assembly.GetTypes() does not return all types

I try to lead the types from an .dll (which is also referenced in the executing project). I call:

    public static void LoadPlugin(string pluginFile)
    {
        Assembly assembly = Assembly.LoadFrom(pluginFile);

        foreach (Type type in assembly.GetTypes())
        {
            // play with it
        }
    }

It loads just a few of them:

public partial class Mathematics : UserControl, IMathematics, IPortable

and

public partial class Welcome : UserControl

but the next one, and some others, are ignored:

public partial class Test : UserControl, ITest, IPortable

They all stand in the same assembly, under the same namespace. The public static void LoadPlugin(string pluginFile) method is located in other assembly that is also referenced in th开发者_如何学JAVAe executing project.

No exceptions are thrown. What could be the issues for not loading all the types? Any ideas?

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Are the types not loaded inheriting from a third assembly that isn't referenced correctly from the calling assembly?