[c#] c# how to add byte to byte array

How to add a byte to the beginning of an existing byte array? My goal is to make array what's 3 bytes long to 4 bytes. So that's why I need to add 00 padding in the beginning of it.

As many people here have pointed out, arrays in C#, as well as in most other common languages, are statically sized. If you're looking for something more like PHP's arrays, which I'm just going to guess you are, since it's a popular language with dynamically sized (and typed!) arrays, you should use an ArrayList:

var mahByteArray = new ArrayList<byte>();

If you have a byte array from elsewhere, you can use the AddRange function.

mahByteArray.AddRange(mahOldByteArray);

Then you can use Add() and Insert() to add elements.

mahByteArray.Add(0x00); // Adds 0x00 to the end.
mahByteArray.Insert(0, 0xCA) // Adds 0xCA to the beginning.

Need it back in an array? .ToArray() has you covered!

mahOldByteArray = mahByteArray.ToArray();

Simple, just use the code below, as I do:

        public void AppendSpecifiedBytes(ref byte[] dst, byte[] src)
    {
        // Get the starting length of dst
        int i = dst.Length;
        // Resize dst so it can hold the bytes in src
        Array.Resize(ref dst, dst.Length + src.Length);
        // For each element in src
        for (int j = 0; j < src.Length; j++)
        {
            // Add the element to dst
            dst[i] = src[j];
            // Increment dst index
            i++;
        }
    }

    // Appends src byte to the dst array
    public void AppendSpecifiedByte(ref byte[] dst, byte src)
    {
        // Resize dst so that it can hold src
        Array.Resize(ref dst, dst.Length + 1);
        // Add src to dst
        dst[dst.Length - 1] = src;
    }

Although internally it creates a new array and copies values into it, you can use Array.Resize<byte>() for more readable code. Also you might want to consider checking the MemoryStream class depending on what you're trying to achieve.

To prevent recopy the array every time which isn't efficient

What about using Stack

csharp> var i = new Stack<byte>();
csharp> i.Push(1);
csharp> i.Push(2); 
csharp> i.Push(3); 
csharp> i; { 3, 2, 1 }

csharp> foreach(var x in i) {
  >       Console.WriteLine(x);
  >     }

3 2 1

Arrays can't be resized, so you need to allocte a new array that is larger, write the new byte at the beginning of it, and use Buffer.BlockCopy to transfer the contents of the old array across.