private string GetNextFileName(string fileName)
{
    int index = 1;

    string nextName = $"{fileName}_{index}.png";

    while (File.Exists(nextName))
    {
        nextName = $"{fileName}_{++index}.png";
    }

    return nextName;
}

private string GetNextFileName(string fileName)
{
    int i = 1;
    string nextName = $"{fileName}_{i}.png";

    for (; i < 100; ++i)
    {
        if (!File.Exists(nextName))
        {
            return nextName;
        }
        nextName = $"{fileName}_{i}.png";
    }

    throw new ApplicationException("Unable to get free filename");
}