datagridview上的异常（nullreferenceexception） [英] Exception (nullreferenceexception) on datagridview

问题描述

有人可以帮我理解我的代码问题吗？它始终是异常，如果我尝试用NULL处理单元格无关紧要。



我的捕获System.Exception错误是：

对象引用未设置为对象的实例







< pre lang =c＃> 尝试
{

worksheet = workbook.ActiveSheet;

worksheet.Name = ExportedFromDatGrid;

int cellRowIndex = 1 ;
int cellColumnIndex = 1 ;


// 遍历每一行并从每列读取值。
for （ int i = -1; i < dataGridView1.Rows.Count - 1 ; i ++）
{
for （ int j = 0 ; j < dataGridView1.Columns.Count; j ++）
{
// Excel索引从1,1开始。由于第一行将具有列标题，因此添加条件检查。
if （cellRowIndex == 1 ）
{
worksheet.Cells [cellRowIndex，cellColumnIndex] = dataGridView1.Columns [j] .HeaderText;
}
其他
{

如果（dataGridView1.Rows [i] .Cells [j] .Value.ToString（）！= null ）
{
worksheet.Cells [cellRowIndex，cellColumnIndex] = dataGridView1.Rows [i] .Cells [j] .Value.ToString（）;
}
else
{
worksheet.Cells [cellRowIndex，cellColumnIndex] = 字符串 .Empty;
}


// 工作：
// worksheet.Cells [cellRowIndex，cellColumnIndex] = dataGridView1.Rows [i] .Cells [j]。 Value.ToString（）;
}
cellColumnIndex ++;
}
cellColumnIndex = 1 ;
cellRowIndex ++;
}

// 获取要保存的Excel的位置和文件名来自用户。
SaveFileDialog saveDialog = new SaveFileDialog（）;
saveDialog.Filter = Excel文件（* .xlsx）| * .xlsx |所有文件（*。 。*）| *。*;
saveDialog.FilterIndex = 2 ;

if （saveDialog.ShowDialog（）== System.Windows.Forms.DialogResult.OK）
{
workbook.SaveAs（saveDialog.FileName）;
MessageBox.Show（ Export Successful）;
}
}
catch （System.Exception ex）
{
MessageBox.Show（例如信息）;
}
最后
{
excel.Quit（）;
workbook = null ;
excel = null ;
}

我尝试过：



我尝试使用IF / ELSE条件来避免程序进入异常，但没有成功。

解决方案

你的 catch 块没有帮助你 - 你几乎丢弃了异常的所有有用细节，并在抛出异常时阻止Visual Studio中断。



但是在猜测中，异常可能是从这一行抛出的：

 if（dataGridView1.Rows [ i] .Cells [j] .Value.ToString（）！= null）

如果 Value 是 null ，当你试图调用 ToString NullReferenceException c> on。



如果价值 不 null ，然后 ToString 将不会返回 null 。



删除该行的 .ToString（）调用。

 if（dataGridView1.Rows [i] .Cells [j] .Value！= null）
 {
 worksheet.Cells [cellRowIndex，cellColumnIndex] = dataGridView1.Rows [i] .Cells [j] .Value.ToString（）; 
} 
 else 
 {
 worksheet.Cells [cellRowIndex，cellColumnIndex] = String.Empty; 
} 

Is anybody can help me understand the issue of my code? It is always going to the exception, does not matter if I try to "treat" the cell with NULL.

My catch System.Exception error is:
"Object reference not set to an instance of an object"

try
 {

     worksheet = workbook.ActiveSheet;

     worksheet.Name = "ExportedFromDatGrid";

     int cellRowIndex = 1;
     int cellColumnIndex = 1;


     //Loop through each row and read value from each column.
     for (int i = -1; i < dataGridView1.Rows.Count - 1; i++)
     {
         for (int j = 0; j < dataGridView1.Columns.Count; j++)
         {
             // Excel index starts from 1,1. As first Row would have the Column headers, adding a condition check.
             if (cellRowIndex == 1)
             {
                 worksheet.Cells[cellRowIndex, cellColumnIndex] = dataGridView1.Columns[j].HeaderText;
             }
             else
             {

                 if (dataGridView1.Rows[i].Cells[j].Value.ToString() != null)
                 {
                     worksheet.Cells[cellRowIndex, cellColumnIndex] = dataGridView1.Rows[i].Cells[j].Value.ToString();
                 }
                 else
                 {
                     worksheet.Cells[cellRowIndex, cellColumnIndex] = String.Empty;
                 }


                 // working:
                 //worksheet.Cells[cellRowIndex, cellColumnIndex] = dataGridView1.Rows[i].Cells[j].Value.ToString();
             }
             cellColumnIndex++;
         }
         cellColumnIndex = 1;
         cellRowIndex++;
     }

     //Getting the location and file name of the excel to save from user.
     SaveFileDialog saveDialog = new SaveFileDialog();
     saveDialog.Filter = "Excel files (*.xlsx)|*.xlsx|All files (*.*)|*.*";
     saveDialog.FilterIndex = 2;

     if (saveDialog.ShowDialog() == System.Windows.Forms.DialogResult.OK)
     {
         workbook.SaveAs(saveDialog.FileName);
         MessageBox.Show("Export Successful");
     }
 }
 catch (System.Exception ex)
 {
     MessageBox.Show(ex.Message);
 }
 finally
 {
     excel.Quit();
     workbook = null;
     excel = null;
 }

What I have tried:

I tried, with no success, to use an IF/ELSE condition to avoid the program entering the "exception".

解决方案

Your catch block isn't helping you - you're throwing away almost all of the useful details of the exception, and preventing Visual Studio from breaking when the exception is thrown.

But at a guess, the exception is probably thrown from this line:

if (dataGridView1.Rows[i].Cells[j].Value.ToString() != null)

If the Value is null, you'll get a NullReferenceException when you try to call ToString on it.

If the Value is not null, then ToString will not return null.

Remove the .ToString() call on that line.

if (dataGridView1.Rows[i].Cells[j].Value != null)
{
    worksheet.Cells[cellRowIndex, cellColumnIndex] = dataGridView1.Rows[i].Cells[j].Value.ToString();
}
else
{
    worksheet.Cells[cellRowIndex, cellColumnIndex] = String.Empty;
}

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