Friday, February 17, 2012

C# 5: Caller Info Attributes

One of the new features that will be available in C# 5 are the Caller Info Attributes.
So what is this animal? If you are/were a C++ developer you probably know the __FILE__ and __LINE__ macros, which expand when compiled to produce a string containing the name of the current source file, and the current line, respectively. Of course C# 5.0 doesn’t support macro’s but the C# team used a different trick to introduce the CallerFilePath, CallerLineNumber and CallerMemberName attributes.
A simple sample:
public static class Trace
{
 public static void WriteLine(string message,
  [CallerFilePath] string file = "",
  [CallerLineNumber] in line = 0,
  [CallerMemberName] string member = "")
 {
  var s = string.Format("{0}:{1} - {2}: {3}", file, line, member, message);

  Console.WriteLine(s);
 }
}
 

If you call the WriteLine method only specifying the message property the compiler would automatically fill out the optional parameters in the WriteLine method with details of where you made the call from.

How can this be useful?

Think about the INotifyPropertyChanged which typically requires the use of magic strings.  Instead of relying on expression trees to get some type safety, we’ll be able to do the following:

protected void RaisePropertyChanged([CallerMemberName] string member = "")
{
    var propertyChanged= PropertyChanged;
    if(propertyChanged != null)
    {
        propertyChanged(new PropertyChangedEventArgs(this, member));
    }
}

and now when you call this method from a property setter, leaving out the optional parameter:

public string ProductName
{
    get
    {
        return _productName;
    }
    set
    {
        _productName=value;
        RaisePropertyChanged();
    }
}

the compiler will fill out the parameter for you, passing “ProductName” to the RaisePropertyChanged method(at least that’s what I hope that it will do, as the feature is not available yet in the current Developer Preview).

Goodbye magic strings!

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