Extending Enums

MTG API v1.4.0 added the ETGModCompatibility class, which has a method named ExtendEnum.
To extend an enum, you can do one of the following:

Option 1: manual extension

You can manually extend any enum you want by calling ETGModCompatibility.ExtendEnum<YourEnumType>(YourPluginClass.GUID, "NameOfYourEnum") You can either save the value returned or you can call ETGModCompatibility.ExtendEnum<YourEnumType>(YourPluginClass.GUID, "NameOfYourEnum") every time you want to get it (calling it with the same values after the first time will only return the saved result and will not add anything)

Option 2: automatic extension

Firstly, you need to add this class to your project:

public class EnumExtensionAttribute : Attribute
{
    public EnumExtensionAttribute(Type extensiontype)
    {
        type = extensiontype;
    }

    public Type type;
}

Secondly, create a new class and name it {NameOfTheEnumYouWantToExtend}E.cs.
Create a public static NameOfTheEnumYouWantToExtend field for each new extended value you want.
Example of such class:

[EnumExtension(typeof(GungeonFlags))]
public static class GungeonFlagsE
{
    public static GungeonFlags TEST_FLAG_1;
    public static GungeonFlags TEST_FLAG_2;
    public static GungeonFlags TEST_FLAG_3;
    public static GungeonFlags TEST_FLAG_4;
}

Then, add this code to your Plugin class's Awake() method:

var asm = Assembly.GetExecutingAssembly();
foreach(var type in asm.GetTypes())
{
    var custom = type.GetCustomAttributes(false);
    if (custom != null)
    {
        var extension = custom.OfType<EnumExtensionAttribute>().FirstOrDefault();
        if(extension != null && extension.type != null && extension.type.IsEnum)
        {
            foreach(var f in type.GetFields(BindingFlags.Public | BindingFlags.Static))
            {
                f.SetValue(null, ETGModCompatibility.ExtendEnum(GUID, f.Name, extension.type));
            }
        }
    }
}

(Note: this code needs using System.Linq; and using System.Reflection;)
Then, when you want to use an extended enum value, you can do {NameOfTheEnumYouWantToExtend}E.ValueName