Develop Reference

Assembly.CreateInstance type conversion returns null

I'm trying to load and use a DLL at runtime and this works fine:
var pluggin = asm2.CreateInstance("ParserTest.Interface", true) as iPluggin;
But this doesn't (I need to loop through the DLL files in a specific folder to find the right ones implementing iPluggin interface):
...
var asm = Assembly.LoadFrom(dll.FullName);
if (asm.GetExportedTypes().FirstOrDefault(q => q.GetInterface(tName) != null) == null) continue;
Project.ProcessList.Add(asm.CreateInstance(tName, true) as iPluggin);
...
Doing some research in debug mode I found out that:
asm.CreateInstance(tName, true)
returns the correct object, but when trying to cast it to iPluggin the result is null.
Any idea why?

I did something similar:
private readonly Type _pluginbaseType = typeof(BasePlugin);
public AssemblyPlugin(Assembly assembly)
{
Type[] _plugins = _assembly.GetExportedTypes()
.Where(t => t.BaseType.IsSubclassOf(_pluginbaseType)
.ToArray();
}
After this you can:
BasePlugin plugin = (BasePlugin)Activator.CreateInstance(pluginType);
You should prefer a base class instead of an interface.

Mistery solved althought I don't understand it.
The DLL that defines de interface (SDK.dll) was among the ones being checked.
I removed it manually in my query and all woks as expected now.
This is my final code:
var plugins =
from fi in di.GetFiles("*.dll").Where(p => p.Name.ToUpper() != "SDK.DLL")
let asm = Assembly.LoadFrom(fi.FullName)
from t in asm.GetExportedTypes()
where t.GetInterface(typeof(iPluggin).Name) != null
select asm.CreateInstance(t.FullName, true) as iPluggin;
Project.ProcessList.AddRange(plugins);

Using reflection to get all classes of certain base type in dll

I have a dll that contains a number of classes that all inherit from a CommandBase class. I'm trying to get instances of all of these classes (CommandA, CommandB, CommandC, etc...) using reflection in C# so that I can call a specific method on each one. Here is what I have so far:
//get assemblies in directory.
string folder = Path.Combine(HttpContext.Current.Server.MapPath("~/"), "bin");
var files = Directory.GetFiles(folder, "*.dll");
//load each assembly.
foreach (string file in files)
{
var assembly = Assembly.LoadFile(file);
if (assembly.FullName == "MyCommandProject")
{
foreach (var type in assembly.GetTypes())
{
if (!type.IsClass || type.IsNotPublic) continue;
if(type is CommandBase)
{
var command = Activator.CreateInstance(type) as CommandBase;
}
}
}
}
I'm having 2 issues. The 1st issue is that the line "if(type is CommandBase") gives the following warning:
The given expression is never of the provided type CommandBase.
The 2nd issue is that I can't figure out how to get an instance of the actual object (CommandA, CommandB, etc...), just converting it to CommandBase isn't enough.

This is the method I use to load up based on an interface.
private static List<T> GetInstances<T>()
{
return (from t in Assembly.GetExecutingAssembly().GetTypes()
where t.GetInterfaces().Contains(typeof (T)) && t.GetConstructor(Type.EmptyTypes) != null
select (T) Activator.CreateInstance(t)).ToList();
}
And here's the same function that pulls back based on base class.
private static IList<T> GetInstances<T>()
{
return (from t in Assembly.GetExecutingAssembly().GetTypes()
where t.BaseType == (typeof(T)) && t.GetConstructor(Type.EmptyTypes) != null
select (T)Activator.CreateInstance(t)).ToList();
}
Of course it would need to be modified slightly to point at the reference you're loading.

You must change
if(type is CommandBase)
to
if(type.IsSubclassOf(typeof(CommandBase)))
If the IsSubclassOf is the converse of IsAssignableFrom. That is, if t1.IsSubclassOf(t2) is true, then t2.IsAssignableFrom(t1) is also true.

