InvalidOperationException：不存在数据时尝试读取无效。 （SQL）

Question

    void ExecuteContent()
{
    StringBuilder sb = new StringBuilder();
    sb.Append("SELECT TOP 1 @UsersID,u.Avatar,t.Date, u.Name, t.ThreadTitle, t.ThreadParagraph");
     sb.Append(" FROM Users as u");
     sb.Append(" INNER JOIN Threads as t ON u.UsersID = t.UsersID");
     sb.Append(" Where @UsersID=t.UsersID");
     sb.Append(" ORDER BY t.Date DESC");


     using (SqlConnection conn = new SqlConnection(AnswerQuestion.connectionString))
     {
         conn.Open();
         SqlCommand sqlComm = new SqlCommand(sb.ToString(), conn);
        MembershipUser CurrentUser = Membership.GetUser();    
         Guid i = (Guid)CurrentUser.ProviderUserKey;
         sqlComm.Parameters.Add("@UsersID", SqlDbType.UniqueIdentifier).Value = i;
         SqlDataReader dr = sqlComm.ExecuteReader();

         UserName = dr["Name"].ToString();//The exception is thrown here
         Image = (Image) dr["Avatar"];
         ThreadTitle = dr["ThreadTitle"].ToString();
         ThreadParagraph = dr["ThreadParagraph"].ToString();
         Time = (DateTime)AllQuestionsPresented.TryParse(dr["Date"].ToString()); 
     }

}

我不明白为什么我得到它。我想做的就是找到最后一个发布该线程的人..我查看了调试，一切似乎都很好。我还查看了 Visual Studio 2010 中的 sql server.. 有数据，但不知何故它没有被读取并且抛出异常...:(

Answer 1

您应该在访问值之前检查 dr.Read()：

if(dr.Read())
{
    UserName = dr["Name"].ToString();//The exception is thrown here
    Image = (Image) dr["Avatar"];
    ThreadTitle = dr["ThreadTitle"].ToString();
    ThreadParagraph = dr["ThreadParagraph"].ToString();
    Time = (DateTime)AllQuestionsPresented.TryParse(dr["Date"].ToString());
}

Answer 2

您必须循环遍历您的阅读器：

while(dr.Read())
{
    serName = dr["Name"].ToString();//The exception is thrown here
         Image = (Image) dr["Avatar"];
         ThreadTitle = dr["ThreadTitle"].ToString();
         ThreadParagraph = dr["ThreadParagraph"].ToString();
         Time = (DateTime)AllQuestionsPresented.TryParse(dr["Date"].ToString()); 
}

或 if(dr.Read()) 如果您只有一个结果

并且
你应该总是用 using: 包围你的 Reader

using(dr)
{
    while(dr.Read())
    {
    }
}

并且你应该在调用 ToString(); 之前做一些空检查。

ThreadParagraph = dr["ThreadParagraph"] == null ? "" : dr["ThreadParagraph"].ToString();

这里有一个在代码中逐字编写查询的小技巧：

var query = @"SELECT TOP 1 @UsersID,u.Avatar,t.Date, u.Name, t.ThreadTitle, t.ThreadParagraph 
    FROM Users as u 
    INNER JOIN Threads as t ON u.UsersID = t.UsersID 
    Where @UsersID=t.UsersID ORDER BY t.Date DESC";