invalidCastException was unhandled by user code

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hai;

I work windows application using C#
my problem is "InvalidCastException was unhandled"
"Object cannot be cast from DBNull to other types."

in my application ,i have a datagridview control and a textbox.
and also i create a table named as Mark, in this table have 4 fields. they are name,mark,mark1,total.

1)Firstly i bind the data in the datagridview. my code is bellow

C#

private void Form4_Load(object sender, EventArgs e)
       {
           SqlDataAdapter da = new SqlDataAdapter("select * from  addr", con);
           DataSet ds = new DataSet();
           da.Fill(ds);
           dataGridView1.DataSource = ds.Tables[0];
       }

2) i do add mark and mark1 and store in total column in datagridview. my code is given bellow

C#

private void dataGridView1_CurrentCellDirtyStateChanged(object sender, EventArgs e)
       {
           c1 = Convert.ToDouble(dataGridView1.CurrentRow.Cells[3].Value);
           a = Convert.ToDouble(dataGridView1.CurrentRow.Cells[1].Value);
           b = Convert.ToDouble(dataGridView1.CurrentRow.Cells[2].Value);
           dataGridView1.CurrentRow.Cells[3].Value = a * b;
           dataGridView1.Columns["Amount"].ReadOnly = true;

           c = 0;
           c = Convert.ToDouble(dataGridView1.CurrentRow.Cells[3].Value) - c1;
           ng = ng + Convert.ToDouble(c);
           c = 0;
           a = 0;
           textBox1.Text = ng.ToString();

       }

the error occured at this line "c1 = Convert.ToDouble(dataGridView1.CurrentRow.Cells[3].Value);"//InvalidCastException was unhandled " this is the error message.

how to over come these problem and how to edit/ add a new value inthis datagrideview.

please help me

thanks in advance.

Posted 21-Mar-12 8:14am

vidya jobin

Updated 21-Mar-12 8:45am

Wes Aday

v2

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Clifford Nelson · Answer 1 · 2012-03-21T08:37:00

Solution 2

You can use the Double.TryParse. This way you can check if there is a problem:

Double x;
if (Double.TryParse(dataGridView1.CurrentRow.Cells[3].Value, out x)
  c1 = x;
else
{
  //Whatever the fallback will be.
}

Posted 21-Mar-12 8:37am

Clifford Nelson

Sergey Alexandrovich Kryukov · Answer 2 · 2012-03-21T08:36:00

It simply means that the type of that Cell[3] content is not double. Just check it up. If it's the string, you can further parse if to double using double.Parse(string) or double.TryParse(string), but it would be a bad code design. You should better have right types in right cells, and, ultimately, right types in the columns of your database.

By the way, you are using hard coded immediate constants for the cell indices; this is always bad, hard to maintain, error-prone.

—SA