Expression to Set Properties on Existing Object Instance

Question

How can I use Expression to set properties on an instance of an object I already have? I don't want to create a new object, so Expression.New where you can pass in member initializers won't work.

This is not a duplicate of How to set property value using Expressions? because that takes an Expression and sets properties via Reflection.

I have an existing object and want to create an Expression where I can set properties. That expression will then get compiled into a delegate to execute.

Possible duplicate of How to set property value using Expressions? — George Alexandria
– George Alexandria, Commented Sep 22, 2017 at 22:43
Expression.Assign into Expression.Property of your object (either as a ConstantExpression or ParameterExpression depending on how you're using it) passing the PropertyInfo of the property you want to assign — pinkfloydx33
– pinkfloydx33, Commented Sep 22, 2017 at 23:04

MBoros · Accepted Answer · 2017-09-23 16:14:19Z

3

what about:

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

public static void Main()
{
    var user = new User();
    var assigner = GetAssigner<User, string>(u => u.Name);
    assigner.Compile()(user, "Joe");
    Console.WriteLine(user.Name);
}

public static Expression<Action<TClass, TValue>> GetAssigner<TClass, TValue>(Expression<Func<TClass, TValue>> propertyAccessor){
    var prop = ((MemberExpression)propertyAccessor.Body).Member;
    var typeParam = Expression.Parameter(typeof(TClass));
    var valueParam = Expression.Parameter(typeof(TValue));
    return Expression.Lambda<Action<TClass, TValue>>(
        Expression.Assign(
            Expression.MakeMemberAccess(typeParam, prop),
            valueParam), typeParam, valueParam);

}

Remember that the expression is just a definition of what should be done, you have to compile it to a delegate to be able to invoke it.

answered Sep 23, 2017 at 16:14

MBoros

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1 Comment

Josh Close Over a year ago

The combination of Expression.Assign with Expression.MakeMemberAccess is exactly what I was looking for.

Josh E · Accepted Answer · 2017-09-22 22:40:24Z

1

Have you looked at using MemberAssignment to set the value of the member(s)?

Represents assignment operation for a field or property of an object.

Important note in the remarks, however: you can't create these directly. Instead,

Use the Bind factory methods to create a MemberAssignment. A MemberAssignment has the BindingType property equal to Assignment.

answered Sep 22, 2017 at 22:40

Josh E

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4 Comments

Josh Close Over a year ago

I have a list of MemberAssignments. How do I create an Expression to assign those to an existing object? I can do it when creating a new object via Expression.New, but I don't know how to do it with an existing object.

Mike Strobel Over a year ago

It’s been a while, but I seem to recall MemberAssignment being specifically for object initializer expressions, like new X { Member = y }. A plain old Assign(Property(x, "Member"), y), should be fine.

Josh E Over a year ago

The docs don't mention object initializer expressions, perhaps you're thinking of MemberInitExpression?

Josh E Over a year ago

@JoshClose pass the property to get/set as a ParameterExpression after all, property setter are really just method calls accepting a parameter corresponding to the value to update

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