未将对象引用设置为对象的实例

Question

我有这样的功能

public void GetTablesWithUpperCaseName()
{
   SqlConnectionStringBuilder objConnectionString = new SqlConnectionStringBuilder();
   objConnectionString.DataSource = txtHost.Text;
   objConnectionString.UserID = txtUsername.Text;
   objConnectionString.Password = txtPassword.Text;
   objConnectionString.InitialCatalog = Convert.ToString(cmbDatabases.SelectedValue);

   SqlConnection sConnection = new SqlConnection(objConnectionString.ConnectionString);

   //To Open the connection.
   sConnection.Open();

   //Query to select table_names that have their names in uppercase letters.
   string selectTablesWithUppercaseName = @"SELECT 
                                              NAME
                                            FROM 
                                              sysobjects 
                                            WHERE 
                                              UPPER(name) COLLATE Latin1_General_BIN = name COLLATE Latin1_General_BIN 
                                              AND 
                                                 OBJECTPROPERTY(ID,N'IsTable')=1
                                              AND 
                                                 OBJECTPROPERTY(ID,N'IsMSShipped')=0 ";
   //Create the command object
   SqlCommand sCommand = new SqlCommand(selectTablesWithUppercaseName, sConnection);

   try
   {
       //Create the dataset
       DataSet dsListOfTablesWithUppercaseName = new DataSet("sysobjects");

       //Create the dataadapter object
       SqlDataAdapter sDataAdapter = new SqlDataAdapter(selectTablesWithUppercaseName, sConnection);

       //Provides the master mapping between the sourcr table and system.data.datatable
       sDataAdapter.TableMappings.Add("Table", "sysobjects");

       //Fill the dataset
       sDataAdapter.Fill(dsListOfTablesWithUppercaseName);

       //Bind the result combobox with foreign key table names
       DataViewManager dvmListOfForeignKeys = dsListOfTablesWithUppercaseName.DefaultViewManager;
       dgResultView.DataSource = dsListOfTablesWithUppercaseName.Tables["sysobjects"];
    }
    catch(Exception ex)
    {
        //All the exceptions are handled and written in the EventLog.
        EventLog log = new EventLog("Application");
        log.Source = "MFDBAnalyser";
        log.WriteEntry(ex.Message);
    }
    finally
    {
       //If connection is not closed then close the connection
       if(sConnection.State != ConnectionState.Closed)
       {
          sConnection.Close();
       }
    }
 }

另一个用于计算从前面的函数生成的行的函数。但是这个功能

空引用异常或对象引用未设置为对象的实例。

任何人都可以帮助我...为什么它仅针对上述功能捕获错误并且对所有其他类似功能正常工作。

private void UpdateLabelText()
{
    SqlConnectionStringBuilder objConnectionString = new SqlConnectionStringBuilder();
    objConnectionString.DataSource = txtHost.Text;
    objConnectionString.UserID = txtUsername.Text;
    objConnectionString.Password = txtPassword.Text;
    objConnectionString.InitialCatalog = Convert.ToString(cmbDatabases.SelectedValue);

    SqlConnection sConnection = new SqlConnection(objConnectionString.ConnectionString);

    //To Open the connection.
    sConnection.Open();

    try
    {
        int SelectedCellTotal = 0;
        int counter;

        // Iterate through the SelectedCells collection and sum up the values.
        for(counter = 0;counter < (dgResultView.SelectedCells.Count);counter++)
        {
            if(dgResultView.SelectedCells[counter].FormattedValueType == Type.GetType("System.String"))
            {
                 string value = null;

                 // If the cell contains a value that has not been commited,
                 if(dgResultView.IsCurrentCellDirty == true)
                 {
                    value = dgResultView.SelectedCells[counter].EditedFormattedValue.ToString();
                 }
                 else
                 {
                    value = dgResultView.SelectedCells[counter].FormattedValue.ToString();
                 }
                 if(value != null)
                 {
                    // Ignore cells in the Description column.
                    if(dgResultView.SelectedCells[counter].ColumnIndex != dgResultView.Columns["TABLE_NAME"].Index)
                    {
                       if(value.Length != 0)
                       {
                          SelectedCellTotal += int.Parse(value);
                       }
                    }
                 }
              }
            }

