Assign Property with an ExpressionTree

Question

I\'m playing around with the idea of passing a property assignment to a method as an expression tree. The method would Invoke the expression so that the property gets assig

Answer 1

May be u mean Expression.Assign which was added in .net 4.0?

Answer 2

A general solution for the expression tree may not contain an assignment operator issue would be to create a local setter Action and call that one by the Expression:

// raises "An expression tree may not contain an assignment operator"
Expression<Action<string>> setterExpression1 = value => MyProperty = value;

// works
Action<string> setter = value => MyProperty = value;
Expression<Action<string>> setterExpression2 = value => setter(value);

This may not be suited to your exact problem, but I'm hoping this helps someone as this question is kind of the best match for googling that error message.

I'm unsure why exactly the compiler disallows this.

Answer 3

Here is a generaic solution that can give you an Action for assignment from an expression to specify the left hand side of assignment, and a value to assign.

public Expression<Action> Assignment<T>(Expression<Func<T>> lvalue, T rvalue)
{
    var body = lvalue.Body;
    var c = Expression.Constant(rvalue, typeof(T));
    var a = Expression.Assign(body, c);
    return Expression.Lambda<Action>(a);
}

with this, the code in the question is simply

ViewModelBase vm = new ViewModelBase();

vm.RunAndRaise(Assignment(() => vm.Value, 1));

If you change the definition like

public void RunAndRaise(Expression<Action> Exp) {
    Exp.Compile()();
    PropertyChanged(Exp.Member.Name);
}

We can also say

//Set the current thread name to "1234"
Assignment(() => Thread.CurrentThread.Name, "1234")).Compile()();

Simple enough, isn't it?

Answer 4

You can't do it this way. First, lambda expressions can be converted only to delegate types or Expression<T>.

If you change the signature of the method (for now ignoring its implementation) to public void RunAndRaise(Expression<Action> Exp), the compiler complains that “An expression tree may not contain an assignment operator”.

You could do it by specifying the property using lambda and the value you want to set it to in another parameter. Also, I didn't figure out a way to access the value of vm from the expression, so you have to put that in another parameter (you can't use this for that, because you need the proper inherited type in the expression):^{see edit}

public static void SetAndRaise<TViewModel, TProperty>(
    TViewModel vm, Expression<Func<TViewModel, TProperty>> exp, TProperty value)
    where TViewModel : ViewModelBase
{
    var propertyInfo = (PropertyInfo)((MemberExpression)exp.Body).Member;
    propertyInfo.SetValue(vm, value, null);
    vm.PropertyChanged(propertyInfo.Name);
}

Another possibility (and one I like more) is to raise the event from setter specifically using lambda like this:

private int m_value;
public int Value
{
    get { return m_value; }
    set
    {
        m_value = value;
        RaisePropertyChanged(this, vm => vm.Value);
    }
}

static void RaisePropertyChanged<TViewModel, TProperty>(
    TViewModel vm, Expression<Func<TViewModel, TProperty>> exp)
    where TViewModel : ViewModelBase
{
    var propertyInfo = (PropertyInfo)((MemberExpression)exp.Body).Member;
    vm.PropertyChanged(propertyInfo.Name);
}

This way, you can use the properties as usual, and you could also raise events for computed properties, if you had them.

EDIT: While reading through Matt Warren's series about implementing IQueryable<T>, I realized I can access the referenced value, which simplifies the usage of RaisePropertyChanged() (although it won't help much with your SetAndRaise()):

private int m_value;
public int Value
{
    get { return m_value; }
    set
    {
        m_value = value;
        RaisePropertyChanged(() => Value);
    }
}

static void RaisePropertyChanged<TProperty>(Expression<Func<TProperty>> exp)
{
    var body = (MemberExpression)exp.Body;
    var propertyInfo = (PropertyInfo)body.Member;
    var vm = (ViewModelBase)((ConstantExpression)body.Expression).Value;
    vm.PropertyChanged(vm, new PropertyChangedEventArgs(propertyInfo.Name));
}