Chapter 73: C# 5.0 Features

Section 73.1: Async & Await

async and await are two operators that are intended to improve performance by freeing up Threads and waiting for operations to complete before moving forward.

Here's an example of getting a string before returning it's length:

//This method is async because:

//1. It has async and Task or Task<T> as modifiers //2. It ends in "Async"

async Task<int> GetStringLengthAsync(string URL){ HttpClient client = new HttpClient();

//Sends a GET request and returns the response body as a string

Task<string> getString = client.GetStringAsync(URL);

//Waits for getString to complete before returning its length string contents = await getString;

return contents.Length;

}

private async void doProcess(){

int length = await GetStringLengthAsync("http://example.com/");

//Waits for all the above to finish before printing the number

Console.WriteLine(length);

}

Here's another example of downloading a file and handling what happens when it's progress has changed and when the download completes (there are two ways to do this):

Method 1:

//This one using async event handlers, but not async coupled with await private void DownloadAndUpdateAsync(string uri, string DownloadLocation){

WebClient web = new WebClient();

//Assign the event handler

web.DownloadProgressChanged += new DownloadProgressChangedEventHandler(ProgressChanged); web.DownloadFileCompleted += new AsyncCompletedEventHandler(FileCompleted);

//Download the file asynchronously web.DownloadFileAsync(new Uri(uri), DownloadLocation);

}

//event called for when download progress has changed

private void ProgressChanged(object sender, DownloadProgressChangedEventArgs e){

//example code int i = 0; i++; doSomething();

}

//event called for when download has finished

private void FileCompleted(object sender, AsyncCompletedEventArgs e){ Console.WriteLine("Completed!")

}

Method 2:

//however, this one does

//Refer to first example on why this method is async

private void DownloadAndUpdateAsync(string uri, string DownloadLocation){ WebClient web = new WebClient();

//Assign the event handler

web.DownloadProgressChanged += new DownloadProgressChangedEventHandler(ProgressChanged);

//Download the file async

web.DownloadFileAsync(new Uri(uri), DownloadLocation);

//Notice how there is no complete event, instead we're using techniques from the first example

}

private void ProgressChanged(object sender, DownloadProgressChangedEventArgs e){ int i = 0;

i++; doSomething();

}

private void doProcess(){

//Wait for the download to finish

await DownloadAndUpdateAsync(new Uri("http://example.com/file")) doSomething();

}

Section 73.2: Caller Information Attributes

C.I.A.s are intended as a simple way of getting attributes from whatever is calling the targeted method. There is really only 1 way to use them and there are only 3 attributes.

Example:

//This is the "calling method": the method that is calling the target method public void doProcess()

{

GetMessageCallerAttributes("Show my attributes.");

}

//This is the target method

//There are only 3 caller attributes

public void GetMessageCallerAttributes(string message,

//gets the name of what is calling this method

[System.Runtime.CompilerServices.CallerMemberName] string memberName = "",

//gets the path of the file in which the "calling method" is in

[System.Runtime.CompilerServices.CallerFilePath] string sourceFilePath = "",

//gets the line number of the "calling method"

[System.Runtime.CompilerServices.CallerLineNumber] int sourceLineNumber = 0)

{

//Writes lines of all the attributes

System.Diagnostics.Trace.WriteLine("Message: " + message); System.Diagnostics.Trace.WriteLine("Member: " + memberName); System.Diagnostics.Trace.WriteLine("Source File Path: " + sourceFilePath); System.Diagnostics.Trace.WriteLine("Line Number: " + sourceLineNumber);

}

Example Output:

//Message: Show my attributes. //Member: doProcess

//Source File Path: c:\Path\To\The\File //Line Number: 13