How to obtain a list of classes that implement an interface?

Question

I'm trying to get a list of classes that implement an interface and then at some later point in the program, instantiate these classes and pass in parameters to their constructors.

In a previous Stack Overflow page, I saw this code that instantiated the classes with an empty constructor:

var preprocessors = from t
                    in Assembly.GetExecutingAssembly().GetTypes()
                    where t.GetInterfaces()
                           .Contains(typeof(Preprocessing))
                       && t.GetConstructor(Type.EmptyTypes) != null
                    select Activator.CreateInstance(t) as Preprocessing;

But I don't want some classes to be instantiated without passing some kind of parameter to the constructor (the parameter is obtained in a for loop so I have to wait until I instantiate it).

I tried doing just this to get the list of classes to be instantiated:

var preprocessors = from t
                    in Assembly.GetExecutingAssembly().GetTypes()
                    select t.GetInterfaces()
                            .Contains(typeof(Preprocessing))

But after doing this, I wasn't sure how to access the classes and instantiate them. Would really appreciate some guidance on this. Thanks!!

Edit: I can't figure out what to put in the Activator.CreateInstance(...) parentheses. I tried putting something like this:

foreach (var sim in similarities)
{
  var a = Activator.CreateInstance(sim, preprocessedData) as Preprocessing;

But that is throwing an error, most likely because preprocessedData is a DenseMatrix object (from the MathNet Numerics library). Is there any way to send a DenseMatrix as parameter and not an array?

You have everything you need in the question - use Activator.CreateInstance() to create an instance of the class — dotnetom, Jul 03 '14 at 19:46
I can't figure out what to put in the Activator.CreateInstance(...) parentheses. I'm doing — covfefe, Jul 03 '14 at 20:13

Vladimir Shmidt · Answer 1 · 2014-07-04T14:00:10.067

3

overload of CreateInstance with params is suitable here

public static Object CreateInstance(
    Type type,
    params Object[] args
)

http://msdn.microsoft.com/en-US/library/wcxyzt4d(v=vs.110).aspx

usage example

var constructorParams = new object[] { 1, "string", new object() }; //here is example of params that you will pass to each plugin constructor
var pluginTypes = Assembly.GetExecutingAssembly().GetTypes().Where(t => typeof(Preprocessing).IsAssignableFrom(t)); //all types of your plugin
var plugins = pluginTypes.Select(pluginType => Activator.CreateInstance(pluginType, constructorParams)); //instanciated plugins

UPDATE

var a = Activator.CreateInstance(sim, new object[] { preprocessedData })

Lets imagine that sim has this implementation:

class sim
{
  public sim(int a, string b, AnotherType c){}
}

so to initiate this class with parametres constructor you have to pass three parametrs like:

var a = Activator.CreateInstance(sim, new object[] { 1230, "string", new AnotherType() })

as a result CLR via reflection will produce you your intanse.

edited Jul 04 '14 at 14:00

answered Jul 03 '14 at 19:53

Vladimir Shmidt

2,651
1
19
21

1

Vladimir, Do you think all SO users know russian? – L.B Jul 03 '14 at 19:56
1

L.B could you be more specific? p.s sorry for misspell – Vladimir Shmidt Jul 03 '14 at 20:03
Vladimir, click your link in question :) – L.B Jul 03 '14 at 20:05
1

oh, sorry. Current location in OS setup as EN so site provide english version even with ru-ru. will change – Vladimir Shmidt Jul 03 '14 at 20:09

Nicholas Carey · Answer 2 · 2014-07-03T20:38:55.827

This should give you a list of types in the current app domain that implement the specified interface:

IEnumerable<Type> TypesImplementingInterface( Type interfaceType , params Type[] desiredConstructorSignature )
{
  if (  interfaceType == null     ) throw new ArgumentNullException(       "interfaceType" ) ;
  if ( !interfaceType.IsInterface ) throw new ArgumentOutOfRangeException( "interfaceType" ) ;

  return AppDomain
         .CurrentDomain
         .GetAssemblies()
         .SelectMany( a => a.GetTypes() )
         .Where( t =>  t.IsAssignableFrom( interfaceType ) )
         .Where( t => !t.IsInterface )
         .Where( t =>  t.GetConstructor( desiredConstructorSignature ) != null )
         ;
}

Once you have that, instantiating instances of the type is easy, something along the lines of this:

T ConstructInstance<T>( Type t , params object[] parameterList )
{
  Type[]          signature   = parameterList
                                .Select( p => p.GetType() )
                                .ToArray()
                                ;
  ConstructorInfo constructor = t.GetConstructor(   signature     ) ;
  T               instance    = constructor.Invoke( parameterList ) ;

  return instance ;
}

in your case, you'd want something like this:

Type[] types = TypesImplementingInterface( typeof(IFoo) , typeof(DenseMatrix) ).ToArray() ;

DenseMatrix dmInstance = ... ;
...
IFoo constructedInstance = ConstructInstance<IFoo>( types[0] , dmInstance ) ;

MJVC · Answer 3 · 2014-07-03T20:34:21.673

The CreateInstance has a signature that takes the constructor parameters:

Activator.CreateInstance Method (Type, Object[])

