One of my assemblies contains the following 'provider' types:

I also have an XML file which holds provider info using the DeviceInfoProvider base class. A simplified version looks like this:

<DeviceInfoProvider Type="SbRioI2CProvider" Assembly="assembly.dll" >
</DeviceInfoProvider>
<DeviceInfoProvider Type="GenericProvider" Assembly="assembly.dll" >
</DeviceInfoProvider>

At runtime, I map XML fields to my variables:

assembly.dll  ⇒ assemblyPath
Type          ⇒ typeName

And after reading the XML, use the following code to instantiate my types:

var assembly = Assembly.LoadFrom(assemblyPath);

var type = (from t in assembly.GetTypes()
            where t.IsPublic && t.Name == typeName
            select t).FirstOrDefault();

if (type != null)
{
    instance = type.GetConstructor(Type.EmptyTypes).Invoke(null);
}

As expected, this generates my objects appropriately.

The problem comes when I try to cast instance as a base class object:

using (var provider = instance as DeviceInfoProvider)
{
    // provider is null!
}

The runtime type of instance is the expected derived class, yet I am unable to successfully cast it to its base type.

What am I missing?

• c#
• casting
• xml-deserialization

Your problem may be that you are creating instances (GenericProvider, SbRioI2CProvider) from types in assembly.dll in the LoadFrom context. Then you are trying to cast by name to a type (DeviceInfoProvider) in that assembly. This implicitly uses the Load context. Types from the same assembly but loaded in different contexts are considered different types by the runtime, so the cast fails and you get null. This article provides some additional explanation of assembly binding contexts.

To get this cast to succeed, you need to get the Assembly that is loaded in the LoadFrom context into the Load context. There are a couple ways to do this. One way is to put the assembly in the GAC. Another is to remove the assembly.dll from the applicationbase so that it will not be found by probing. Then use the AppDomain.AssemblyResolve event to load the Assembly that you got via LoadFrom.

Invoking the constructor of a reflected type doesn't create an instance of it.

To create an instance of a reflected type, call Activator.CreateInstance.

It looks like this line should be:

if (type != null) {
    instance = Activator.CreateInstance(type)
}

This will cause instance to be of type object, but now you can cast it to whatever you want.

See: http://msdn.microsoft.com/en-us/library/wccyzw83.aspx