Convert the string to the equivalent hex of that string - vb.net

Question

I would like to convert the string that I've inputted to it's equivalent hex.
For example:
I have the string D7 and I would like it to be converted to hex D7 so that it can be read by the software that can only read hex values.
bla-bla-bla = Error, because there's no hex equivalent of it
aBc = ABC because there's a hex equivalent of it.
123 = 123 because there's a hex equivalent of it.
12 AB 3.14 = Error, because there's no hex equivalent of it
3.F1 = Error, because there's no hex equivalent of it
Though I'm not sure about that, but I guess that will be the result. As long as there's no hex equivalent of each text then it will be error.
Edit: I've tried to convert the C# code of Dmitry, but it is still not working. I'll try it again on Monday
Here's the code
Dim source As String = "abc789Def"
Dim Sb As New StringBuilder(source.Length)
For Each c As Char In source
If ((c >= "0" AndAlso c <= "9") Or (c >= "a" AndAlso c <= "f") Or (c >= "A" AndAlso c <= "F")) Then
Sb.Append(Char.ToUpper(c))
Dim result As String = Sb.ToString
Console.WriteLine("result " & result)
Else
Console.Write("Error")
End If
Next

Answer 1

It seems, that you want to have the string to be in upper case if all the characters in it are in either ['0'..'9'] or ['a'..'f'] or ['A'..'F'] range (C# code):
String source = "abc789Def";
if (source.All(c => (c >= '0' && c <= '9') ||
(c >= 'a' && c <= 'f') ||
(c >= 'A' && c <= 'F'))) {
String result = source.ToUpper();
//TODO: put a relevant code here
}
else {
// error: at least one character can't be converted
}
Edit: no Linq solution:
String source = "abc789Def";
StringBuilder Sb = new StringBuilder(source.Length);
foreach (Char c in source)
if ((c >= '0' && c <= '9') ||
(c >= 'a' && c <= 'f') ||
(c >= 'A' && c <= 'F'))
Sb.Append(Char.ToUpper(c));
else {
// Error: at least one character can't be converted
return;
}
String result = Sb.ToString();
// Put relevant code here

Answer 2

The ability to validate a Hex number is provided by the TryParse method on integer types using the System.Globalization.NumberStyles.HexNumber style.
Dim inputValue As String = "23"
Dim returnValue As String = Nothing
If ValidateAndFormatHex(inputValue, returnValue) Then
Console.WriteLine(returnValue)
Else
Console.WriteLine("Error")
End If
Private Shared Function ValidateAndFormatHex(input As String, ByRef formattedValue As String) As Boolean
input = input.Trim().ToUpperInvariant
Dim result As Int32
Dim parses As Boolean = Int32.TryParse(input, System.Globalization.NumberStyles.HexNumber, System.Globalization.CultureInfo.InvariantCulture, result)
If parses Then formattedValue = input
Return parses
End Function