替换项目 DataGridViewComboBoxCell c#。

Question

我想在两列之间创建一个依赖关系 每行 的DataGridView中。我在运行时添加了两列（品牌，模型）（从数据库中选择数据）。我的目标是当品牌单元格发生变化时，在models单元格中加载新项目。填充数据。

private void autoOptionsForm_Load(object sender, EventArgs e)
{
    Connection.ds.Tables.Clear();
    Connection.adap = new SqlDataAdapter("SELECT * FROM autoOptions", Connection.conn);
    Connection.adap.Fill(Connection.ds, "autoOptions");
    aoGV.DataSource = Connection.ds.Tables[0];         
    // Additing ComboBox column
    DataGridViewComboBoxColumn carCB = new DataGridViewComboBoxColumn();
    carCB.Name = "Model";
    aoGV.Columns.Add(carCB);

    DataGridViewComboBoxColumn brandCB = new DataGridViewComboBoxColumn();
    brandCB.Name = "Brand";
    aoGV.Columns.Add(brandCB);
    foreach (DataGridViewRow row in aoGV.Rows) {
        if (row.Index == aoGV.Rows.Count - 1) continue;
        //Additing CB for Models
     SqlCommand cmd = new SqlCommand(@"SELECT Model FROM Cars WHERE 
                        Brand_ID = (SELECT Brand_ID FROM Cars WHERE id='" + row.Cells["Car_ID"].Value + "')", Connection.conn);
     SqlDataReader sqlReader = cmd.ExecuteReader();
     while (sqlReader.Read()) ((DataGridViewComboBoxCell)row.Cells["Model"]).Items.Add(sqlReader["Model"].ToString());

        //Current Model ComboBox
     cmd = new SqlCommand(@"SELECT Model FROM Cars WHERE id = "+ row.Cells["Car_ID"].Value, Connection.conn);
     sqlReader = cmd.ExecuteReader();
     sqlReader.Read();
     int iId = ((DataGridViewComboBoxCell)row.Cells["Model"]).Items.IndexOf(sqlReader["Model"].ToString());

     ((DataGridViewComboBoxCell)row.Cells["Model"]).Value = ((DataGridViewComboBoxCell)row.Cells["Model"]).Items[iId];

        //Additing CB for Brands
     cmd = new SqlCommand(@"SELECT Brand_name FROM Brands", Connection.conn);
      sqlReader = cmd.ExecuteReader();
      while (sqlReader.Read()) ((DataGridViewComboBoxCell)row.Cells["Brand"]).Items.Add(sqlReader["Brand_name"].ToString());

      cmd = new SqlCommand(@"SELECT Brand_name FROM Brands WHERE id = 
                             (SELECT Brand_ID FROM Cars WHERE id = " + row.Cells["Car_ID"].Value + ")", Connection.conn);
      sqlReader = cmd.ExecuteReader();
      sqlReader.Read();
      iId = ((DataGridViewComboBoxCell)row.Cells["Brand"]).Items.IndexOf(sqlReader["Brand_name"].ToString());
      ((DataGridViewComboBoxCell)row.Cells["Brand"]).Value = ((DataGridViewComboBoxCell)row.Cells["Brand"]).Items[iId];
    }
}

我的方法是用代码解决。

private void aoGV_CellEndEdit(object sender, DataGridViewCellEventArgs e)
{
    if (aoGV.Columns[aoGV.SelectedCells[0].ColumnIndex].Name != "Brand") return;
    DataGridViewRow row = aoGV.Rows[aoGV.SelectedCells[0].RowIndex];
    SqlCommand cmd = new SqlCommand(@"SELECT Model FROM Cars WHERE 
                        Brand_ID = (SELECT id FROM Brands WHERE Brand_name = '"+row.Cells["Brand"].Value.ToString()+"')", Connection.conn);
    SqlDataReader sqlReader = cmd.ExecuteReader();
    ((DataGridViewComboBoxCell)row.Cells["Model"]).Items.Clear();
    while (sqlReader.Read()) ((DataGridViewComboBoxCell)row.Cells["Model"]).Items.Add(sqlReader["Model"].ToString());
}

当我试图清除单元格中的项目时，出现了一个错误，为什么会出现这个错误？为什么会出现这样的错误呢？有什么其他的方法可以解决这个问题吗？

Answer 1

我知道这是个老问题，但还是要回答一下，也许对下一个问题有帮助。

我刚刚也遇到了同样的问题，稍微调试了一下，发现问题出在对象的值变量上.值变量保存着combobox的当前选择值，但是当你清除项目时，那么这个值就会变得无效，这就导致了错误.在清除之前将值变量设置为null就可以消除错误。