get and set object property using Expression trees instead of Reflection

c# dynamic-method expression-trees reflection

Question

I would like to dynamically get and set an objects properties as follows:

public class Person
{
    public string Name {get; set; }
}

public class Testing
{
    public void Run()
    {
        var p = new Person();

        SetValue(p, "Name", "Henry");

        var name = GetValue(p, "Name");
    }
}

Please could I get help creating the GetValue and SetValue methods using dynamic method (or expression trees)?

I am intending to save compiled expressions in a dictionary, to speed up future get/set calls.

1
1
2/25/2016 8:03:03 PM

Accepted Answer

Do you really want to use expression trees? for this simple scenario I would try to compile directly into a DynamicMethod using Reflection.Emit API's by getting an IL Generator. But .. for expression trees, i wrote a helper for you:

 public class PropertyManager : DynamicObject
    {
        private static Dictionary<Type, Dictionary<string, GetterAndSetter>> _compiledProperties = new Dictionary<Type, Dictionary<string, GetterAndSetter>>();

        private static Object _compiledPropertiesLockObject = new object();

        private class GetterAndSetter
        {
            public Action<object, Object> Setter { get; set; }

            public Func<Object, Object> Getter { get; set; }
        }

        private Object _object;

        private Type _objectType;

        private PropertyManager(Object o)
        {
            _object = o;
            _objectType = o.GetType();
        }

        public static dynamic Wrap(Object o)
        {
            if (o == null)
                return null; // null reference will be thrown

            var type = o.GetType();

            EnsurePropertySettersAndGettersForType(type);

            return new PropertyManager(o) as dynamic;
        }

        private static void EnsurePropertySettersAndGettersForType(Type type)
        {
            if (false == _compiledProperties.ContainsKey(type))
                lock (_compiledPropertiesLockObject)
                    if (false == _compiledProperties.ContainsKey(type))
                    {
                        Dictionary<string, GetterAndSetter> _getterAndSetters;
                        _compiledProperties[type] = _getterAndSetters = new Dictionary<string, GetterAndSetter>();

                        var properties = type.GetProperties(System.Reflection.BindingFlags.Instance | System.Reflection.BindingFlags.Public | System.Reflection.BindingFlags.NonPublic);

                        foreach (var property in properties)
                        {
                            var getterAndSetter = new GetterAndSetter();
                            _getterAndSetters[property.Name] = getterAndSetter;

                            // burn getter and setter

                            if (property.CanRead)
                            {
                                // burn getter

                                var param = Expression.Parameter(typeof(object), "param");

                                Expression propExpression = Expression.Convert(Expression.Property(Expression.Convert(param, type), property), typeof(object));

                                var lambda = Expression.Lambda(propExpression, new[] { param });

                                var compiled = lambda.Compile() as Func<object, object>;
                                getterAndSetter.Getter = compiled;
                            }

                            if (property.CanWrite)
                            {
                                var thisParam = Expression.Parameter(typeof(Object), "this");
                                var theValue = Expression.Parameter(typeof(Object), "value");

                                var isValueType = property.PropertyType.IsClass == false && property.PropertyType.IsInterface == false;

                                Expression valueExpression;
                                if (isValueType)
                                    valueExpression = Expression.Unbox(theValue, property.PropertyType);
                                else
                                    valueExpression = Expression.Convert(theValue, property.PropertyType);

                                var thisExpression = Expression.Property (Expression.Convert(thisParam, type), property);


                                Expression body = Expression.Assign(thisExpression, valueExpression);

                                var block = Expression.Block(new[]
                                {
                                    body,
                                    Expression.Empty ()
                                });

                                var lambda = Expression.Lambda(block, thisParam, theValue);

                                getterAndSetter.Setter = lambda.Compile() as Action<Object, Object>;
                            }
                        }
                    }
        }

        public override bool TryGetMember(GetMemberBinder binder, out object result)
        {
            var memberName = binder.Name;
            result = null;
            Dictionary<string, GetterAndSetter> dict;
            GetterAndSetter getterAndSetter;
            if (false == _compiledProperties.TryGetValue(_objectType, out dict)
                || false == dict.TryGetValue(memberName, out getterAndSetter))
            {
                return false;
            }

            if (getterAndSetter.Getter == null)
                throw new NotSupportedException("The property has no getter!");

            result = getterAndSetter.Getter(_object);
            return true;
        }

        public override bool TrySetMember(SetMemberBinder binder, object value)
        {
            var memberName = binder.Name;

            Dictionary<string, GetterAndSetter> dict;
            GetterAndSetter getterAndSetter;
            if (false == _compiledProperties.TryGetValue(_objectType, out dict)
                || false == dict.TryGetValue(memberName, out getterAndSetter))
            {
                return false;
            }

            if (getterAndSetter.Setter == null)
                throw new NotSupportedException("The property has no getter!");

            getterAndSetter.Setter(_object, value);

            return true;
        }
    }

And this is how you can use it:

Person p = new Person();
p.Name = "mama";
var wrapped = PropertyManager.Wrap(p);

var val = wrapped.Name; // here we are using our compiled method ...

It is very obvious that you can extract my compilation logic to use strings instead of letting DLR giving the property name for you :)

3
3/2/2016 10:01:07 PM


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