async await return Task

Question

Can somebody explain what does this means into a synchronous method  If I try to change the method to async then VS complain about it   This works   public Task MethodName          return Task FromResult lt object gt  null       This doesn t work   public async Task MethodName          return Task FromResult lt object gt  null       So basically I would like to know what exactly this means  Task FromResult lt object gt  null

User · Answer

Adding the async keyword is just syntactic sugar to simplify the creation of a state machine. In essence, the compiler takes your code;

public async Task MethodName()
{
     return null;
}

And turns it into;

public Task MethodName()
{
     return Task.FromResult<object>(null);
}

If your code has any await keywords, the compiler must take your method and turn it into a class to represent the state machine required to execute it. At each await keyword, the state of variables and the stack will be preserved in the fields of the class, the class will add itself as a completion hook to the task you are waiting on, then return.

When that task completes, your task will be executed again. So some extra code is added to the top of the method to restore the state of variables and jump into the next slab of your code.

See What does async & await generate? for a gory example.

This process has a lot in common with the way the compiler handles iterator methods with yield statements.

User · Answer

You need to use the await keyword when use async and your function return type should be generic Here is an example with return value   public async Task lt object gt  MethodName         return await Task FromResult lt object gt  null       Here is an example with no return value   public async Task MethodName         await Task CompletedTask      Read these   TPL  http   msdn microsoft com en-us library dd460717 v vs 110  aspx and Tasks  http   msdn microsoft com en-us library system threading tasks v vs 110  aspx  Async  http   msdn microsoft com en-us library hh156513 aspx Await  http   msdn microsoft com en-us library hh156528 aspx

User · Answer

async methods are different than normal methods  Whatever you return from async methods are wrapped in a Task   If you return no value void  it will  be wrapped in Task  If you return int it will be wrapped in Task lt int gt  and so on   If your async method needs to return int you d mark the return type of the method as Task lt int gt  and you ll return plain int not the Task lt int gt   Compiler will convert the int to Task lt int gt  for you   private async Task lt int gt  MethodName         await SomethingAsync        return 42   Note we return int not Task lt int gt  and that compiles     Sameway  When you return Task lt object gt  your method s return type should be Task lt Task lt object gt  gt   public async Task lt Task lt object gt  gt  MethodName          return Task FromResult lt object gt  null    This will compile     Since your method is returning Task  it shouldn t return any value  Otherwise it won t compile   public async Task MethodName          return   This should work but return is redundant and also method is useless      Keep in mind that async method without an await statement is not async

User · Answer

This is a Task that is returning a Task of type String  C  anonymous function or in other word a delegation is used  Func        public static async Task lt string gt  MyTask                   C  anonymous AsyncTask         return await Task FromResult lt string gt    Func lt string gt        gt                           your code here             return   string result here

User · Answer

In order to get proper responses back from async methods  you need to put await while calling those task methods  That will wait for converting it back to the returned value type rather task type      E g var content   await StringAsyncTask             where public async Task lt String   StringAsyncTask

[c#] async await return Task

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