Getting the constructor of an Interface Type through reflection?
- by Will Marcouiller
I have written a generic type: IDirectorySource<T> where T : IDirectoryEntry, which I'm using to manage Active Directory entries through my interfaces objects: IGroup, IOrganizationalUnit, IUser.
So that I can write the following:
IDirectorySource<IGroup> groups = new DirectorySource<IGroup>(); // Where IGroup implements `IDirectoryEntry`, of course.`
foreach (IGroup g in groups.ToList()) {
listView1.Items.Add(g.Name).SubItems.Add(g.Description);
}
From the IDirectorySource<T>.ToList() methods, I use reflection to find out the appropriate constructor for the type parameter T. However, since T is given an interface type, it cannot find any constructor at all!
Of course, I have an internal class Group : IGroup which implements the IGroup interface. No matter how hard I have tried, I can't figure out how to get the constructor out of my interface through my implementing class.
[DirectorySchemaAttribute("group")]
public interface IGroup {
}
internal class Group : IGroup {
internal Group(DirectoryEntry entry) {
NativeEntry = entry;
Domain = NativeEntry.Path;
}
// Implementing IGroup interface...
}
Within the ToList() method of my IDirectorySource<T> interface implementation, I look for the constructor of T as follows:
internal class DirectorySource<T> : IDirectorySource<T> {
// Implementing properties...
// Methods implementations...
public IList<T> ToList() {
Type t = typeof(T)
// Let's assume we're always working with the IGroup interface as T here to keep it simple.
// So, my `DirectorySchema` property is already set to "group".
// My `DirectorySearcher` is already instantiated here, as I do it within the DirectorySource<T> constructor.
Searcher.Filter = string.Format("(&(objectClass={0}))", DirectorySchema)
ConstructorInfo ctor = null;
ParameterInfo[] params = null;
// This is where I get stuck for now... Please see the helper method.
GetConstructor(out ctor, out params, new Type() { DirectoryEntry });
SearchResultCollection results = null;
try {
results = Searcher.FindAll();
} catch (DirectoryServicesCOMException ex) {
// Handling exception here...
}
foreach (SearchResult entry in results)
entities.Add(ctor.Invoke(new object() { entry.GetDirectoryEntry() }));
return entities;
}
}
private void GetConstructor(out ConstructorInfo constructor, out ParameterInfo[] parameters, Type paramsTypes) {
Type t = typeof(T);
ConstructorInfo[] ctors = t.GetConstructors(BindingFlags.CreateInstance
| BindingFlags.NonPublic
| BindingFlags.Public
| BindingFlags.InvokeMethod);
bool found = true;
foreach (ContructorInfo c in ctors) {
parameters = c.GetParameters();
if (parameters.GetLength(0) == paramsTypes.GetLength(0)) {
for (int index = 0; index < parameters.GetLength(0); ++index) {
if (!(parameters[index].GetType() is paramsTypes[index].GetType()))
found = false;
}
if (found) {
constructor = c;
return;
}
}
}
// Processing constructor not found message here...
}
My problem is that T will always be an interface, so it never finds a constructor.
Might somebody guide me to the right path to follow in this situation?
