WiX Custom Action Project - BadImageFormatException

I'm developing my first custom action but I can't get the resulting .CA.dll file to load. Heres the process at its simplest, and the result:

I create a custom action project and keep all the defaults. The class looks like this:

using Microsoft.Deployment.WindowsInstaller;
namespace CustomAction
{
public class CustomActions
{
[CustomAction]
    public static ActionResult CustomAction1(Session session)
    {
        session.Log("Begin CustomAction1");

        return ActionResult.Success;
    }
}
}

I then build the assembly (either in debug or release) which creates me a CustomAction.CA.dll file. I then try to run this test:

[TestMethod]
    public void LoadAssembly()
    {
        Assembly.LoadFrom(@"D:\CustomAction\bin\Debug\CustomAction.CA.dll");
    }

And get the error: System.BadImageFormatException: Could not load file or assembly 'file:///D:\CustomAction\bin\Debug\CustomAction.CA.dll' or one of its dependencies. The module was expected to contain an assembly manifest.

Neither can I reference the custom action from my WiX project. Getting really frustrated!

Edit: Had a look and when I run through VS test manager I'm getting the following in the application event log:

TmiDataManager.TryConvertPropertyValueToDisplayText: Failed to convert property value using the property descriptor's type converter. System.FormatException: Input string was not in a correct format.

at System.Number.StringToNumber(String str, NumberStyles options, NumberBuffer& number, NumberFormatInfo info, Boolean parseDecimal)

at System.Number.ParseInt32(String s, NumberStyles style, NumberFormatInfo info)

at System.String.System.IConvertible.ToInt32(IFormatProvider provider)

at System.Convert.DefaultToType(IConvertible value, Type targetType, IFormatProvider Provider)

at System.String.System.IConvertible.ToType(Type type, IFormatProvider provider)

at System.ComponentModel.EnumConverter.ConvertTo(ITypeDescriptorContext context, CultureInfo culture, Object value, Type destinationType)

at System.ComponentModel.TypeConverter.ConvertToString(Object value)"

Further edit: I can load the normal CustomAction.dl开发者_运维技巧l via Assembly.LoadFrom, so maybe this is a different issue to the BadImageFormat thing? Could there be any reason a blank action with no further dependencies wouldn't load into my WiX project?

CustomAction.CA.dll is a native-code DLL that contains the managed-code assembly and knows how to run as a custom action inside MSI. The managed-code assembly is "CustomAction.dll."