Keypress Event in C#

Key Press Event

 

This event fires or executes whenever a user presses a key in the TextBox.

 

You should understand some important points before we are entering into the actual example.

1. e.Keychar is a property that stores the character pressed from the Keyboard. Whenever a key is pressed the character is stored under the e object in a property called Keychar and the character appears on the TextBox after the end of keypress event.
2. e.Handled is a property then assigned to true, it deletes the character from the keychar property.

 

Indexof(): it returns the index number of the first occurence of the specified letter in the string.

 

Note: if the character doesn't exist in the string it returns -1.

 

Example 1:

 

Form8.cs[Design]

 

 

 

Form8.cs (Code Behind file) 

private void textBox1_KeyPress(object sender, KeyPressEventArgs e)
{
string st = "0123456789"+(char)8;
if(st.IndexOf(e.KeyChar)==-1)
{
MessageBox.Show("please enter digits only");
e.Handled = true;
}
}

Note:

 

The ASCII value of a Backspace is 8. In our keyboard, every key has an ASCII value, but it is very difficult to remember each and every ASCII value.

 

Microsoft has introduced a class called keys. By using this Key class you can easily determine the ASCII value.

 

Example 2

(The same example but I am using a predefind function.)

 

Microsoft provides a predifined function called IsDigit(). By using this function you can easily validate the data, whether a digit was entered or not. 

private void textBox1_KeyPress(object sender, KeyPressEventArgs e)
{
if (!(Char.IsDigit(e.KeyChar) || (e.KeyChar == (char)Keys.Back)))
{
MessageBox.Show("please enter digits only");
e.Handled = true;
}
}

Thank you.