InvalidOperationException：無數據存在時無效嘗試讀取。 （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()); 
    } 

} 

我不明白我爲什麼得到它。我所要做的就是讓最後一個發佈帖子的人..我看着調試，而事情看起來很好。我也看了視覺工作室2010年的SQL服務器..有數據,,但不知何故它沒有被讀取和拋出異常...... :(

Answer 1

您必須循環通過你的讀者：

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())如果你只有一個結果

而且 你笑你總是圍繞着你的閱讀器用一個：

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

你應該做一些空的檢查，然後再調用ToString（）;

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

而且這裏有一個小技巧來編寫查詢代碼，使用逐字：

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"; 

Answer 2

你應該檢查dr.Read（）之前訪問值：

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()); 
}