System.NullReferenceException添加DateTime SQL参数

Question

我对如何做到这一点感到困惑，因为我对通过c#访问server非常陌生。我想用startTime和EndTime之间的时间的条件来查询表测试。但是，当我试图添加时间参数时，它会说我有一个空引用异常。

DateTime date = DateTime.Now;

    SqlConnection con = new SqlConnection("Data Source=.\\SQLSERVER;Initial Catalog=quizMaker;Integrated Security=True");
    SqlCommand com;     

subjects = "Subject-3";



        con.Open();
        SqlParameter time = new SqlParameter("@time", SqlDbType.DateTime);
        time.Value = date;
        com.Parameters.Add(time); //error pops up here
        SqlParameter subjected = new SqlParameter("@subject", SqlDbType.VarChar, 20);
        subjected.Value = subjects;
        com = new SqlCommand("Select * from quiz where StartTime<=@time and EndTime>=@time and Subject_ID = @subject", con);


        com.ExecuteNonQuery();
        con.Close();

Answer 1

Com未分配，com为null。首先将参数添加到com中，然后执行com =新SqlCommand

com.Parameters.Add(time); //<-- com = null
com = new SqlCommand("Select * from quiz where StartTime<=@time and EndTime>=@time and Subject_ID = @subject", con); //<-- com has a value (!= null)

它只需做其他明智的事情:地点

com = new SqlCommand("Select * from quiz where StartTime<=@time and EndTime>=@time and Subject_ID = @subject", con); //<-- com has a value (!= null)

在此之前

com.Parameters.Add(time); //<-- com = null