private override

11Dec/089

static reflection – Method Guards

Another bit of magic with static reflection. Refer to my last post for a little more explanation of static reflection, and a sweet implementation (IMO) of INotifyPropertyChanged using static reflection.

In the past several years, on several projects, I've seen many lines of code at the beginning of public interface methods that look like:

    public class WebServer
    {
        public void BootstrapServer(int port, string rootDirectory, string serverName)
        {
            if( rootDirectory == null )
            {
                throw new ArgumentNullException( "rootDirectory" );
            }
            if( serverName == null )
            {
                throw new ArgumentNullException( "serverName" );
            }

            // Bootstrap the server
        }
    }

I realize the need, since we don't have real DbC in .NET (yet), but I REALLY dislike this and feel it to be a pretty bad smell.  It feels unDRY, obscures essence, and I just plain don't like it. ;)

So, here is an implementation that makes it tolerable, if not enjoyable (because of the coolness of using static reflection), because I think it actually adds to the essence and overall readability of a given method.

This is what the above code can now look like:

    public class WebServer
    {
        public void BootstrapServer( int port, string rootDirectory, string serverName )
        {
            Guard.IsNotNull( () => rootDirectory );
            Guard.IsNotNull( () => serverName );

            // Bootstrap the server
        }
    }

And here is the implementation:

    public static class Guard
    {
        public static void IsNotNull<T>(Expression<Func<T>> expr)
        {
            // expression value != default of T
            if (!expr.Compile()().Equals(default(T)))
                return;

            var param = (MemberExpression) expr.Body;
            throw new ArgumentNullException(param.Member.Name);
        }
    }

Lovin it!  Hope you do too.

8Dec/087

static reflection – INotifyPropertyChanged

I'm not going to delve into the definition of static reflection here, but you'll see how it's implemented, and how I found it can help me with implementing INotifyPropertyChanged.

Now, I've never been a fan of INotifyPropertyChanged myself, mostly because it feels wet (read: not DRY).  I'm working on a WPF app at the moment, and have come to realize, if I embrace INotifyPropertyChanged, I get to take full advantage of the excellent WPF databinding story.

Here is what a normal use of INotifyPropertyChanged looks like:

    public class Person : INotifyPropertyChanged
    {
        private string _firstName;
        private string _lastName;

        public event PropertyChangedEventHandler PropertyChanged;

        public string FirstName
        {
            get { return _firstName; }
            set
            {
                if (_firstName != value)
                {
                    _firstName = value;
                    FirePropertyChanged("FirstName");
                }
            }
        }

        public string LastName
        {
            get { return _lastName; }
            set
            {
                if (_lastName != value)
                {
                    _lastName = value;
                    FirePropertyChanged("LastName");
                }
            }
        }

        private void FirePropertyChanged(string propertyName)
        {
            if (PropertyChanged != null)
            {
                PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
            }
        }
    }

Here is what the static reflection use of INotifyPropertyChanged looks like:

    public class PersonStaticReflection : NotifyPropertyChanged
    {
        private string _firstName;
        private string _lastName;

        public string FirstName
        {
            get { return _firstName; }
            set { SetProperty(()=> FirstName, ()=> _firstName, value); }
        }

        public string LastName
        {
            get { return _lastName; }
            set
            {
                SetProperty( () => LastName, () => _lastName, value, () =>
                   {
                       // do something useful here
                   });
            }
        }
    }

Doesn't that just feel DRYer?  It does to me!

There are several important pieces to note:

  • Inheriting from NotifyPropertyChanged, this is the class that holds the SetProperty method, and hides the static reflection magic.
  • We're calling SetProperty with three arguments in the FirstName property
    • first: a LINQ Expression pointing to the property we're changing
    • second: a LINQ Expression pointing to the field to backing the property
    • third: the new, incoming value
  • We're calling SetProperty with a fourth argument in the LastName property
    • fourth: an action that will get executed only if the values were different
  • The SetProperty method, is going to look at the current value of the field, and the incoming value.  If the two are different, it will assign the new value to the field, and then fire the NotifyPropertyChanged event, with the name of the property given via the first argument.  I ended up pulling this into its own class so I could use it as the layer supertype in my View Model layer. Here is the implementation:

    public class NotifyPropertyChanged : INotifyPropertyChanged
    {
        public event PropertyChangedEventHandler PropertyChanged;

        protected void SetProperty < T>( Expression < Func> propExpr, Expression> fieldExpr, T value )
        {
            SetProperty(propExpr, fieldExpr, value, ()=> { });
        }

        protected void SetProperty < T>( Expression < Func> propExpr, Expression < Func < T>> fieldExpr, T value, Action doIfChanged )
        {
            var prop = (PropertyInfo)((MemberExpression)propExpr.Body).Member;
            var field = (FieldInfo)((MemberExpression)fieldExpr.Body).Member;

            var currVal = prop.GetValue( this, null );

            if( currVal == null && value == null )
                return;
            if( currVal == null || !currVal.Equals( value ) )
            {
                field.SetValue( this, value );
                doIfChanged();

                if( PropertyChanged != null )
                    PropertyChanged( this, new PropertyChangedEventArgs( prop.Name ) );
            }
        }
    }

Questions/comments/likes/dislikes? Let me know.