How to sequential multilevel async process with C#?

Question

I have a daily scheduler sequential scenario that needs to run every midnight:

1. Check_Tenant_Expiry and Get its return value (true/false)
2. Run_Daily_Report (pass the returning value from Check_Tenant_Expiry)

I expect to do Check_Tenant_Expiry and after it completed it will continue with Run_Daily_Report, I use the code below

bool _xxx = await Check_Tenant_Expiry().ContinueWith(t =>
{
    Run_Daily_Report(_xxx); // THE WARNING GOES HERE
}, TaskContinuationOptions.OnlyOnRanToCompletion);

public static async Task<bool> Check_Tenant_Expiry()
{ 
   bool _ret = false;
   ...
   ...
   return await Task.FromResult(_ret);
}

public static async Task Run_Daily_Report()
{ .... }

questions:

1. Why I got a warning on Run_Daily_Report:

Severity Code Description Project File Line Suppression State Warning CS4014 Because this call is not awaited, execution of the current method continues before the call is completed. Consider applying the 'await' operator to the result of the call.

2. I am not sure that I already have correct logic for the sequential multilevel process like first I need to run Check_Tenant_Expiry() until it finished completely and then continue with Run_Daily_Report()

I need advice.

Many thanks in advance Don

Answer 1

I expect to do Check_Tenant_Expiry and after it completed it will continue with Run_Daily_Report

You shouldn't use ContinueWith for this. ContinueWith is a low-level method with dangerous default behavior. At the very least, you must pass a TaskScheduler. In addition, ContinueWith doesn't understand asynchronous code, so if you pass async code in, you'll also need to use Unwrap.

Instead, you should just use await, which is simpler, more maintainable, and does the right thing by default:

bool _xxx = await Check_Tenant_Expiry();
await Run_Daily_Report(_xxx);

On a side note, this seems suspect:

return await Task.FromResult(_ret);

That's literally a less efficient form of this:

return _ret;

I suspect that the await Task.FromResult was added to avoid a compiler warning telling you that the method will run synchronously. If you were in fact getting that warning, then the proper solution is to either make the method properly asynchronous (not adding an await just to make the warning go away), or to remove the async and leave the method synchronous.

Answer 2

Try this

await Check_Tenant_Expiry().ContinueWith( async (t) =>
{
     await Run_Daily_Report(); 
}, TaskContinuationOptions.OnlyOnRanToCompletion);

Answer 3

1. You made a function with async keyword, which means you told compiler excute "Run_Daily_Reporty" method by asynchronously. But you didn't execute the method with await keyword, which means It's run by synchronously. That's why compiler told you "it's right?"

Try this

// You must use async keyword when you want to use await keyword in lambda function.
await Check_Tenant_Expiry().ContinueWith(async(t) =>
{
    await Run_Daily_Report(t.Result);
}, TaskContinuationOptions.OnlyOnRanToCompletion);

2. I think you made code the way you wanted to work.

I think you might not know the async/await keywords yet. They can make your asynchronous code looks like synchronous code that we've been making every day, which means it's way more easy to understand. How about making code like below. It's going to make the same result with the code you made.

bool result = await Check_Tenant_Expiry();
await Run_Daily_Report(result);