public override void GenerateCode(CodeGenerationManager manager, object obj)
{
    if (manager == null)
    {
        throw new ArgumentNullException("The CodeGenerationManager object is null.");
    }

    if (obj == null)
    {
        throw new ArgumentNullException("The object to generate code for is null.");
    }

    // Cast obj to your custom activity for access to its members.
    Activity1 customActivity = obj as Activity1;

    if (customActivity == null)
    {
        throw new ArgumentException("The obj variable cannot be cast to the Activity type.");
    }

    // Retrieve a type provider object.
    ITypeProvider typeProvider = (ITypeProvider)manager.GetService(typeof(ITypeProvider));

    if (typeProvider == null)
    {
        throw new InvalidOperationException();
    }

    // TODO: Use types in the System.CodeDOM namespace to generate 
    // code. 
}

[ActivityCodeGenerator(typeof(CustomCodeGenerator))]