how can i tell if a user entered in ONLY a letter a-z and or a digit...

ive tried..

Dim mytext As Char
If mytext.IsLetterOrDigit(txtDistrictDB.Text) = True Then
MsgBox("yes")
Else
MsgBox("no")
End If

but if i type in a character such as !@#$%^&*()_ it returns true also.....

it doesn't work if i try to get the characters in a text box, but if i type in one character than it works.. is there a way around it rather than looping through each character?