c# winform，对象不能从dbnull转换为其他类型

Question

I have checkbox in my DataGridView to delete one or multiple rows from my DataGridView already loaded data from database. DataSource is dataBindingSource. I want to check first if the user checked one or multiple checkboxes and here is my code to check.

int total = dgvRegister.Rows.Cast<DataGridViewRow>().Where(x => Convert.ToBoolean(x.Cells["Delete"].Value) == true).Count();
if (total > 0)
{
// Do something

}

But I'am geting this error "object cannot be cast from dbnull to other types". I have gogled but not s omuch to help. Please help me to solv this problem and thanks a lot in advance.

Here is the hole code

private void btnDeActivateComputers_Click(object sender, EventArgs e)
    {
        try
        {
            using (CompEntities db = new CompEntities())
            {
                if (dgvRegister.Rows.Count > 0)
                {
                    int total = dgvRegister.Rows.Cast<DataGridViewRow>().Where(x => Convert.ToBoolean(x.Cells["Delete"].Value) == true).Count();
                    if (total > 0)
                    {
                         
                        if (MessageBox.Show("Do you want to delete?", "Deleting", MessageBoxButtons.YesNo, MessageBoxIcon.Question) == DialogResult.Yes)
                        {
                            for (int i = dgvRegister.RowCount - 1; i >= 0; i--)
                            {
                                DataGridViewRow row = dgvRegister.Rows[i];
                                if (Convert.ToBoolean(row.Cells["Delete"].Value) == true)
                                {
                                    int ComId = Convert.ToInt32(dgvRegister.Rows[row.Index].Cells[0].Value.ToString());
                                    var deletecomp = db.Comp.Where(x => x.ComId == cId).FirstOrDefault();
                                    if (deletecomp != null)
                                    {
                                        db.Comp.Remove(deletecomp);


                                    }


                                    db.SaveChanges();

                                }
                            }
                        }


                        

                    }
                    else
                    {
                        MessageBox.Show("Please Check at least one checkbox !", "Information", MessageBoxButtons.OK,
                        MessageBoxIcon.Error,
                        MessageBoxDefaultButton.Button1);
                    }
                }
                 
            }

        }
        catch (Exception ex)
        {
            MessageBox.Show(ex.Message);
            Exception inner = ex.InnerException;
            while (inner != null)
            {
                MessageBox.Show(inner.Message);
                inner = inner.InnerException;
            }
        }
        
    }

Answer 1

Here is a pattern to following using Entity Framework Core and C#9, some adjustments are needed if using say .NET Framework 4.8 but the principles are the same as noted below.

Model Categories where Checked property is not part of the underlying table. [NotMapped] tells Entity Framework to exclude this property while [DefaultValue(false)] provides false as the default value.

public class Categories
{
    [NotMapped]
    [DefaultValue(false)]
    public bool Checked { get; set; }
    public int CategoryId { get; set; }
    public string CategoryName { get; set; }
}

In the form, create a BindingSource which is used for all operations once assigned as the DataSource to the DataGridView. As mentioned in various comments, there is zero need to reference data via the DataGridView.

Notes

• Code is C#9, minor adjustments will be needed if not using .NET Core.
• In the button to remove records a lambda statement is used to get checked items
• A wrapper for MessageDialog is used to ask permission to remove data
• A class is provided to where you would remove the records but doesn't actually remove records, you need to write that code.
• DataGridView columns were created in the DataGridView designer and each column DataPropertyName set to properties from the model

Form code with two classes that belong in their own files.

namespace StackoverflowCodeSample
{
    public partial class Form1 : Form
    {
        private readonly BindingSource _bindingSource = new();

        public Form1()
        {
            InitializeComponent();

            dataGridView1.AutoGenerateColumns = false;
            Shown += OnShown;

        }

        private void OnShown(object sender, EventArgs e)
        {
            using var context = new NorthwindContext();

            _bindingSource.DataSource = context.Categories.ToList();
            dataGridView1.DataSource = _bindingSource;

        }

        private void GetCheckedButton_Click(object sender, EventArgs e)
        {

            var checkedCategories =
                ((List<Categories>)_bindingSource.DataSource)
                .Where(category => category.Checked)
                .ToList();


            if (checkedCategories.Count > 0)
            {
                /*
                 * Prompt to remove selected entities
                 * Remove from database table, save
                 * if save changes is successful
                 *    Remove from BindingSource
                 * else
                 *    inform user of issues
                 */

                if (Dialogs.Question($"Remove {checkedCategories.Count} items?"))
                {
                    var (success, exception) = DataOperations
                        .RemoveRange(checkedCategories);

                    if (success)
                    {
                        for (int index = 0; index < checkedCategories.Count; index++)
                        {
                            _bindingSource.Remove(checkedCategories[index]);
                        }
                    }
                    else
                    {
                        MessageBox.Show($"Removal failed with\n{exception.Message}");
                    }
                }
            }
        }
    }

    public class DataOperations
    {
        public static (bool success, Exception exception) RemoveRange(List<Categories> list)
        {
            try
            {
                // remove items here
                // Set each entity state
                // perform SaveChanges
                return (true, null);
            }
            catch (Exception e)
            {
                return (false, e);
            }
        }
    }
    public static class Dialogs
    {
        public static bool Question(string text)
        {
            return (MessageBox.Show(
                text,
                "Question",
                MessageBoxButtons.YesNo,
                MessageBoxIcon.Question,
                MessageBoxDefaultButton.Button2) == DialogResult.Yes);
        }
    }
}