Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
Dim r, s, c
Const pi = 3.14159
Val(InputBox("请输入 r "))
s = pi * r * r
c = 2 * pi * r
TextBox1.Text = s
TextBox2.Text = c
End Sub
问题说是 r在被赋值前引用导致null异常

向楼主看齐啊1

应该说Dim r，s，c As integer吧

目测是
Val(InputBox("请输入 r "))
的问题

r=val(inputbox("请输入r"))

6楼正解