If you want something different, you could have a member variable of type Dictionary(Of String, Func(Of Double, Double, Double))
to associate a string operator with the actual logic for the operator:
Private _ops = New Dictionary(Of String, Func(Of Double, Double, Double))() From {
{"+", Function(x, y) x + y},
{"-", Function(x, y) x - y},
{"*", Function(x, y) x * y},
{"/", Function(x, y) x / y}
}
And then use that in your button click handler:
Dim op = _ops(opType)
txtField.Text = CStr(op(CDbl(opStore), CDbl(txtField.Text))