Unity3D NonNull attribute + colouring PropertyDrawer

NonNullAttribute.cs

using UnityEngine;

namespace UGS.unityutil.attributes {

	/// <summary>
	/// Checks non-null (and non-empty if applied to a string, non-zero if a scalar or Vector, etc) in a custom inspector.
	/// Colours red if any problems.
	/// </summary>
	public sealed class NonNullAttribute : PropertyAttribute {
	}
}

NonNullAttributePropertyDrawer.cs

using UnityEditor;
using UnityEngine;

// Place in Editor sub-directory
namespace UGS.unityutil.attributes {
	[CustomPropertyDrawer(typeof (NonNullAttribute))] // n.b. will need 2nd param 'true' if wish to use for subclasses of NonNullAttribute
	public sealed class NonNullAttributePropertyDrawer : PropertyDrawer {

		public override void OnGUI(Rect position, SerializedProperty property, GUIContent label) {
			var oc = GUI.backgroundColor;
			var result = isProblem(property);
			if (result.HasValue) {
				if (result.Value) {
					GUI.backgroundColor = ColourBad;
					if (string.IsNullOrEmpty(label.tooltip)) {
						label.tooltip = (property.propertyType == SerializedPropertyType.String) ? "Must not be empty." : "Must not be null.";
					}
				}
			} else { // no result = unhandled
				GUI.backgroundColor = ColourUnhandled;
				if (string.IsNullOrEmpty(label.tooltip)) {
					label.tooltip = AttributeName + " cannot be applied to this type";
				} else {
					label.tooltip += "\n" + AttributeName + " cannot be applied to this type";
				}
			}
			EditorGUI.PropertyField(position, property, label, true);
			GUI.backgroundColor = oc;
		}

		public override float GetPropertyHeight(SerializedProperty property, GUIContent label) {
			return EditorGUI.GetPropertyHeight(property);
		}

		private bool? isProblem(SerializedProperty property) {
			switch (property.propertyType) {
				case SerializedPropertyType.String:
					return string.IsNullOrEmpty(property.stringValue);

				case SerializedPropertyType.Vector2:
					return 0f >= property.vector2Value.sqrMagnitude;

				case SerializedPropertyType.Vector2Int:
					return 0f >= property.vector2IntValue.sqrMagnitude;

				case SerializedPropertyType.Vector3:
					return 0f >= property.vector3Value.sqrMagnitude;

				case SerializedPropertyType.Vector3Int:
					return 0f >= property.vector3IntValue.sqrMagnitude;

				case SerializedPropertyType.Vector4:
					return 0f >= property.vector4Value.sqrMagnitude;

				case SerializedPropertyType.Float:
					return 0f == property.floatValue;

				case SerializedPropertyType.Integer:
					return 0 == property.intValue;

				case SerializedPropertyType.Color:
					return NullColour == property.colorValue;

				case SerializedPropertyType.LayerMask:
					return 0 == property.intValue;

				case SerializedPropertyType.ObjectReference:
					return null == property.objectReferenceValue;
			}

			return null; // indicate not handled
		}

		private static readonly string AttributeName = "WARNING: " + typeof (NonNullAttribute).Name;

		private static readonly Color ColourBad = new Color(1f, 0.5f, 0.5f);

		private static readonly Color ColourUnhandled = new Color(1f, 0f, 0.8f);

		private static readonly Color NullColour = new Color(0f, 0f, 0f, 0f);

	}
}