Assign Default vlaue to DataGridViewTextBoxCell in Windows apps.

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C#

ADO.NET

i am populating Database table by data present in DatagridView cells. When Any of the cells are left blank it gives Exception. i have to assign default value to that cells.
here is my code to Save Data.

string strContName = GVcontacts.Rows[i].Cells[0].Value.ToString();
string strContNo1 = GVcontacts.Rows[i].Cells[1].Value.ToString();
string strContNo2 = GVcontacts.Rows[i].Cells[2].Value.ToString();
string strEmail = GVcontacts.Rows[i].Cells[3].Value.ToString();

string strFillSuppContacts = "Insert into supplier_contacts(supplier_code, Contact_Name, Contact_No1, Contact_No2, Email_ID)"
+ "values('"+lblCode.Text+"','" + strContName + "','" + strContNo1 + "','" + strContNo2 + "','" + strEmail + "')";
MySqlCommand cmdFillSuppContacts = new MySqlCommand(strFillSuppContacts, con);

cmdFillSuppContacts.ExecuteNonQuery();