the1mason Posts - Answer Overflow

the1mason

•Created by the1mason on 4/20/2025 in #help

typeof(T).IsAssignableFrom does not work as I expected

There is a method in a generic class PluginLoader<TPlugin> : IPluginLoader<TPlugin>:

    private static TPlugin? CreatePlugin(Assembly assembly)
    {
        var types = assembly.GetTypes().Where(t => typeof(TPlugin).IsAssignableFrom(t)).ToArray();
        return types.Length switch
        {
            > 1 => throw new ApplicationException(
                $"Found more than 1 type, implementing IPlugin in assembly {assembly.FullName}"),
            < 1 => throw new ApplicationException(
                $"Found 0 types, implementing IPlugin in assembly {assembly.FullName}"),
            _ => (TPlugin?)Activator.CreateInstance(types.First()) ?? default
        };
    }

    private static TPlugin? CreatePlugin(Assembly assembly)
    {
        var types = assembly.GetTypes().Where(t => typeof(TPlugin).IsAssignableFrom(t)).ToArray();
        return types.Length switch
        {
            > 1 => throw new ApplicationException(
                $"Found more than 1 type, implementing IPlugin in assembly {assembly.FullName}"),
            < 1 => throw new ApplicationException(
                $"Found 0 types, implementing IPlugin in assembly {assembly.FullName}"),
            _ => (TPlugin?)Activator.CreateInstance(types.First()) ?? default
        };
    }

There is also an interface IWebPlugin with implementation Plugin1 This method is called with an assembly, containing Plugin1 and tries to create an instance of it. It worked before I made PluginLoader generic, but now typeof(TPlugin).IsAssignableFrom(t) returns false. Plugin1.GetInterface("IWebPlugin") returnsIWebPlugin, typeof(TPlugin gets IWebPlugin too, but the original condition fails nevertheless. Here's my result of playing with the debugger

 var debugTypes = assembly.GetTypes()
  {System.RuntimeType[1]}
    [0]: {Plugin1.Plugin1}

  var iWebPlugin = debugTypes[0].GetInterface("IWebPlugin")
  {RainCrab.Plugins.AspNet.IWebPlugin}


  var iaf = iWebPlugin.IsAssignableFrom(debugTypes[0])
  true

  var iafGeneric = typeof(TPlugin).IsAssignableFrom(debugTypes[0])
  false

  typeof(TPlugin)
  {RainCrab.Plugins.AspNet.IWebPlugin}

 var debugTypes = assembly.GetTypes()
  {System.RuntimeType[1]}
    [0]: {Plugin1.Plugin1}

  var iWebPlugin = debugTypes[0].GetInterface("IWebPlugin")
  {RainCrab.Plugins.AspNet.IWebPlugin}


  var iaf = iWebPlugin.IsAssignableFrom(debugTypes[0])
  true

  var iafGeneric = typeof(TPlugin).IsAssignableFrom(debugTypes[0])
  false

  typeof(TPlugin)
  {RainCrab.Plugins.AspNet.IWebPlugin}

Why if Plugin1 implements IWebPlugin and typeof(TPlugin) is IWebPlugin, Plugin1 is not assignable from IWebPlugin?

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