Trim padded values when converting a byte array to String

Question

An external DLL that I'm using in my C# program gives me a pointer to a location in memory. From there, I read in the following 200 bytes at that location to a byte array, and then convert it to a String.

Problem: For the most part my code works but it also reads in the values that the DLL used as padding (seems to be 205).

Attempt: I tried trimming the string by using examples from this Stack Overflow post, but the padding remained. My code is as follows:

const int BYTES_PER_DEVICE_NAME = 200;
const char PADDING_VALUE = (char)205;
const char NULL_VALUE = (char)0;

IntPtr nameAddress = (IntPtr)myDLL.GetPointer();

// Only run if the pointer is not to a null value
if (nameAddress != IntPtr.Zero)
{
    byte[] nameAsBytes = new byte[BYTES_PER_DEVICE_NAME];
    Marshal.Copy(nameAddress, nameAsBytes, 0, BYTES_PER_DEVICE_NAME);
    string nameAsString = System.Text.Encoding.UTF8.GetString(nameAsBytes).TrimEnd(new char[] { PADDING_VALUE, NULL_VALUE });

    // Add the string to deviceNames, a string array
    deviceNames[n] = nameAsString;
}

As seen below, even though I used TrimEnd() the string nameAsString still ended up containing the null and padded values.

I'm currently trying to see if I can find the index of \0 in my nameAsBytes byte array, and only parse the bytes up until that index. But still, what am I doing wrong with TrimEnd()?

John Wu · Accepted Answer · 2017-03-08 17:18:36Z

Once the byte array is converted to a .NET string it is in UTF16 encoding, and char(205) won't have the effect you are looking for. Any value greater than 127 may end up mapping to a different code point post-conversion.

Try removing the 205's before converting to a string.

const int BYTES_PER_DEVICE_NAME = 200;
const char PADDING_VALUE = (char)205;
const char NULL_VALUE = (char)0;

IntPtr nameAddress = (IntPtr)myDLL.GetPointer();

// Only run if the pointer is not to a null value
if (nameAddress != IntPtr.Zero)
{
    byte[] nameAsBytes = new byte[BYTES_PER_DEVICE_NAME];
    Marshal.Copy(nameAddress, nameAsBytes, 0, BYTES_PER_DEVICE_NAME);
    byte[] cleanedBytes = nameAsBytes.Where(a => a != PADDING_VALUE); //Filter out the bad bytes
    string nameAsString = System.Text.Encoding.UTF8.GetString(cleanedBytes);

    // Add the string to deviceNames, a string array
    deviceNames[n] = nameAsString;
}

PaulF · Accepted Answer · 2017-03-08 17:12:59Z

1

You can use the UTF8Encoding.GetString Method (Byte[], Int32, Int32) method : https://msdn.microsoft.com/en-us/library/kzb9f993(v=vs.110).aspx

byte[] nameAsBytes = new byte[BYTES_PER_DEVICE_NAME];
Marshal.Copy(nameAddress, nameAsBytes, 0, BYTES_PER_DEVICE_NAME);
int strlen;
for (strlen = 0; strlen < nameAsBytes.Length;strlen++)
{
  if (nameAsBytes[strlen] == 0)
    break;
}
string nameAsString = System.Text.Encoding.UTF8.GetString(nameAsBytes, 0, strlen);

answered Mar 8, 2017 at 17:12

PaulF

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