Set Default value in Windows Forms DataGridView Cell if Blank, NULL or Empty using C# and VB.Net

I have filled gridview.In that some cells are blank due to no data were found from datasource.

I want to filled that blank cells with Value 0.

Now after filling datagridview i wrote following code

foreach (DataGridViewRow row in dataGridView1.Rows)
{
    foreach (DataGridViewCell celval in row.Cells)
    {
        if (celval.Value == null)
        {
            celval.Value = 0; 
        }
    }
}

this code has no error but doesn't worked.

i doesn't filling blank cell with 0.

i have already used break point it checks for if condition but got condition false.

please solve this problem