How to save a char[25] when it contains only 6 chars C#

Question

I have a class that contains as follow

public class foo
{
    public long id= 0;
    public char[] TestId = new char[25];// new char[25];
    public char[] GUID = new char[64]; //new char[64];
    public char[] sign = new char[61];//new char[61];
    public int DemandDate = 0;
    public int CompleteDate = 0;
    public int DownloadDate = 0;

}
    static public void WriteFile(string fileName, List<ClientInfo> list)
    {
        using (FileStream fs = new FileStream(fileName, FileMode.Create))
        {
            using (var writer = new BinaryWriter(fs, Encoding.UTF8, false))
            {
                foreach(ClientInfo ci in list)
                {
                    WriteClientInfo(writer, offset, ociv);
                    offset += header.recordSize;
                }
            }
        }
    }

    private static void WriteClientInfo(BinaryWriter writer, int offset, ClientInfo ci)
    {
        try
        {
            writer.Write(ci.ID);
            writer.Write(ci.TestId, 0, 25);
            writer.Write(ci.GUID, 0, 64 );
            writer.Write(ci.Sign, 0, 61);
            writer.Write(ci.DemandDate);
            writer.Write(ci.CompleteDate);
            writer.Write(ci.DownloadDate);
        }
        catch (Exception ex)
        {

        }
    }

when I serialize this ci.TestId will crash, stating that the index is out of range. I know that the value in ci.TestId = FD3394, but I need to be able to save the length of 25 char, not the actual length of the ci.TestId. Because I read this structure from a program that is written in C++, and it has to be the exact size, of the length of the char in the definition, not what is used. I modify it in C# but needs to be read back in C++ as well.

Answer 1

You can use the PadLeft() or PadRight() method like this

TestId = TestId.PadLeft(25);

The left side will be filled with spaces.

You have to convert it to a string and then re-convert it to char[]

TestId = _testId.PadLeft(25).ToCharArray();