This is a common error that you get when trying to display a field with no value (null), from a table. If for example you have a field where you store the age of the employee, but you don’t know the age of all employees, and thus those fields are set to NULL. Trying to retrieve this in your C# application and convert it to a string or another data type will result in this “cannot cast from DBNull” error. To prevent this simply check if the value is DBNull:

if(myDataReader["EmployeeAge"] != System.DBNull.Value)
{
   label1.Text = Convert.ToString(myDataReader["EmployeeAge"]);
}

In this example we use a DataReader. If the value of EmployeeAge is not DBNull, we convert it to a string and display it in the label. However, if the value is NULL, we don’t do anything, leaving the label blank. Of course, if you wish you can display a string such as “No age information” using an else block.