I'm doing a simple week end project. In this i need to know a event. Which is A form has text box and label which is student roll no. And name. After enter the 6 digit student roll no in the the textbox corresponding roll no 's name have to display frm a label. With out pressing any button. Which event is suitable for this case.
There's more options, but you may want to start with a simple TextChanged event: C# TextChanged Event
Although not mentioned there, it is a good practice to always check your textbox value for an empty string or clearly invalid data before doing any expensive computation like accessing a database to fetch your student's name.
void rollNumber_TextChanged(object sender, EventArgs e)
if (rollNumber.Text.Trim() == String.Empty || rollNumber.Text.Length < 5)
rollName.Text = GetStudent(rollNumber.Text);
Optionally you can use data binding and data validation via IDataErrorInfo.