Introduction

This project will show a different way how to
- enter ONLY integer value in a TextBox
- enter ONLY the characters you want and avoid certain invalid characters in a TextBox
- highlight the E-Mail address in a TextBox in colour
- and checks whether the E-Mail address is a valid or a invalid E-Mail address
  through the class 'System.Text.RegularExpressions.Regex()'

Let's start with TextBox 'textboxInteger'.
As mentioned before you can enter here ONLY integer values. Other characters will not be accepted.

Here is the method for input integer

private void textboxInteger_KeyPress(object sender, System.Windows.Forms.KeyPressEventArgs e)
{
     if (!Char.IsDigit(e.KeyChar) && e.KeyChar !=(char)8)
    {
        this.statusBar1.Text="incorrect key...";
        e.Handled=true; // input is not passed on to the control(TextBox)
    } else
            this.statusBar1.Text="OK..";
}
As you can see we check whether the char you typed in the TextBox
is a decimal digit or backspace-key for deleting the chars or not .
If it's NOT we won't pass on to the control so that you can't see it in the TextBox(e.Handled=true).
Otherwise we display the Statusbar Text saying "OK.." which means you entered correct decimal digits.

TextBox 'textboxChars'
We want here to avoid entering the user certain invalid characters(i.e. 'a','b','c','#','*','1')
To solve this I implemented 3 methods in the Form1.cs
* mTextboxCharsKeyPressWithForeach
* mTextboxCharsKeyPressWithIF
* mTextboxCharsKeyPressWithSwitch that I prefer
private void mTextboxCharsKeyPressWithSwitch(object sender, System.Windows.Forms.KeyPressEventArgs e)
{
    this.statusBar1.Text="OK...";
    switch(e.KeyChar)
    {  
        case 'a':
        case 'b':
        case 'c':
        case '#':
        case '*':
        case '1': //this is char not int  
                 e.Handled=true; //it indicates the event is handled.  
                 this.errorProvider1.SetError(this.textboxChars, "not allowed chars: 'a','b','c','#','*','1'");  
                 this.statusBar1.Text="not allowed char..."+e.KeyChar;  
                 break;
       default:  
                 this.errorProvider1.SetError(this.textboxChars, ""); //clear error 
                 break;
    } //switch
}