cockClient2/Assets/Editor/DropdownDrawer.cs

using UnityEditor;
using UnityEngine;

[CustomPropertyDrawer(typeof(DropdownAttribute))]
public class DropdownDrawer : PropertyDrawer
{
    public override void OnGUI(Rect position, SerializedProperty property, GUIContent label)
    {
        DropdownAttribute dropdownAttribute = attribute as DropdownAttribute;

        if (property.propertyType == SerializedPropertyType.String)
        {
            int selectedIndex = Mathf.Max(0, System.Array.IndexOf(dropdownAttribute.options, property.stringValue));
            selectedIndex = EditorGUI.Popup(position, label.text, selectedIndex, dropdownAttribute.options);
            property.stringValue = dropdownAttribute.options[selectedIndex];
        }
        else
        {
            EditorGUI.LabelField(position, label.text, "Use Dropdown with string.");
        }
    }
}