Unity Define Symbols Utility

DefineSymbolUtil.cs

using System.Text;
using UnityEditor;
using UnityEngine;

namespace Cleverous
{
    public static class DefineSymbolUtil
    {
        public const string Symbol = "YOUR_DEFINE_SYMBOL";

        [MenuItem("Tools/YOUR TOOL/Install Define Symbol", priority = 900)]
        public static void InstallDefines()
        {
            RemoveDefine(Symbol);
            AddDefine(Symbol);
        }

        public static void InstallDefines()
        {
            RemoveDefine(Symbol);
            AddDefine(Symbol);
        }

        [UnityEditor.Callbacks.DidReloadScripts]
        public static void Verify()
        {
            NamedBuildTarget bt = NamedBuildTarget.FromBuildTargetGroup(EditorUserBuildSettings.selectedBuildTargetGroup);
            string symbols = PlayerSettings.GetScriptingDefineSymbols(bt);
            if (symbols.Contains(Symbol)) return;

            Debug.Log($"Define Symbol '{Symbol}' was not detected so it has been installed. Unity will recompile.");
            InstallDefines();
        }

        public static void AddDefine(string def)
        {
            NamedBuildTarget bt = NamedBuildTarget.FromBuildTargetGroup(EditorUserBuildSettings.selectedBuildTargetGroup);
            string symbols = PlayerSettings.GetScriptingDefineSymbols(bt);

            StringBuilder sb = new();
            sb.Append(symbols);

            if (sb.Length == 0) sb.Append(def + ";");
            else sb.Append($"{(sb[^1] == ';' ? "" : ";")}{def}");

            PlayerSettings.SetScriptingDefineSymbols(bt, sb.ToString());
        }
        public static void RemoveDefine(string def)
        {
            NamedBuildTarget bt = NamedBuildTarget.FromBuildTargetGroup(EditorUserBuildSettings.selectedBuildTargetGroup);
            string symbols = PlayerSettings.GetScriptingDefineSymbols(bt);
            PlayerSettings.SetScriptingDefineSymbols(bt, symbols.Replace(def, ""));
        }
    }
}

Updated to fix bug and account for cases where no defines initially exist.

Updated for Unity 6