CODEWITHSUNDEEP
c#.netexpression-trees
RM.
RM.

Reputation: 2043

How to set a property value from an expression tree?

I would like to set the property value referenced in an expression tree.

using System;
using System.Diagnostics;
using System.Linq.Expressions;
using System.Reflection;

namespace ConsoleApp8
{
    class TestObject
    {
        public double X { get; set; }
    }

    class Program
    {
        static Action<double> GetSetterForX(Expression<Func<double>> expression)
        {
            var body = expression.Body;
            var operand = body as MemberExpression;
            var propertyInfo = (PropertyInfo) (operand.Member);
            var setter = propertyInfo.GetSetMethod(true);

            // At this point I have the setter. But how do I get access to the testObject?

            return null;
        }

        static void Main(string[] args)
        {
            var testObject = new TestObject();
            var setter = GetSetterForX(() => testObject.X);
            setter.Invoke(5);
            Debug.Assert(testObject.X == 5);
        }
    }
}

I can get the setter but can not find a way to get access to the instance (testObject). Is there a way?

Note that this is a simplified example. I will use much more complex expressions with many property references inside and I would like to be able to set all of them (individually).

UPDATE

To clarify. I would like to have a setter returned which only takes a double and it assigns testObject's X property. This should be possible without the need to explicitly pass in the reference to testObject in the setter. I can do the same to get a getter but not for setter. Here is the getter code:

static Func<double> GetGetterForX(Expression<Func<double>> expression)
{
    var body = expression.Body;
    var operand = body as MemberExpression;
    var result = new Func<double>(() => (double) GetValue(operand));

    return result;
}

private static object GetValue(MemberExpression member)
{
    var objectMember = Expression.Convert(member, typeof(object));
    var getterLambda = Expression.Lambda<Func<object>>(objectMember);
    var getter = getterLambda.Compile();
    return getter();
}

The getter returned is always working on the testObject instance. No need to pass in the testObject again.

Upvotes: 6

Views: 4695

Answers (2)

Ivan Stoev
Ivan Stoev

Reputation: 205619

As soon as the input lambda expression represents member accessor, you can use Expression.Assign passing the input lambda expression body and parameter representing the value, e.g.

static Action<double> GetSetterForX(Expression<Func<double>> expression)
{
    var parameter = Expression.Parameter(typeof(double), "value");
    var body = Expression.Assign(expression.Body, parameter);
    var lambda = Expression.Lambda<Action<double>>(body, parameter);
    return lambda.Compile();
}

Upvotes: 5

Fourat
Fourat

Reputation: 2447

You need to return MethodInfo instead of Action<double>, also the Invoke(object obj, object[] params) takes the original object and the parameters :

static MethodInfo GetSetterForX(Expression<Func<double>> expression)
{
    var body = expression.Body;
    var operand = body as MemberExpression;
    var propertyInfo = (PropertyInfo)(operand.Member);
    var setter = propertyInfo.GetSetMethod(true);
    return setter;
}

public static void Main()
{
    var testObject = new TestObject();
    var setter = GetSetterForX(() => testObject.X);
    setter.Invoke(testObject, new object[]{5});
    Debug.Assert(testObject.X == 5);
}

Fiddle : https://dotnetfiddle.net/CHJGbk

Upvotes: 2

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