Change type is CommandBase to typeof(CommandBase).IsAssignableFrom(type)

That's because your type variable is a Type, not a CommandBase.
You want
if(type == typeof(CommandBase))
(Thanks Greg for the correction)

Determine if a type is static

Let's say I have a Type called type.
I want to determine if I can do this with my type (without actually doing this to each type):
If type is System.Windows.Point then I could do this:
Point point1 = new Point();
However if type is System.Environment then this will not fly:
Environment environment1 = new Environment(); //wrong
So if I am iterating through every visible type in an assembly how do I skip all the types that will fail to create an instance like the second one? I'm kind of new to reflection so I'm not that great with the terminology yet. Hopefully what I'm trying to do here is pretty clear.

static classes are declared abstract and sealed at the IL level. So, you can check IsAbstract property to handle both abstract classes and static classes in one go (for your use case).
However, abstract classes are not the only types you can't instantiate directly. You should check for things like interfaces (without the CoClass attribute) and types that don't have a constructor accessible by the calling code.

type.IsAbstract && type.IsSealed
This would be a sufficient check for C# since an abstract class cannot be sealed or static in C#. However, you'll need to be careful when dealing with CLR types from other languages.

you can search for public contructors like this,
Type t = typeof(Environment);
var c = t.GetConstructors(BindingFlags.Public);
if (!t.IsAbstract && c.Length > 0)
{
//You can create instance
}
Or if you only interested in parameterless constructor you can use
Type t = typeof(Environment);
var c = t.GetConstructor(Type.EmptyTypes);
if (c != null && c.IsPublic && !t.IsAbstract )
{
//You can create instance
}

Type t = typeof(System.GC);
Console.WriteLine(t.Attributes);
TypeAttributes attribForStaticClass = TypeAttributes.AutoLayout | TypeAttributes.AnsiClass | TypeAttributes.Class |
TypeAttributes.Public | TypeAttributes.Abstract | TypeAttributes.Sealed | TypeAttributes.BeforeFieldInit;
Console.WriteLine((t.Attributes == attribForStaticClass));
I guess, this should work.

This is a way to get all public contstuctors of all types in an assembly.
var assembly = AppDomain.CurrentDomain.GetAssemblies()[0]; // first assembly for demo purposes
var types = assembly.GetTypes();
foreach (var type in types)
{
var constructors = type.GetConstructors();
}

How enumerate all classes with custom class attribute?

Question based on MSDN example.
Let's say we have some C# classes with HelpAttribute in standalone desktop application. Is it possible to enumerate all classes with such attribute? Does it make sense to recognize classes this way? Custom attribute would be used to list possible menu options, selecting item will bring to screen instance of such class. Number of classes/items will grow slowly, but this way we can avoid enumerating them all elsewhere, I think.

Yes, absolutely. Using Reflection:
static IEnumerable<Type> GetTypesWithHelpAttribute(Assembly assembly) {
foreach(Type type in assembly.GetTypes()) {
if (type.GetCustomAttributes(typeof(HelpAttribute), true).Length > 0) {
yield return type;
}
}
}

Well, you would have to enumerate through all the classes in all the assemblies that are loaded into the current app domain. To do that, you would call the GetAssemblies method on the AppDomain instance for the current app domain.
From there, you would call GetExportedTypes (if you only want public types) or GetTypes on each Assembly to get the types that are contained in the assembly.
Then, you would call the GetCustomAttributes extension method on each Type instance, passing the type of the attribute you wish to find.
You can use LINQ to simplify this for you:
var typesWithMyAttribute =
from a in AppDomain.CurrentDomain.GetAssemblies()
from t in a.GetTypes()
let attributes = t.GetCustomAttributes(typeof(HelpAttribute), true)
where attributes != null && attributes.Length > 0
select new { Type = t, Attributes = attributes.Cast<HelpAttribute>() };
The above query will get you each type with your attribute applied to it, along with the instance of the attribute(s) assigned to it.
Note that if you have a large number of assemblies loaded into your application domain, that operation could be expensive. You can use Parallel LINQ to reduce the time of the operation (at the cost of CPU cycles), like so:
var typesWithMyAttribute =
// Note the AsParallel here, this will parallelize everything after.
from a in AppDomain.CurrentDomain.GetAssemblies().AsParallel()
from t in a.GetTypes()
let attributes = t.GetCustomAttributes(typeof(HelpAttribute), true)
where attributes != null && attributes.Length > 0
select new { Type = t, Attributes = attributes.Cast<HelpAttribute>() };
Filtering it on a specific Assembly is simple:
Assembly assembly = ...;
var typesWithMyAttribute =
from t in assembly.GetTypes()
let attributes = t.GetCustomAttributes(typeof(HelpAttribute), true)
where attributes != null && attributes.Length > 0
select new { Type = t, Attributes = attributes.Cast<HelpAttribute>() };
And if the assembly has a large number of types in it, then you can use Parallel LINQ again:
Assembly assembly = ...;
var typesWithMyAttribute =
// Partition on the type list initially.
from t in assembly.GetTypes().AsParallel()
let attributes = t.GetCustomAttributes(typeof(HelpAttribute), true)
where attributes != null && attributes.Length > 0
select new { Type = t, Attributes = attributes.Cast<HelpAttribute>() };

