【Unity エラー】CS0246 : 名前空間の宣言不足

以下の名前空間の宣言が必要。

ケース１：リストを使う時

using System.Collections.Generic;

エラー内容

error CS0246: The type or namespace name `List`1' could not be found. 
Are you missing a using directive or an assembly reference?

• ケース 1 UI を使うとき

using UnityEngine.UI;

エラー内容

error CS0246: The type or namespace name `Toggle' could not be found. 
Are you missing a using directive or an assembly reference?

エラー内容

error CS0246: The type or namespace name `Text' could not be found. 
Are you missing a using directive or an assembly reference?

• ケース2 DataTime を使うとき

using System;

エラー内容

error CS0246: The type or namespace name `DateTime' could not be found. 
Are you missing a using directive or an assembly reference?