Mark a Field as Required in Unity

RequireReferenceAttribute.cs

using System;
using UnityEngine;

[AttributeUsage(AttributeTargets.Field, Inherited = true, AllowMultiple = false)]
public class RequireReferenceAttribute : PropertyAttribute
{
	public readonly Color highlight;

	public RequireReferenceAttribute()
	{
		// Red, but not blow-your-eyes-out red.
		highlight = new Color(1f, 0f, 0f, 0.75f);
	}

	public RequireReferenceAttribute(float r, float g, float b, float a = 0.75f)
	{
		highlight = new Color(r, g, b, a);
	}
}

RequireReferenceDrawer.cs

using UnityEditor;
using UnityEngine;

[CustomPropertyDrawer(typeof(RequireReferenceAttribute))]
public class RequireReferenceDrawer : PropertyDrawer
{
	bool bDidShowError = false;

	public override void OnGUI(Rect position, SerializedProperty property, GUIContent label)
	{
		if (property.propertyType != SerializedPropertyType.ObjectReference)
		{
			if (!bDidShowError)
			{
				string errorStr = string.Format("The [RequireReference] attribute must be used with an object reference. See the '{0}' field on object '{1}'.", property.displayName, property.serializedObject.targetObject.name);
				Debug.LogError(errorStr);
				bDidShowError = true;
			}
		}
		else if (property.objectReferenceValue == null)
		{
			Color originalColor = GUI.color;

			Rect labelPos = position;
			labelPos.xMax = labelPos.xMin + EditorGUIUtility.labelWidth;

			RequireReferenceAttribute requireAttribute = (RequireReferenceAttribute)base.attribute;

			GUI.color = requireAttribute.highlight;
			GUI.Box(labelPos, string.Empty);
			GUI.color = originalColor;
		}

		EditorGUI.PropertyField(position, property, label);
	}
}