C# version of Base64 Encoding from VB6

Question

I need to encode a string using Base64. I have a VB6 algorithm and I need to write a C# version exactly. I was able to get it right 99% but the last character is wrong and I'm not sure what exactly is going wrong.

Here is the string to encode:

¼9ÀPÊ¨!Œaøc4vøÝFãgïÒ_£¡˜è/[t•Ôý56wBpïcÃ.Cð`

VB6 Encoding gives the following string:

vDnAUMqoFiGMYfhjNHb43UbjZ+/SX6MBoZjoL1t0ldT9NTZ3QnDvYwMTww4uQ/Ag

C# encoding gives the following string:

vDnAUMqoFiGMYfhjNHb43UbjZ+/SX6MBoZjoL1t0ldT9NTZ3QnDvYwMTww4uQ/Ax

As you can see everything is the same except the last letter.

VB6 Algorithm :

Const sBASE_64_CHARACTERS = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/"


Function Base64encode(ByVal asContents)
    Dim lnPosition
    Dim lsResult
    Dim Char1
    Dim Char2
    Dim Char3
    Dim Char4
    Dim Byte1
    Dim Byte2
    Dim Byte3
    Dim SaveBits1
    Dim SaveBits2
    Dim lsGroupBinary
    Dim lsGroup64

    If Len(asContents) Mod 3 > 0 Then asContents = asContents & String(3 - (Len(asContents) Mod 3), " ")
    lsResult = ""

    For lnPosition = 1 To Len(asContents) Step 3
        lsGroup64 = ""
        lsGroupBinary = Mid(asContents, lnPosition, 3)

        Byte1 = Asc(Mid(lsGroupBinary, 1, 1)): SaveBits1 = Byte1 And 3
        Byte2 = Asc(Mid(lsGroupBinary, 2, 1)): SaveBits2 = Byte2 And 15
        Byte3 = Asc(Mid(lsGroupBinary, 3, 1))

        Char1 = Mid(sBASE_64_CHARACTERS, ((Byte1 And 252) \ 4) + 1, 1)
        Char2 = Mid(sBASE_64_CHARACTERS, (((Byte2 And 240) \ 16) Or (SaveBits1 * 16) And &HFF) + 1, 1)
        Char3 = Mid(sBASE_64_CHARACTERS, (((Byte3 And 192) \ 64) Or (SaveBits2 * 4) And &HFF) + 1, 1)
        Char4 = Mid(sBASE_64_CHARACTERS, (Byte3 And 63) + 1, 1)
        lsGroup64 = Char1 & Char2 & Char3 & Char4

        lsResult = lsResult + lsGroup64
    Next

    Base64encode = lsResult
End Function

Here is the C# equivalent :

public const string sBASE_64_CHARACTERS = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";
    public static string Base64encode(string asContents)
    {
        int lnPosition;
        string lsResult;
        string Char1;
        string Char2;
        string Char3;
        string Char4;
        int Byte1;
        int Byte2;
        int Byte3;
        int SaveBits1;
        int SaveBits2;
        string lsGroupBinary = null;
        string lsGroup64 = null;

        if (Microsoft.VisualBasic.Strings.Len(asContents) % 3 > 0)
            asContents = asContents + (3 - (Microsoft.VisualBasic.Strings.Len(asContents) % 3));
        lsResult = "";

        for (lnPosition = 1; lnPosition <= Microsoft.VisualBasic.Strings.Len(asContents); lnPosition += 3)
        {
            lsGroup64 = "";
            lsGroupBinary = Microsoft.VisualBasic.Strings.Mid(asContents, lnPosition, 3);

            Byte1 = Microsoft.VisualBasic.Strings.Asc(Microsoft.VisualBasic.Strings.Mid(lsGroupBinary, 1, 1));
            SaveBits1 = Byte1 & 3;
            Byte2 = Microsoft.VisualBasic.Strings.Asc(Microsoft.VisualBasic.Strings.Mid(lsGroupBinary, 2, 1));
            SaveBits2 = Byte2 & 15;
            Byte3 = Microsoft.VisualBasic.Strings.Asc(Microsoft.VisualBasic.Strings.Mid(lsGroupBinary, 3, 1));