Other answers reference GetCustomAttributes. Adding this one as an example of using IsDefined
Assembly assembly = ...
var typesWithHelpAttribute =
from type in assembly.GetTypes()
where type.IsDefined(typeof(HelpAttribute), false)
select type;

As already stated, reflection is the way to go. If you are going to call this frequently, I highly suggest caching the results, as reflection, especially enumerating through every class, can be quite slow.
This is a snippet of my code that runs through all the types in all loaded assemblies:
// this is making the assumption that all assemblies we need are already loaded.
foreach (Assembly assembly in AppDomain.CurrentDomain.GetAssemblies())
{
foreach (Type type in assembly.GetTypes())
{
var attribs = type.GetCustomAttributes(typeof(MyCustomAttribute), false);
if (attribs != null && attribs.Length > 0)
{
// add to a cache.
}
}
}

This is a performance enhancement on top of the accepted solution. Iterating though all classes can be slow because there are so many. Sometimes you can filter out an entire assembly without looking at any of its types.
For example if you are looking for an attribute that you declared yourself, you don't expect any of the system DLLs to contain any types with that attribute. The Assembly.GlobalAssemblyCache property is a quick way to check for system DLLs. When I tried this on a real program I found I could skip 30,101 types and I only have to check 1,983 types.
Another way to filter is to use Assembly.ReferencedAssemblies. Presumably if you want classes with a specific attribute, and that attribute is defined in a specific assembly, then you only care about that assembly and other assemblies that reference it. In my tests this helped slightly more than checking the GlobalAssemblyCache property.
I combined both of these and got it even faster. The code below includes both filters.
string definedIn = typeof(XmlDecoderAttribute).Assembly.GetName().Name;
foreach (Assembly assembly in AppDomain.CurrentDomain.GetAssemblies())
// Note that we have to call GetName().Name. Just GetName() will not work. The following
// if statement never ran when I tried to compare the results of GetName().
if ((!assembly.GlobalAssemblyCache) && ((assembly.GetName().Name == definedIn) || assembly.GetReferencedAssemblies().Any(a => a.Name == definedIn)))
foreach (Type type in assembly.GetTypes())
if (type.GetCustomAttributes(typeof(XmlDecoderAttribute), true).Length > 0)

In case of the Portable .NET limitations, the following code should work:
public static IEnumerable<TypeInfo> GetAtributedTypes( Assembly[] assemblies,
Type attributeType )
{
var typesAttributed =
from assembly in assemblies
from type in assembly.DefinedTypes
where type.IsDefined(attributeType, false)
select type;
return typesAttributed;
}
or for a large number of assemblies using loop-state based yield return:
public static IEnumerable<TypeInfo> GetAtributedTypes( Assembly[] assemblies,
Type attributeType )
{
foreach (var assembly in assemblies)
{
foreach (var typeInfo in assembly.DefinedTypes)
{
if (typeInfo.IsDefined(attributeType, false))
{
yield return typeInfo;
}
}
}
}

