convert ASCII to char

[husky_doggy]

I have a string of ASCII in HEX. ( 5A65726f)
so first i split them into 2s >>>> 5A 65 72 6f
then i change the hex to integer

using these integer i convert them to char using chr()
it did the job great, i can see the correct word i want..but it give me an error saying " make sure it has the right format" ( error shown below)

Private Sub Button2_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button2.Click
        Dim hexnum As String
        Dim converttohex As String
        Dim hextonum As Integer
        Dim chara As Char
        hexnum = txtReceivedMessage.Text
        For i = 1 To Len(hexnum) Step 2

            converttohex = Mid(hexnum, i, 2)

            hextonum = Convert.ToInt32(converttohex, 16) '<<<<< ERROR here
            chara = Chr(hextonum)
            TextBox1.Text += chara
        Next i

please help...
i thought it was the for loop problem...since i will go up to 7 while len(hexnum) is 8 so it would loop again but there is nothing after 8th HEX?
so i tried For i = 1 To Len(hexnum)-1 Step 2 ...this time...textbox didnt show anything...+ the same error came up...

[Atheist]

All arrays in .NET are zero-based, so your loop should go from 0 to hexnum.Length-1.

[husky_doggy]

All arrays in .NET are zero-based, so your loop should go from 0 to hexnum.Length-1.

right...
but for the Mid() function ...the arguement must be greater than 0
how should i do it now?

[Atheist]

The Mid function is just kept for legacy support in VB.NET, use the strings Substring method instead.

[husky_doggy]

ok ..after I looked at it in detail....i found the string my device send to me application has some invalid char hiding...
so i use string.trim()

DONE!
Thank you Atheist anyway~

       Dim hexnum As String
        Dim converttohex As String
        Dim hextonum As Integer
        Dim chara As Char
        hexnum = txtReceivedMessage.Text.trim() <<<!!!
        For i = 0 To Len(hexnum)-1 Step 2

            converttohex = hexnum.Substring(i, 2)

            hextonum = Convert.ToInt32(converttohex, 16) 
            chara = Chr(hextonum)
            TextBox1.Text += chara
        Next i