DataGridView導入Excel時，出現了“未將對象引用設置到對象的實例”的解決辦法

print?for (int i = 0; i < iRows - 1; i++) 
 { 
     for (int j = 1, k = 0; j < iCols; j++) 
     { 
         if (dgvStudent.Columns[j].Visible) 
         { 
            
                 dimArray[i + 1, k] = dgvStudent.Rows[i].Cells[j].Value.ToString(); 
                 k++; 
          
         } 
     } 
 } 

               for (int i = 0; i < iRows - 1; i++)
                {
                    for (int j = 1, k = 0; j < iCols; j++)
                    {
                        if (dgvStudent.Columns[j].Visible)
                        {
                         
                                dimArray[i + 1, k] = dgvStudent.Rows[i].Cells[j].Value.ToString();
                                k++;
                       
                        }
                    }
                }[csharp] view plaincopyprint?//其中public System.Windows.Forms.DataGridView dgvStudent;，dimArray為Excel中的單元格，即 

//其中public System.Windows.Forms.DataGridView dgvStudent;，dimArray為Excel中的單元格，即[csharp] view plaincopyprint?//ws.get_Range(ws.Cells[1, 1], ws.Cells[iRows, iCols]).Value2 = dimArray; 

//ws.get_Range(ws.Cells[1, 1], ws.Cells[iRows, iCols]).Value2 = dimArray;[csharp] view plaincopyprint?//ws 為Microsoft.Office.Interop.Excel.Worksheet  

//ws 為Microsoft.Office.Interop.Excel.Worksheet
這是從Datagridview導入Excel時的代碼，但是出現了“未將對象引用設置到對象的實例”這個錯誤，我以為是不是我手動添加列時添加的不正確，所以我把函數 dgvStudent.Columns.Add(列名字符串變量, 列頭名稱字符串變量);改為了

[csharp]
DataGridViewTextBoxColumn column = new DataGridViewTextBoxColumn();  
               column.HeaderText = sh; 
               column.Name = quesId; 
               column.ReadOnly = false; 
               dgvStudent.Columns.Add(column); 

 DataGridViewTextBoxColumn column = new DataGridViewTextBoxColumn();
                column.HeaderText = sh;
                column.Name = quesId;
                column.ReadOnly = false;
                dgvStudent.Columns.Add(column);不用DataGridViewColumn column = new DataGridViewColumn();由於會出現CellType為空的錯誤。

但是還會有“未將對象引用設置到對象的實例”錯誤，因此不是手動添加列的錯誤。此時定位到Datagridview單元格與Excell的關系，

[csharp]
for (int i = 0; i < iRows - 1; i++) 
               { 
                   for (int j = 1, k = 0; j < iCols; j++) 
                   { 
                       if (dgvStudent.Columns[j].Visible) 
                       { 
                           if (dgvStudent.Rows[i].Cells[j].ValueType == typeof(string)) 
                           { 
                               dimArray[i + 1, k] = dgvStudent.Rows[i].Cells[j].Value.ToString(); 
                               k++; 
                           } 
                           else//很重要，因為DataGridView單元格有null的時候  
                           { 
                               if (dgvStudent[j, i].Value == null) 
                               { 
                                   dgvStudent[j, i].Value = string.Empty; 
                               } 
                               dimArray[i + 1, k] = dgvStudent[j, i].Value.ToString(); 
                               k++; 
                           } 
                       } 
                   } 
               } 

 for (int i = 0; i < iRows - 1; i++)
                {
                    for (int j = 1, k = 0; j < iCols; j++)
                    {
                        if (dgvStudent.Columns[j].Visible)
                        {
                            if (dgvStudent.Rows[i].Cells[j].ValueType == typeof(string))
                            {
                                dimArray[i + 1, k] = dgvStudent.Rows[i].Cells[j].Value.ToString();
                                k++;
                            }
                            else//很重要，因為DataGridView單元格有null的時候
                            {
                                if (dgvStudent[j, i].Value == null)
                                {
                                    dgvStudent[j, i].Value = string.Empty;
                                }
                                dimArray[i + 1, k] = dgvStudent[j, i].Value.ToString();
                                k++;
                            }
                        }
                    }
                }此時，我了解到原來是DataGridView單元格值為null，無法轉化為string。