避免在准备好的语句中出现空变量错误

问题描述

        var publicFunctions = new PublicFunctionsController();

        if (!publicFunctions.CheckSession(Session["id"])) { RedirectToAction("Index","Home"); };

        

        var src = DateTime.Now;
        var date = new DateTime(src.Year,src.Month,src.Day,src.Hour,src.Minute,00);

        var arrow = OverUnder == "Over" ? $"EndTime >= {date}" : OverUnder == "Under" ? $"EndTime <= {date}" : "";
        var userCategories = Categoriesb == "" ? "" : $"Categories IN ({Categoriesb})";
        var lastId = $"Id > {Id}";
        var firstAnd = arrow == "" ? "" : "AND";
        var secondAnd = userCategories == "" ? "" : "AND";
        var sortByViews = ifViews == 1 ? "ORDER BY Views DESC" : "";

        //var strin = $"SELECT TOP 5 StatementTitle,Statement,ArgumentTitle,Argument,Explanation,Categories,EndTime,SessionId,Id FROM Statements WHERE {arrow} {firstAnd} {userCategories} {secondAnd} {lastId} ORDER BY Id DESC {sortByViews}";
        //the above works when used as query but below does not

        var queryString = "SELECT TOP 5 StatementTitle,Id FROM Statements WHERE @arrow @firstAnd @userCategories @secondAnd @lastId ORDER BY Id DESC @sortByViews";

        List<SingleStatementBlog> data = new List<SingleStatementBlog>();

        var con = publicFunctions.Connection();

        con.open();

        sqlCommand command = new sqlCommand(queryString,con); //figure out or for like clause
        command.Parameters.AddWithValue("@arrow",arrow);
        command.Parameters.AddWithValue("@firstAnd",firstAnd);
        command.Parameters.AddWithValue("@userCategories",userCategories);
        command.Parameters.AddWithValue("@secondAnd",secondAnd);
        command.Parameters.AddWithValue("@lastId",lastId);
        command.Parameters.AddWithValue("@sortByViews",sortByViews);

        sqlDataReader reader = command.ExecuteReader();
        //error at first and

当我使用参数运行查询时，我收到一条错误消息，提示“在 firstAnd 时出错”，但是当我在没有参数的情况下运行查询时，它可以工作（如评论中所示）。我想知道空字符串“”是否是失败的原因......

解决方法

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