EDIT

This is an example:

interface ITest
{
    string Say();
}

class Test1 : ITest
{
    private string message;

    public Test1(string message)
    {
        this.message = message;
    }

    public string Say()
    {
        return this.message;
    }
}

class Test2 : ITest
{
    private string message;

    public Test2(string message)
    {
        this.message = message;
    }

    public string Say()
    {
        return this.message;
    }
}

void Main()
{
    string[] args = new string[] { "Hello", "World" };
    var tests = Assembly.GetExecutingAssembly().GetTypes()
        .Where(t => t.GetInterfaces().Contains(typeof(ITest)))
        .Select((t, i) => Activator.CreateInstance(t, args[i]) as ITest);

    foreach (var item in tests)
    {
        Console.WriteLine(item.Say());
    }
}

I'm doing var a = Activator.CreateInstance(sim, preprocessedData) as Preprocessing. Where preprocessedData is a DenseMatrix object from the MathNet.Numerics library, but it is not accepting that as parameter. — covfefe, Jul 03 '14 at 20:21
Maybe because Preprocessing doesn't have a constructor accepting that as a parameter. Did you wrote that class or it's defined in a third party library? — MJVC, Jul 04 '14 at 13:16

score 0 · Answer 4 · answered Jul 03 '14 at 20:48

The reflection mechanism presented by others may fail if the you have a large set of assemblies to look through - the .Net VM lazy loads, assemblies aren't loaded until they are actually referenced, that is, a class is explicitly referenced from that assembly.

If your program references 20 assemblies and has only actually loaded 2 of them, AppDomain.GetAssemblies() won't return the other 18 and you'll never bother looking through them for implementors of your interface.

There are various ways of forcing the assemblies to be loaded. You could look through your call stack, such as I do below, or you could look through your current assemblies and force-load their referenced assemblies, etc.

Below is a complete class that attempts to deal with these problems:

public class PluginMgr<T> where T : class
{
    public PluginMgr()
    {
        this.Plugins = new List<T>();
    }

    /// <summary>
    /// The list of plugin instances that were found and created.
    /// </summary>
    public List<T> Plugins { get; private set; }

    /// <summary>
    /// Scans loaded assemblies for classes that implement the interface specified by the type 
    /// parameter, then instantiates and stores any classes that are found.
    /// </summary>
    public void Initialize()
    {
        ForceLoadAssemblies();
        FindPlugins();
    }

    /// <summary>
    /// Attempts to force the VM to load all assemblies that are referenced by any assembly 
    /// implicated in the call stack. Referenced assemblies are not loaded until reference 
    /// resolution happens that depends on that assembly; if an assembly X has a reference to
    /// another assembly Y, but X never uses anything from Y, then Y is never loaded; that is to 
    /// say, the .Net assembly loader uses 'lazy loading'.
    /// 
    /// This is necessary because our plugin sniffing logic won't find the plugins that don't
    /// have their defining assembly loaded. If we forcibly load the assembly, then we'll guarentee
    /// that we find the assembly.
    /// </summary>
    private void ForceLoadAssemblies()
    {
        StackFrame[] frames;
        AssemblyName[] refedAssemblies;
        Assembly assembly;

        frames = new StackTrace().GetFrames();

        foreach (StackFrame frame in frames)
        {
            assembly = frame.GetMethod().DeclaringType.Assembly;
            refedAssemblies = assembly.GetReferencedAssemblies();

            foreach (AssemblyName refedAssembly in refedAssemblies)
            {
                Assembly.Load(refedAssembly);
            }
        }
    }


    /// <summary>
    /// Scan through every loaded assembly to find types that are implementors of our
    /// given interface.
    /// </summary>
    private void FindPlugins()
    {
        Assembly[] assemblies = AppDomain.CurrentDomain.GetAssemblies();
        Type interfaceType = typeof(T);

        foreach( Assembly assembly in assemblies )
        {
            Type[] types = assembly.GetExportedTypes();
            foreach( Type type in types )
            {
                if( type.GetInterfaces().Contains( interfaceType ) )
                {
                    T plugin = Activator.CreateInstance( type ) as T;
                    this.Plugins.Add( plugin );
                }
            }
        }
    }

You ar right about assemblies being loaded or not. So I've tried your code but it didn't work for me. If I have any use of the clases, they are all loaded and I can find them. If I don't use one of them, your plugin manager cannot find it. — Antonio Rodríguez, Sep 30 '20 at 12:49

score 0 · Accepted Answer · answered Jul 11 '14 at 15:20

Thank you for all the responses, they went a long way in helping me figure this out.

Some of the other solutions here may also be equivalent to what I did, but this ended up working for me.

var preprocessors = from t in Assembly.GetExecutingAssembly().GetTypes()
                            where t.GetInterfaces().Contains(typeof(Preprocessing))
                            select Activator.CreateInstance(t, originalData) as Preprocessing;  // Create instance of class with originalData as parameter

// Obtain a list of all types in the assembly (will be instantiated in the foreach loop)
var similarities = Assembly.GetExecutingAssembly().GetTypes();

This is how I created an instance and passed it parameters in a for-loop:

foreach (var sim in similarities)
{
   if (sim.GetInterfaces().Contains(typeof(Similarity)))   // Check if the assembly types are of type Similarity
   {
      // Create instance of similarity class with preprocessedData as a parameter
      var similarityObject = Activator.CreateInstance(sim, preprocessedData) as Similarity;

How to obtain a list of classes that implement an interface?

5 Answers5