C# Get constants from nested classes with reflection

Use GetNestedTypes() instead with the correct BindingFlags for public constants:

var nestedTypes = typeof(MainFoo).GetNestedTypes(BindingFlags.Public);
foreach (Type type in nestedTypes)
{
    FieldInfo[] constants = type.GetFields(BindingFlags.Public | BindingFlags.Instance | BindingFlags.Static);
    // <do stuff here>
}