Asp.net 静态变量是线程安全的吗 C＃

我想创建一个存储数据表的类，这将阻止我的应用程序在每次检索时导入详细信息列表.因此这应该做一次，我相信下面的代码会这样做，但我不确定它是否是线程安全的.

下面的代码位于我的三层应用程序的业务层部分，它将数据表返回到表示层.

public class BusinessLayerHandler
{
    public static DataTable unitTable;
    public static DataTable currencyTable;

    public static DataTable GetUnitList()
    {
        //import lists each time the application is run
        unitTable = null;
        if (unitTable == null)
        {
            return unitTable = DatabaseHandler.GetUnitList();
        }
        else
        {
            return unitTable;
        }
    }

    public static DataTable GetCurrencyList()
    {
        //import lists each time the application is run
        currencyTable = null;
        if (currencyTable == null)
        {
            return currencyTable = DatabaseHandler.GetCurrencyList();
        }
        else
        {
            return currencyTable;
        }
    }

感谢任何帮助，如果有更好的方式缓存数据表，请让我知道.

更新:

多亏了你的意见，如果我理解正确，这是建议的方法:

public class BusinessLayerHandler
{
    private static DataTable unitTable;
    private static DataTable currencyTable;

    private static readonly object unitTableLock = new object();
    private static readonly object currencyTableLock = new object();

    public static DataTable GetUnitList()
    {
        //import lists each time the application is run
        //unitTable = null;

        lock (unitTableLock)
        {
            if (unitTable == null)   
            {
                return unitTable = DatabaseHandler.GetUnitList();
            }
        }
        return unitTable;
    }

    public static DataTable GetCurrencyList()
    {
        //import lists each time the application is run
        lock (currencyTableLock)
        {
            if (currencyTable == null)
            {
                return currencyTable = DatabaseHandler.GetCurrencyList();
            }
        }
        return currencyTable;
    }
}