如何解决将文本分配给标签时，我会得到未设置为的对象引用和对象实例
|| 我构建了一个包含标签的简单自定义控件。在自定义控件的代码背后，我试图设置标签的文本，但出现空引用错误。有任何想法吗？protected void Page_Load(object sender,EventArgs e)
{
SqlConnection connection = new SqlConnection(GetConnectionString());
connection.Open();
SqlCommand cmd = new SqlCommand(\"DisplayCustomerReviews\",connection);
cmd.CommandType = CommandType.StoredProcedure;
XmlReader reader = cmd.ExecuteXmlReader();
//while (reader.Read())
while (!reader.EOF)
{
reader.ReadToFollowing(\"review\");
reader.MoveToAttribute(\"Name\");
string Name = reader.Value;
reader.MoveToAttribute(\"Message\");
string Message = reader.Value;
reader.MoveToAttribute(\"Rating\");
string Rating = reader.Value;
reader.MoveToAttribute(\"Date\");
string Date = reader.Value;
reader.MoveToAttribute(\"Time\");
string Time = reader.Value;
CreateReviewPanel(Name,Message,Rating,Date,Time);
}
reader.Close();
connection.Close();
}
private string GetConnectionString()
{
return ConfigurationManager.AppSettings[\"SQLConn\"];
}
private void CreateReviewPanel(string Name,string Message,string Rating,string Date,string Time)
{
// need to check against nulls or empty strings to avoid
// extra reviewPanel being created at end of XML read.
if (Name != \"\")
{
Guid panelID = Guid.NewGuid();
Panel reviewPanel = new Panel();
reviewPanel.ID = panelID.ToString();
nameLabel.Text = Name;
messageLabel.Text = Message;
dateLabel.Text = Date;
timeLabel.Text = Time;
switch (Rating)
{
case \"1\":
ratingImage.ImageUrl = \"~/images/one-star.gif\";
break;
case \"2\":
ratingImage.ImageUrl = \"~/images/two-stars.gif\";
break;
case \"3\":
ratingImage.ImageUrl = \"~/images/three-stars.gif\";
break;
case \"4\":
ratingImage.ImageUrl = \"~/images/four-stars.gif\";
break;
case \"5\":
ratingImage.ImageUrl = \"~/images/five-stars.gif\";
break;
}
}
}
}
解决方法
nameLabel是页面上控件的ID吗？如果不是，则需要执行以下操作：
Label nameLabel = this.lblSomeControlID;
protected void Page_Load(object sender,EventArgs e) {
string conn = GetConnectionString();
if (String.IsNullOrEmpty(conn)) {
Console.WriteLine(\"No Connection String!\");
return;
}
try {
SqlConnection connection = new SqlConnection(conn);
connection.Open();
SqlCommand cmd = new SqlCommand(\"DisplayCustomerReviews\",connection);
cmd.CommandType = CommandType.StoredProcedure;
XmlReader reader = cmd.ExecuteXmlReader();
while (!reader.EOF) {
reader.ReadToFollowing(\"review\");
reader.MoveToAttribute(\"Name\");
string Name = reader.Value;
reader.MoveToAttribute(\"Message\");
string Message = reader.Value;
reader.MoveToAttribute(\"Rating\");
string Rating = reader.Value;
reader.MoveToAttribute(\"Date\");
string Date = reader.Value;
reader.MoveToAttribute(\"Time\");
string Time = reader.Value;
CreateReviewPanel(Name,Message,Rating,Date,Time);
}
reader.Close();
connection.Close();
} catch (Exception err) {
Console.WriteLine(\"Possible Problems:\");
Console.WriteLine(\"DisplayCustomerReviews was not found or spelled incorrectly\");
Console.WriteLine(\"One or more of the attributes listed above do not exist.\");
Console.WriteLine(err.Message);
}
}
private string GetConnectionString() {
string connStr = null;
try {
connStr = ConfigurationManager.AppSettings[\"SQLConn\"];
} catch (Exception err) {
Console.WriteLine(\"AppSettings is incorrect or does not contain SQLConn\");
Console.WriteLine(err.Message);
}
return connStr;
}
private void CreateReviewPanel(string Name,string Message,string Rating,string Date,string Time) {
if (!String.IsNullOrEmpty(Name)) //need to check against nulls or empty strings to avoid extra reviewPanel being created at end of XML read.
{
Guid panelID = Guid.NewGuid();
Panel reviewPanel = new Panel();
try {
reviewPanel.ID = panelID.ToString();
} catch (Exception) {
Console.WriteLine(\"Panel Object does not contain an ID property.\");
Console.WriteLine(err.Message);
}
nameLabel.Text = Name;
messageLabel.Text = Message;
dateLabel.Text = Date;
timeLabel.Text = Time;
try {
switch (Rating) {
case \"1\":
ratingImage.ImageUrl = \"~/images/one-star.gif\";
break;
case \"2\":
ratingImage.ImageUrl = \"~/images/two-stars.gif\";
break;
case \"3\":
ratingImage.ImageUrl = \"~/images/three-stars.gif\";
break;
case \"4\":
ratingImage.ImageUrl = \"~/images/four-stars.gif\";
break;
case \"5\":
ratingImage.ImageUrl = \"~/images/five-stars.gif\";
break;
}
Console.WriteLine(\"File Check: \",ratingImage.ImageUrl);
Console.WriteLine(File.Exists(ratingImage.ImageUrl));
} catch (Exception err) {
Console.WriteLine(\"This control type does not contain an ImageUrl property\");
Console.WriteLine(err.Message);
}
}
}
private void CreateReviewPanel(string Name,string Time)
{
if (Name != \"\") //need to check against nulls or empty strings to avoid extra reviewPanel being created at end of XML read.
{
Guid panelID = Guid.NewGuid();
ReviewPanel reviewPanel = new ReviewPanel();
reviewPanel.ID = panelID.ToString();
reviewPanel.nameLabel.Text = Name;
reviewPanel.messageLabel.Text = Message;
reviewPanel.dateLabel.Text = Date;
reviewPanel.timeLabel.Text = Time;
switch (Rating)
{
case \"1\":
reviewPanel.ratingImage.ImageUrl = \"~/images/one-star.gif\";
break;
case \"2\":
reviewPanel.ratingImage.ImageUrl = \"~/images/two-stars.gif\";
break;
case \"3\":
reviewPanel.ratingImage.ImageUrl = \"~/images/three-stars.gif\";
break;
case \"4\":
reviewPanel.ratingImage.ImageUrl = \"~/images/four-stars.gif\";
break;
case \"5\":
reviewPanel.ratingImage.ImageUrl = \"~/images/five-stars.gif\";
break;
}
}
}
}
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