            // Set the labels to reflect the current state of the DataGridView.
            lblDisplay.Text = "There are Total " + dgResultView.RowCount + cmbOperations.SelectedItem.ToString();
        }
        catch(Exception ex)
        {
            //All the exceptions are handled and written in the EventLog.
            EventLog log = new EventLog("Application");
            log.Source = "MFDBAnalyser";
            log.WriteEntry(ex.Message);
        }
        finally
        {
            //If connection is not closed then close the connection
            if(sConnection.State != ConnectionState.Closed)
            {
                sConnection.Close();
            }
        }
    }

此外， lblDisplay.Text 没有占用适当的空间。

等待回复

Answer 1

好的，我真的不知道为什么您会收到“空引用异常” - 但仍然有几点需要说明：

• 我将使用sys.tables而不是sysobjects指定要查询的对象类型

• 总是把你的一次性用品SqlConnection和SqlCommand成using(.....) { ...... }块。这样，您将不需要任何finally {..}块，并且 .NET 将在不再需要这些对象时妥善处理这些对象

• DataSet当你只有一张桌子时，为什么要使用 a ？这只是不必要的开销 - 使用 aDataTable代替！

• 不要SqlConnection太早打开- 等到最后一刻，打开它，执行查询，立即再次关闭它

• 实际上，在使用 时SqlDataAdapter，您根本不需要SqlConnection自己打开- 它SqlDataAdapter会为您执行此操作（并在读取完数据后再次关闭它）

• 也不会从与绑定到UI元素的数据库混合数据的检索-这是一个非常不好的做法。从该GetTablesWithUpperCaseName方法中，您应该DataTable向调用者（UI）返回一些东西（如 a ）并让 UI 处理绑定过程

• 沿着相同的路线：该方法不应该从 UI 元素（如文本框）本身获取内容 - 将这些值作为方法参数传递，以获得更清晰的代码 - 你可能有一天实际上能够在另一个项目中重用

这就是我认为你的第一种方法应该看起来像

    public DataTable GetTablesWithUpperCaseName(string server, string database, 
                                                string username, string password)
    {
        // Create the datatable
        DataTable dtListOfTablesWithUppercaseName = new DataTable("tableNames");

        SqlConnectionStringBuilder objConnectionString = new SqlConnectionStringBuilder();
        objConnectionString.DataSource = server;;
        objConnectionString.UserID = username;
        objConnectionString.Password = password;
        objConnectionString.InitialCatalog = database;

        // Define the Query against sys.tables - much easier and cleaner!
        string selectTablesWithUppercaseName =
            "SELECT NAME FROM sys.tables WHERE UPPER(name) COLLATE Latin1_General_BIN = name COLLATE Latin1_General_BIN AND is_msshipped = 0";

        // put your SqlConnection and SqlCommand into using blocks!
        using (SqlConnection sConnection = new SqlConnection(objConnectionString.ConnectionString))
        using (SqlCommand sCommand = new SqlCommand(selectTablesWithUppercaseName, sConnection))
        {
            try
            {
                // Create the dataadapter object
                SqlDataAdapter sDataAdapter = new SqlDataAdapter(selectTablesWithUppercaseName, sConnection);

                // Fill the datatable - no need to open the connection, the SqlDataAdapter will do that all by itself 
                // (and also close it again after it is done)
                sDataAdapter.Fill(dtListOfTablesWithUppercaseName);
            }
            catch (Exception ex)
            {
                //All the exceptions are handled and written in the EventLog.
                EventLog log = new EventLog("Application");
                log.Source = "MFDBAnalyser";
                log.WriteEntry(ex.Message);
            }
        }

        // return the data table to the caller
        return dtListOfTablesWithUppercaseName;
    }