How to add numbers in a text file together with file name

Question

I want to check the size of text file i.e. if the file size is greater then 500kb then create another file with numbers.

Example:

Currently my file is saved like : NetApiLog_2019_04_25.txt 

 

Here is the code to save file-

public void writeLogFile(string errorln)
{
FileStream fs = null;
try
{
var common = (ICommon)new CommonAttribute();
var netVpbxApiLogFlag = common.GetConfigurationByKeyName(string.Empty, "NetVpbxApiLogFlag");
if (Convert.ToBoolean(netVpbxApiLogFlag))
{
var logFilePath = common.GetConfigurationByKeyName(string.Empty, "NetApiLogFilePath");
string FileName = string.Format("{0}_{1}.txt", logFilePath,
DateTime.Today.ToString("yyyy_MM_dd"));

if (!Directory.Exists(logFilePath))
{
Directory.CreateDirectory(logFilePath);
}

fs = new FileStream(FileName, FileMode.Append, FileAccess.Write);
using (StreamWriter sw = new StreamWriter(fs))
{
fs = null;
sw.WriteLine("Start VPBX Api.");
sw.WriteLine("-------------------------------------------------------");
sw.WriteLine(string.Format("{0}:{1}", DateTime.Now.ToString(), errorln));
}
}
}
catch (Exception ex)
{
throw ex;
}
finally
{
if (fs != null)
{
fs.Dispose();
}
}
}

If the file size is greater then 500kb then create the file like : NetApiLog_1_2019_04_25.txt

if the size of file text file  NetApiLog_1_2019_04_25.txt greater then 500 kb then the name should be NetApiLog_2_2019_04_25.txt.

 

 

Can anyone help me to how to write in c# asp .net.??