C＃：对象引用未设置到对象

Question

我碰到下面的错误的实例：

Object reference not set to an instance of an object

这是我的C夏普代码：

 DataTable tableAcces = dsAcces.Tables["dsPrinterAcces"]; 
     DataTable tableMDF = dsAcces.Tables["dsPrinterMDF"]; 
     DataRow newrow = null; 


     foreach(DataRow dr in tableAcces.Rows) 
     { 
      newrow = tableMDF.NewRow(); 

     newrow["PRINTER_ID"] = dr["PRINTER_ID"]; 
     newrow["MERK"] = dr["MERK"]; 
     newrow["MODEL"] = dr["MODEL"]; 
     newrow["LOKAAL_ID"] = dr["LOKAAL_ID"]; 

     tableMDF.Rows.Add(newrow); 
     } 

     daMDF.Update(dsMDF, "dsPrinterMDF"); 
     lblSucces.Text = "Gelukt. De tabel printers is overgezet."; 
    } 

在这一行中，他抛出的错误：

newrow = tableMDF.NewRow(); 

非常感谢，

Vincent

Answer 1

tableMDF为空。你需要找出为什么dsAcces.Tables["dsPrinterMDF"]在顶部返回null。

Answer 2

似乎dsAcces.Tables [“dsPrinterMDF”]返回null而不是表。也许这张表没有找到，或者你有一个错字。

Answer 3

看起来像tableMDF为空，检查是否有表dsPrinterMDF。

Answer 4

dsAcces.Tables["dsPrinterMDF"] 

看起来是空的，所以确保它提供了一个DataTable。我猜标识符dsPrinterMDF不存在（例如具有不同的大小写）

Answer 5

这里是我的所有代码：

ng connStringAcces = ConfigurationManager.ConnectionStrings["RandAppAcces"].ToString(); 
    connAcces = new OleDbConnection(connStringAcces); 


    daAcces = new OleDbDataAdapter(); 


    string sqlSelectAcces = "SELECT * FROM tblPrinter"; 
    OleDbCommand cmdAcces = new OleDbCommand(sqlSelectAcces, connAcces); 
    daAcces.SelectCommand = cmdAcces; 

    string connStringMDF = ConfigurationManager.ConnectionStrings["RandAppMDF"].ToString(); 
    connMDF = new SqlConnection(connStringMDF); 

    daMDF = new SqlDataAdapter(); 

    string sqlSelectMDF = "SELECT * FROM tblPrinter"; 
    SqlCommand cmdMDF = new SqlCommand(sqlSelectMDF, connMDF); 
    daMDF.SelectCommand = cmdMDF; 

    string sqlInsertMDF = @"INSERT INTO tblPrinter(PRINTER_ID, MERK, MODEL, LOKAAL_ID)" + 
    @"VALUES (@PRINTER_ID, @MERK, @MODEL, @LOKAAL_ID)"; 

    cmdMDF = new SqlCommand(sqlInsertMDF, connMDF); 

    cmdMDF.Parameters.Add("@PRINTER_ID", SqlDbType.Int, 0, "PRINTER_ID"); 
    cmdMDF.Parameters.Add("@MERK", SqlDbType.NVarChar, 100, "MERK"); 
    cmdMDF.Parameters.Add("@MODEL", SqlDbType.NVarChar, 100, "MODEL"); 
    cmdMDF.Parameters.Add("@LOKAAL_ID", SqlDbType.Int, 0, "LOKAAL_ID"); 

    daMDF.InsertCommand = cmdMDF; 

    dsMDF = new DataSet(); 
    dsAcces = new DataSet(); 

    daMDF.Fill(dsMDF, "dsPrinterMDF"); 
    daAcces.Fill(dsAcces, "dsPrinterAcces"); 

    DataTable tableAcces = dsAcces.Tables["dsPrinterAcces"]; 
    DataTable tableMDF = dsAcces.Tables["dsPrinterMDF"]; 
    DataRow newrow = null; 


    foreach(DataRow dr in tableAcces.Rows) 
    { 
     newrow = tableMDF.NewRow(); 

    newrow["PRINTER_ID"] = dr["PRINTER_ID"]; 
    newrow["MERK"] = dr["MERK"]; 
    newrow["MODEL"] = dr["MODEL"]; 
    newrow["LOKAAL_ID"] = dr["LOKAAL_ID"]; 

    tableMDF.Rows.Add(newrow); 
    }