How to write a sequence of bytes from a file to a byte array without padding the array with null bytes?

Question

I have

[13,132,32,75,22,61,50,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0]

I want

[13,132,32,75,22,61,50]

I have an array of bytes size 1048576 that I have written to using a file stream. Starting at a particular index in this array until the end of the array are all null bytes. There might be 100000 bytes with values and 948576 null bytes at the end of the array. When I don't know the size of a file how do I efficiently create a new array of size 100000 (ie same as total bytes in unknown file) and write all bytes from that file to the byte array?

byte[] buffer = new byte[0x100000];
int numRead = await fileStream.ReadAsync(buffer, 0, buffer.length); // byte array is padded with null bytes at the end

Answer 1

You're stating in the comments that you're just decoding the byte array into a string, so why not read the file contents as a string, such as:

var contents = File.ReadAllText(filePath, Encoding.UTF8);
// contents holds all the text in the file at filePath and no more

or if you want to use a stream:

using (var sr = new StreamReader(path)) 
{
    // Read one character at a time:
    var c = sr.Read();

    // Read one line at a time:
    var line = sr.ReadLine();

    // Read the whole file
    var contents = sr.ReadToEnd();
}

If you, however, insist on going through a buffer you cannot avoid part of the buffer being empty (having null-bytes) when you reach the end of the file but that's where the return value of ReadAsync saves the day:

byte[] buffer = new byte[0x100000];
int numRead = await fileStream.ReadAsync(buffer, 0, buffer.length);

var sectionToDecode = new byte[numRead];
Array.Copy(buffer, 0, sectionToDecode, 0, numRead);
// Now sectionToDecode has all the bytes that were actually read from the file