Linq expressions and extension methods to get property name

Question

I was looking at this post that describes a simple way to do databinding between POCO properties: Data Binding POCO Properties

One of the comments by Bevan included a simple Binder class that can be used to accomplish such data binding. It works great for what I need but I would like to implement some of the suggestions that Bevan made to improve the class, namely:

• Checking that source and target are assigned
• Checking that the properties identified by sourcePropertyName and targetPropertyName exist
• Checking for type compatibility between the two properties

Also, given that specifying properties by string is error prone, you could use Linq expressions and extension methods instead. Then instead of writing

Binder.Bind( source, "Name", target, "Name")

you could write

source.Bind( Name => target.Name);

I'm pretty sure I can handle the first three (though feel free to include those changes) but I have no clue how to use Linq expressions and extension methods to be able to write code without using property name strings.

Any tips?

Here is the original code as found in the link:

public static class Binder
{

    public static void Bind(
        INotifyPropertyChanged source,
        string sourcePropertyName,
        INotifyPropertyChanged target,
        string targetPropertyName)
    {
        var sourceProperty
            = source.GetType().GetProperty(sourcePropertyName);
        var targetProperty
            = target.GetType().GetProperty(targetPropertyName);

        source.PropertyChanged +=
            (s, a) =>
            {
                var sourceValue = sourceProperty.GetValue(source, null);
                var targetValue = targetProperty.GetValue(target, null);
                if (!Object.Equals(sourceValue, targetValue))
                {
                    targetProperty.SetValue(target, sourceValue, null);
                }
            };

        target.PropertyChanged +=
            (s, a) =>
            {
                var sourceValue = sourceProperty.GetValue(source, null);
                var targetValue = targetProperty.GetValue(target, null);
                if (!Object.Equals(sourceValue, targetValue))
                {
                    sourceProperty.SetValue(source, targetValue, null);
                }
            };
    }
}

Answer 1

The following will return a property name as a string from a lambda expression:

public string PropertyName<TProperty>(Expression<Func<TProperty>> property)
{
  var lambda = (LambdaExpression)property;

  MemberExpression memberExpression;
  if (lambda.Body is UnaryExpression)
  {
    var unaryExpression = (UnaryExpression)lambda.Body;
    memberExpression = (MemberExpression)unaryExpression.Operand;
  }
  else
  {
    memberExpression = (MemberExpression)lambda.Body;
  }

  return memberExpression.Member.Name;
}

Usage:

public class MyClass
{
  public int World { get; set; }
}

...
var c = new MyClass();
Console.WriteLine("Hello {0}", PropertyName(() => c.World));

UPDATE

public static class Extensions
{
    public static void Bind<TSourceProperty, TDestinationProperty>(this INotifyPropertyChanged source, Expression<Func<TSourceProperty, TDestinationProperty>> bindExpression)
    {
        var expressionDetails = GetExpressionDetails<TSourceProperty, TDestinationProperty>(bindExpression);
        var sourcePropertyName = expressionDetails.Item1;
        var destinationObject = expressionDetails.Item2;
        var destinationPropertyName = expressionDetails.Item3;

        // Do binding here
        Console.WriteLine("{0} {1}", sourcePropertyName, destinationPropertyName);
    }

    private static Tuple<string, INotifyPropertyChanged, string> GetExpressionDetails<TSourceProperty, TDestinationProperty>(Expression<Func<TSourceProperty, TDestinationProperty>> bindExpression)
    {
        var lambda = (LambdaExpression)bindExpression;

        ParameterExpression sourceExpression = lambda.Parameters.FirstOrDefault();
        MemberExpression destinationExpression = (MemberExpression)lambda.Body;

        var memberExpression = destinationExpression.Expression as MemberExpression;
        var constantExpression = memberExpression.Expression as ConstantExpression;
        var fieldInfo = memberExpression.Member as FieldInfo;
        var destinationObject = fieldInfo.GetValue(constantExpression.Value) as INotifyPropertyChanged;

        return new Tuple<string, INotifyPropertyChanged, string>(sourceExpression.Name, destinationObject, destinationExpression.Member.Name);
    }
}

Usage:

public class TestSource : INotifyPropertyChanged
{
    public event PropertyChangedEventHandler PropertyChanged;

    public string Name { get; set; }        
}

public class TestDestination : INotifyPropertyChanged
{
    public event PropertyChangedEventHandler PropertyChanged;

    public string Id { get; set; }    
}

class Program
{        
    static void Main(string[] args)
    {
        var x = new TestSource();
        var y = new TestDestination();

        x.Bind<string, string>(Name => y.Id);
    }    
}

Answer 2

This question is very similar to: Retrieving Property name from lambda expression

(Cross-posting answer from https://stackoverflow.com/a/17220748/1037948)

I don't know if you need to bind to "subproperties", but inspecting the lambda.Body for Member.Name will only return the "final" property, not a "fully-qualified" property.

ex) o => o.Thing1.Thing2 would result in Thing2, not Thing1.Thing2.

This is problematic when trying to use this method to simplify EntityFramework DbSet.Include(string) with expression overloads.

So you can "cheat" and parse the Expression.ToString instead. Performance seemed comparable in my tests, so please correct me if this is a bad idea.

The Extension Method

/// <summary>
/// Given an expression, extract the listed property name; similar to reflection but with familiar LINQ+lambdas.  Technique @via https://stackoverflow.com/a/16647343/1037948
/// </summary>
/// <remarks>Cheats and uses the tostring output -- Should consult performance differences</remarks>
/// <typeparam name="TModel">the model type to extract property names</typeparam>
/// <typeparam name="TValue">the value type of the expected property</typeparam>
/// <param name="propertySelector">expression that just selects a model property to be turned into a string</param>
/// <param name="delimiter">Expression toString delimiter to split from lambda param</param>
/// <param name="endTrim">Sometimes the Expression toString contains a method call, something like "Convert(x)", so we need to strip the closing part from the end</pa ram >
/// <returns>indicated property name</returns>
public static string GetPropertyName<TModel, TValue>(this Expression<Func<TModel, TValue>> propertySelector, char delimiter = '.', char endTrim = ')') {

    var asString = propertySelector.ToString(); // gives you: "o => o.Whatever"
    var firstDelim = asString.IndexOf(delimiter); // make sure there is a beginning property indicator; the "." in "o.Whatever" -- this may not be necessary?

    return firstDelim < 0
        ? asString
        : asString.Substring(firstDelim+1).TrimEnd(endTrim);
}//--   fn  GetPropertyNameExtended

(Checking for the delimiter might even be overkill)