Difference between calling an async method and Task.Run an async method

Question

I have a method in my view model

private async void SyncData(SyncMessage syncMessage)
{
    if (syncMessage.State == SyncState.SyncContacts)
    {
        this.SyncContacts(); 
    }
}

private async Task SyncContacts()
{
    foreach(var contact in this.AllContacts)
    {
       // do synchronous data analysis
    }

    // ...

    // AddContacts is an async method
    CloudInstance.AddContacts(contactsToUpload);
}

When I call SyncData from the UI commands and I'm syncing a large chunk of data UI freezes. But when I call SyncContacts with this approach

private void SyncData(SyncMessage syncMessage)
{
    if (syncMessage.State == SyncState.SyncContacts)
    {
        Task.Run(() => this.SyncContacts()); 
    }
}

Everything is fine. Should not they be the same? I was thinking that not using await for calling an async method creates a new thread.

Answer 1

Should not they be the same? I was thinking that not using await for calling an async method creates a new thread.

No, async does not magically allocate a new thread for it's method invocation. async-await is mainly about taking advantage of naturally asynchronous APIs, such as a network call to a database or a remote web-service.

When you use Task.Run, you explicitly use a thread-pool thread to execute your delegate. If you mark a method with the async keyword, but don't await anything internally, it will execute synchronously.

I'm not sure what your SyncContacts() method actually does (since you haven't provided it's implementation), but marking it async by itself will gain you nothing.

Edit:

Now that you've added the implementation, i see two things:

1. I'm not sure how CPU intensive is your synchronous data analysis, but it may be enough for the UI to get unresponsive.

2. You're not awaiting your asynchronous operation. It needs to look like this:

   private async Task SyncDataAsync(SyncMessage syncMessage)
   {
       if (syncMessage.State == SyncState.SyncContacts)
       {
           await this.SyncContactsAsync(); 
       }
   }
   
   private Task SyncContactsAsync()
   {
       foreach(var contact in this.AllContacts)
       {
          // do synchronous data analysis
       }
   
       // ...
   
       // AddContacts is an async method
       return CloudInstance.AddContactsAsync(contactsToUpload);
   }