "*.dll"))
{
try
{
Assembly.LoadFrom(dllPath);
}
catch
{
// ignore
}
}
_loadedAllDlls = true;
}
foreach (Assembly asm in AppDomain.CurrentDomain.GetAssemblies())
{
try
{
if (localOnly && Path.GetDirectoryName(asm.CodeBase) != localDirectoryName)
{
continue;
}
Type[] typesInAssembly;
try
{
typesInAssembly = exportedOnly ? asm.GetExportedTypes() : asm.GetTypes();
}
catch
{
continue;
}
foreach (Type t in typesInAssembly)
{
try
{
if (assignable.IsAssignableFrom(t) && assignable != t)
{
list.Add(t);
}
}
catch (Exception ex)
{
Log.Debug(
typeof(ReflectionUtil),
String.Format(
"Error searching for types assignable to type {0} searching assembly {1} testing {2}{3}",
assignable.FullName,
asm.FullName,
t.FullName,
FlattenReflectionTypeLoadException(ex)),
ex);
}
}
}
catch (Exception ex)
{
// ignore dynamic module error, no way to check for this condition first
// http://groups.google.com/group/microsoft.public.dotnet.languages.csharp/browse_thread/thread/7b02223aefc6afba/c8f5bd05cc8b24b0
if (!(ex is NotSupportedException && ex.Message.Contains("not supported in a dynamic")))
{
Log.Debug(
typeof(ReflectionUtil),
String.Format(
"Error searching for types assignable to type {0} searching assembly {1} from {2}{3}",
assignable.FullName,
asm.FullName,
asm.CodeBase,
FlattenReflectionTypeLoadException(ex)),
ex);
}
}
}
return list.ToArray();
}
It sounds like you have a plugin architecture and you want to allow other components to provide implementations of IProcessor without having to update some master config file. If that's the case, then I think you are best suited by using MEF (managed extensibility framework) (Website).
This is a framework designed to allow such behavior. Once you establish a catalog of assemblies to load components from, importing the collection of IProcessor instances is as easy as the following
var processors = container.GetExportedValues<IProcessor>();
There are many tutorials online for MEF that should get you started.