This is another version of the code provided by Trade-Ideas philip,
I've condensed the code to linq, plugged it into a nice static function which you can just drop in the project.
Original:
https://stackoverflow.com/a/41411243/4122889
I've also added AsParallel() - on my machine with enough cores etc, and with a 'normally' sized project (which is completely subjective), this was the fastest/
Without AsParallel() this took 1,5 seconds for about 200 results, and with it, it took about a couple milliseconds - therefore this seems the fastest to me.
Note that this skips the assemblies in the GAC.
private static IEnumerable<IEnumerable<T>> GetAllAttributesInAppDomain<T>()
{
var definedIn = typeof(T).Assembly.GetName().Name;
var assemblies = AppDomain.CurrentDomain.GetAssemblies();
var res = assemblies.AsParallel()
.Where(assembly => (!assembly.GlobalAssemblyCache) && ((assembly.GetName().Name == definedIn) ||
assembly.GetReferencedAssemblies()
.Any(a => a.Name == definedIn))
)
.SelectMany(c => c.GetTypes())
.Select(type => type.GetCustomAttributes(typeof(T), true)
.Cast<T>()
)
.Where(c => c.Any());
return res;
}
Usage:
var allAttributesInAppDomain = GetAllAttributesInAppDomain<ExportViewAttribute>();
Note if you have only 1 attribute per class, so not multiple, its easier to flatten the result from IEnumerable<IEnumerable<T>> to IEnumerable<T> like so:
var allAttributesInAppDomainFlattened = allAttributesInAppDomain.SelectMany(c => c);
Remember, this uses IEnumerable so call ToList() to actually run the function.

We can improve on Andrew's answer and convert the whole thing into one LINQ query.
public static IEnumerable<Type> GetTypesWithHelpAttribute(Assembly assembly)
{
return assembly.GetTypes().Where(type => type.GetCustomAttributes(typeof(HelpAttribute), true).Length > 0);
}

How to find all the classes which implement a given interface?

Under a given namespace, I have a set of classes which implement an interface. Let's call it ISomething. I have another class (let's call it CClass) which knows about ISomething but doesn't know about the classes which implement that interface.
I would like that CClass to look for all the implementation of ISomething, instantiate an instance of it and execute the method.
Does anybody have an idea on how to do that with C# 3.5?

A working code-sample:
var instances = from t in Assembly.GetExecutingAssembly().GetTypes()
where t.GetInterfaces().Contains(typeof(ISomething))
&& t.GetConstructor(Type.EmptyTypes) != null
select Activator.CreateInstance(t) as ISomething;
foreach (var instance in instances)
{
instance.Foo(); // where Foo is a method of ISomething
}
Edit Added a check for a parameterless constructor so that the call to CreateInstance will succeed.

You can get a list of loaded assemblies by using this:
Assembly assembly = System.Reflection.AppDomain.CurrentDomain.GetAssemblies()
From there, you can get a list of types in the assembly (assuming public types):
Type[] types = assembly.GetExportedTypes();
Then you can ask each type whether it supports that interface by finding that interface on the object:
Type interfaceType = type.GetInterface("ISomething");
Not sure if there is a more efficient way of doing this with reflection.

A example using Linq:
var types =
myAssembly.GetTypes()
.Where(m => m.IsClass && m.GetInterface("IMyInterface") != null);

foreach (Type t in Assembly.GetCallingAssembly().GetTypes())
{
if (t.GetInterface("ITheInterface") != null)
{
ITheInterface executor = Activator.CreateInstance(t) as ITheInterface;
executor.PerformSomething();
}
}

You could use something like the following and tailor it to your needs.
var _interfaceType = typeof(ISomething);
var currentAssembly = System.Reflection.Assembly.GetExecutingAssembly();
var types = GetType().GetNestedTypes();
foreach (var type in types)
{
if (_interfaceType.IsAssignableFrom(type) && type.IsPublic && !type.IsInterface)
{
ISomething something = (ISomething)currentAssembly.CreateInstance(type.FullName, false);
something.TheMethod();
}
}
This code could use some performance enhancements but it's a start.

Maybe we should go this way
foreach ( var instance in Assembly.GetExecutingAssembly().GetTypes().Where(a => a.GetConstructor(Type.EmptyTypes) != null).Select(Activator.CreateInstance).OfType<ISomething>() )
instance.Execute();