输入半径，计算圆周长和圆面积（保留小数点后两位小数）

进一步要求，为了保证程序运行正确，对输入的半径要进行合法性检查。数据输入结束有两种方法，

分别编事件过程对数据进行检验：

1：按Tab键，检查数据的合法性，利用TextBox1_LostFocus事件。

2：按回车键，当TextBox1_KeyPress事件中的返回参数e.KeyChar的ASC()值为13时表示输入结束。

Public Class Form1

    Private Sub TextBox1_KeyPress(ByVal sender As Object,ByVal e As System.Windows.Forms.KeyPressEventArgs) Handles TextBox1.KeyPress
        If Asc(e.KeyChar) = 13 Then
            If Not IsNumeric(TextBox1.Text) Then
                MsgBox("输入数字有非数字字符，请重新输入","数据检验")
                TextBox1.Text = ""
                TextBox1.Focus()
            End If
        End If
    End Sub

    Private Sub TextBox1_LostFocus(ByVal sender As Object,ByVal e As System.EventArgs) Handles TextBox1.LostFocus
        If Not IsNumeric(TextBox1.Text) Then
            MsgBox("输入数字有非数字字符，请重新输入","数据检验")
            TextBox1.Text = ""
            TextBox1.Focus()
        End If
    End Sub

    Private Sub Button1_Click(ByVal sender As System.Object,ByVal e As System.EventArgs) Handles Button1.Click
        Dim r!,c!
        r = TextBox1.Text
        c = 2 * 3.1415926 * r
        TextBox2.Text = Format(c,"0.00")
    End Sub

    Private Sub Button2_Click(ByVal sender As System.Object,ByVal e As System.EventArgs) Handles Button2.Click
        Dim r!,s!
        r = TextBox1.Text
        s = 3.1415926 * r * r
        TextBox3.Text = Format(s,"0.00")
    End Sub

End Class