C# 中的静态函数，并发问题

我的应用程序中有许多静态函数，它们在整个应用程序中被多次调用。随着用户数量的增加，这些函数可能会被多次调用。我只是想知道这些函数是否会产生任何并发问题。我的应用程序中的一些静态函数甚至与数据库交互。我确保大部分静态函数只使用局部变量。我的一些静态函数如下。请查看并建议是否需要进行任何更改。

public static bool ISValidString(string CheckString)
{
    if (CheckString != null)
        CheckString = CheckString.Trim();
    if (String.IsNullOrEmpty(CheckString) || String.IsNullOrWhiteSpace(CheckString) || CheckString == "-" ||
        CheckString == "NA" || CheckString == "." || CheckString == "--Select--" || CheckString == "0" || CheckString == "0.00" || CheckString == "00")
        return false;
    else
        return true;
}

    public static DataTable ProcessMaster(string Condition, string Master)
    {
        DataTable dt = new DataTable();
        dt = (((DataSet)HttpContext.Current.Application[Master]).Tables[0].Select(Condition).Length>0)
            ? ((DataSet)HttpContext.Current.Application[Master]).Tables[0].Select(Condition).CopyToDataTable()
            : ((DataSet)HttpContext.Current.Application[Master]).Tables[0].Clone();
        return dt;
    }


    private static void GetPayTypes()
    {
        HttpContext.Current.Application["PayTypeMaster"] = DBFactory.GetMasters(null, "GetPayTypes");
    }
    public static DataTable GetPayTypes(string Filter = null, bool ForceRefresh = false)
    {
        if (HttpContext.Current.Application["PayTypeMaster"] == null || ForceRefresh == true)
            GetPayTypes();

        string Condition;
        if (Factory.ISValidString(Filter))
            Condition = "PayTypeID like '%" + Filter + "%' or PayTypeDesc like '%" + Filter + "%'";
        else
            Condition = Filter;
        return Factory.ProcessMaster(Condition, "PayTypeMaster");
    }

    public static DataSet GetMasters(string Filter, string MasterName)
    {
        CompanyParams myparam = new CompanyParams();
        try
        {
            myparam.AddParameter("@TranType", MasterName);
            myparam.AddParameter("@Filter", Filter);
            return CompanyDB.GetData(SPNameMasters, myparam.Params);
        }
        catch (Exception ex)
        {
            throw ex;
        }
        finally
        {

        }
    }