How will stop parallel execution of two async function and force to one after another.

Hi,

I have two async function one is iterative and another one is recursive. Both are performing very heavy task. Fallowing are both function. Both function I was calling from a third function one after another. First i was calling WritePST() after that AddDirectory(). But both are executing parallel. How can I force WritePST() function to wait until its do not completes its execution and then AddDirectory() should start its execution.

public async void XYZ()
{
    ................
    ....................
    WritePST(mbxPath);
    AddDirectory(folder, sDir)
}

public async void WritePST(string mbxPath)
{
        if (!string.IsNullOrEmpty(mbxPath))
    {
            if ((attr & FileAttributes.Directory) == FileAttributes.Directory){
    }
            else{
                   //Doing Some Work.......
                    while(message != null)
                   {
                await Task.delay(2);
                progressBar.DoubleValue += 1; 
                        //Doing Some Work.......
                  }
                 var dir = new DirectoryInfo(delPath);
                 dir.Attributes = dir.Attributes & ~FileAttributes.ReadOnly;
                 dir.Delete(true);
             }
        }
    }

private async void AddDirectory(FolderInfo folder, string sDir)
{
    try{
        foreach (string f in Directory.GetFiles(sDir, "*.eml"))
        {
            await Task.delay(2);
            progressBar.DoubleValue += 1; 
            MailMessage eml = MailMessage.Load(f);
            folder.AddMessage(MapiMessage.FromMailMessage(eml));
        }
        foreach (string d in Directory.GetDirectories(sDir))
        {
            FolderInfo fi = folder.AddSubFolder(new DirectoryInfo(d).Name);
            AddDirectoryT(fi, d);
        }
    }
    catch (System.Exception excpt){
    Console.WriteLine(excpt.Message); 
    }
}