InvalidOperationException:当不存在数据时，读取尝试无效.(SQL)

本文关键字：无效 SQL 读取 不存在 数据 InvalidOperationException | 更新日期: 2023-09-27 17:59:09

    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()); 
     }
}

我不明白我为什么收到它。我所要做的就是找到最后一个发帖子的人。。我看了调试，一切似乎都很好。我还查看了visualstudio 2010中的sql服务器。有数据，但不知怎么的，它没有被读取，并且抛出了一个异常…：(

InvalidOperationException:当不存在数据时，读取尝试无效.(SQL)

在访问值之前，您应该检查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());
}

您必须通过阅读器循环：

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

而且你应该总是围绕你的阅读器使用：

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

在调用ToString((之前，您应该进行一些null检查；

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

这里有一个用代码编写查询的小技巧，逐字逐句使用：

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";
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