            Char1 = Microsoft.VisualBasic.Strings.Mid(sBASE_64_CHARACTERS, ((Byte1 & 252) / 4) + 1, 1);
            Char2 = Microsoft.VisualBasic.Strings.Mid(sBASE_64_CHARACTERS, (((Byte2 & 240) / 16) | (SaveBits1 * 16) & 0xff) + 1, 1);
            Char3 = Microsoft.VisualBasic.Strings.Mid(sBASE_64_CHARACTERS, (((Byte3 & 192) / 64) | (SaveBits2 * 4) & 0xff) + 1, 1);
            Char4 = Microsoft.VisualBasic.Strings.Mid(sBASE_64_CHARACTERS, (Byte3 & 63) + 1, 1);
            lsGroup64 = Char1 + Char2 + Char3 + Char4;

            lsResult = lsResult + lsGroup64;
        }

        return lsResult;
    }
}

What am I doing wrong?

Why would you ever use the Microsoft.VisualBasic namespace in C#, and why can't you just use the built-in base64 conversion? — Matti Virkkunen
– Matti Virkkunen, Commented Jan 2, 2013 at 18:28
Is there an easy way of converting that string to Base64? Any out of the Box function. I've tried UTF8, ASCII encoding etc., but nothing comes close to VB6 encoding. — Preetham Reddy
– Preetham Reddy, Commented Jan 2, 2013 at 18:28
@InvisibleDev See Convert.ToBase64String Convert.FromBase64String — L.B
– L.B, Commented Jan 2, 2013 at 18:32
@MattiVirkkunen There is some good stuff not related to VB that for some reason ended up in Microsoft.VisualBasic. The TextFieldParser class comes to mind as one example. And any time you're trying to exactly mimic old VB6 behavior in a c# port, this namespace can be very useful and a valid tool to reproduce behavior from things like the Val() function. — Joel Coehoorn
– Joel Coehoorn, Commented Jan 2, 2013 at 18:34

prprcupofcoffee · Accepted Answer · 2013-01-02 18:40:13Z

5

The original code pads the string with spaces such that the length is an integral multiple of 3. The translated code doesn't do this properly. This line isn't translated correctly:

If Len(asContents) Mod 3 > 0 Then asContents = asContents & String(3 - (Len(asContents) Mod 3), " ")

You have it as

if (Microsoft.VisualBasic.Strings.Len(asContents) % 3 > 0)
    asContents = asContents + (3 - (Microsoft.VisualBasic.Strings.Len(asContents) % 3));

but it should be

if (Microsoft.VisualBasic.Strings.Len(asContents) % 3 > 0)
    asContents = asContents + Microsoft.VisualBasic.Strings.String(3 - (Microsoft.VisualBasic.Strings.Len(asContents) % 3), " ");

The code you have, instead of appending spaces, appends a digit.

answered Jan 2, 2013 at 18:40

prprcupofcoffee

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7 Comments

Preetham Reddy Over a year ago

Hi David, Thankyou for pointing it out. But the problem is there is no 'Microsoft.VisualBasic.Strings.String'. What do you think is the best way to implement that function?

Preetham Reddy Over a year ago

I've implemented using the following code and it seems to work. if (Microsoft.VisualBasic.Strings.Len(asContents) % 3 > 0) { for (int i = 0; i <= Microsoft.VisualBasic.Strings.Len(asContents) % 3; i++) { asContents += " "; } }

prprcupofcoffee Over a year ago

@InvisibleDev - you're right. Offhand, I don't recall which namespace the String method is in. However, Microsoft.VisualBasic.Strings.Space(number) would also do the trick.

Matti Virkkunen Over a year ago

Just to note, if you want to construct a string by repeating a certain character, one of the constructors for string do just that.

Alexei Levenkov Over a year ago

+1. @InvisibleDev, simple String.Length should be perfectly enough in your case, and adding spaces can be done by just new String(' ', 3) and than using normal ConvertToBase64 call - shorter and safe...

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