如何为各种构造函数做Activator.CreateInstance？

Question

I need to get my subclasses of DataContext and I found the below function for that lying around somewhere and it sure finds my subclass but I can't instantiate it :(

  public static IEnumerable<T> GetSubclassesFor<T>(Assembly assembly)
  {
   return (assembly.GetTypes()
    .Where(t => t.BaseType == (typeof (T)))
    .Select(t => (T) Activator.CreateInstance(t, new object[] {"asdasd"})))
    .ToList();
  }

I get the following error message:

System.Reflection.TargetInvocationException : Ett undantagsfel har inträffat i målet för en aktivering. ----> System.TypeInitializationException : Typinitieraren för PlaynGO.Cashier.Data.CashierDC utlöste ett undantag. ----> System.NullReferenceException : Objektreferensen har inte angetts till en instans av ett objekt. vid System.RuntimeMethodHandle. InvokeConstructor(IRuntimeMethodInfo method, Object[] args, ref SignatureStruct signature, RuntimeType declaringType) vid System.Reflection.RuntimeConstructorInfo.Invoke(BindingFlags invokeAttr, Binder binder, Object[] parameters, CultureInfo culture) vid System.RuntimeType.CreateInstanceImpl(BindingFlags bindingAttr, Binder binder, Object[] args, CultureInfo culture, Object[] activationAttributes) vid System.Activator.CreateInstance(Type type, Object[] args) vid PlaynGO.Dbml.Reflexion.b _3(Type t) i Reflexion.cs: line 23 vid System.Linq.Enumerable.WhereSelectArrayIterator 2.MoveNext() vid System.Collections.Generic.List 1..ctor(IEnumerable 1 collection) vid System.Linq.Enumerable.ToList(IEnumerable 1 source) vid PlaynGO.Dbml.Reflexion.GetInstances(Assembly assembly) i Reflexion.cs: line 23 vid PlaynGO.Dbml.UnitTests.TestReflection.TestGettingTypes() i TestReflection.cs: line 21 --TypeInitializationException vid PlaynGO.Cashier.Data.CashierDC..ctor(String connection) --NullReferenceExc eption vid PlaynGO.Cashier.Data.CashierDC..cctor()

The constructor I want to call is the following:

    public CashierDC(string connection) :
   base(connection, mappingSource)

MappingSource is directly instantiated and is an instance field. Where do I go wrong? What do I have to do to make this work?

PS. This is .NET 4.0

Answer 1

I think your error is something else. If Activator.CreateInstance can't find your constructor you get a MissingMethodException not a TargetInvocationException . I suspect there's something else wrong in the actual class you're trying to instantiate.

Answer 2

The TargetInvocationException indicates that the constructor that it is invoking has thrown an exception. Perhaps this is due to the actual value that you are passing in for the connection string. You might want to try using the debugger and set a break point in the constructor taking a connection string and (1) make sure that it is being invoked and (2) determine where the exception is occurring.