Unity 3D layout group switches orientation automatically between landscape and portrait layouts so it either acts like a VerticalLayoutGroup or a HorizontalLayoutGroup.

DynamicLayoutGroup.cs

using UnityEngine;
using UnityEngine.UI;
using UnityEngine.EventSystems;
using System.Collections;
using System.Collections.Generic;

using SDD.Extensions;
using SDD.Events;

namespace SDD.UI {

  /// <summary>
  /// Layout group switches orientation automatically between landscape and
  /// portrait layouts so it either acts like a VerticalLayoutGroup or a
  /// HorizontalLayoutGroup.
  /// </summary>
  [AddComponentMenu("Layout/Dynamic Layout Group", 150)]
  public class DynamicLayoutGroup : HorizontalOrVerticalLayoutGroup {

    /// <summary>
    /// When is the layout vertical? In portrait or landscape.
    /// </summary>
    [SerializeField]
    public ScreenOrientation verticalWhen = ScreenOrientation.Portrait;

    public bool IsVertical { get { return GetIsVertical(); }}

    private bool GetIsVertical() {
      bool isVertical;

      if (UnityEngine.Screen.width > UnityEngine.Screen.height) {
        //orientation = ScreenOrientation.Landscape;
        isVertical = (verticalWhen == ScreenOrientation.Landscape) ? true : false;
      }
      else {
        //orientation = ScreenOrientation.Portrait;
        isVertical = (verticalWhen == ScreenOrientation.Portrait) ? true : false;
      }

      //Log.Debug(string.Format("DynamicLayoutGroup.OnRectTransformDimensionsChange() isVertical={0}a, ID={1}", isVertical, GetInstanceID()));
      return isVertical;
    }

    public override void CalculateLayoutInputHorizontal() {
      //Log.Debug(string.Format("DynamicLayoutGroup.CalculateLayoutInputHorizontal() IsVertical={0}, ID={1}", IsVertical, GetInstanceID()));

      base.CalculateLayoutInputHorizontal();
      CalcAlongAxis(0, isVertical: IsVertical);
    }

    public override void CalculateLayoutInputVertical() {
      //Log.Debug(string.Format("DynamicLayoutGroup.CalculateLayoutInputVertical() IsVertical={0}, ID={1}", IsVertical, GetInstanceID()));

      CalcAlongAxis(1, isVertical: IsVertical);
    }

    public override void SetLayoutHorizontal() {
      //Log.Debug(string.Format("DynamicLayoutGroup.SetLayoutHorizontal() IsVertical={0}, ID={1}", IsVertical, GetInstanceID()));

      SetChildrenAlongAxis(0, isVertical: IsVertical);
    }

    public override void SetLayoutVertical() {
      //Log.Debug(string.Format("DynamicLayoutGroup.SetLayoutVertical() IsVertical={0}, ID={1}", IsVertical, GetInstanceID()));

      SetChildrenAlongAxis(1, isVertical: IsVertical);
    }
  }
}

DynamicLayoutGroupEditor.cs

using UnityEditor;
using UnityEngine;
using UnityEngine.UI;

namespace SDD.UI {

  /// <summary>
  /// Override the Unity UI custom editor for HorizontalOrVerticalLayoutGroup
  /// and do exactly nothing. The only purpose of this class is to expose the
  /// public ```VerticalWhen```. The inherited editor prevents editing new
  /// publics in descendant classes without doing a full widget layout here
  /// (too much work!).
  /// </summary>
  /// <remarks>
  /// Place in ```Editor``` folder
  /// </remarks>
  [CustomEditor(typeof(SDD.UI.DynamicLayoutGroup), true)]
  [CanEditMultipleObjects]
  public class DynamicLayoutGroupEditor : UnityEditor.Editor {
  }
}

ScreenOrientation.cs

namespace SDD {

  /// <summary>
  /// Orientation of the screen regardless of which way is up
  /// </summary>
  public enum ScreenOrientation {
    Landscape,
    Portrait
  }
}