Hello I have several text boxes that I will input answers into and I want them to catch the values if they are incorrect or correct.

Example: the user will input a Letter for the answer, A or a, and I was curious to how the correct input can detect if the answer is correct.

If the answer is A B C or D then no message should display but when it's some other value this message will come up.

How do I use true or false in this code?

Code:
Dim dbl1 As Double
dbl1 = CDbl(txt1.Text)

If dbl1 Not 'A OR a' Then
'Display message
MessageBox.Show("Enter A, B, C, or D")