Assign Property with an ExpressionTree

问题:

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 assigned properly, and then sniff out the property name that was just assigned so I can raise the PropertyChanged event. The idea is that I'd like to be able to use slim auto-properties in my WPF ViewModels and still have the PropertyChanged event fired off.

I'm an ignoramus with ExpressionTrees, so I'm hoping someone can point me in the right direction:

public class ViewModelBase {
    public event Action<string> PropertyChanged = delegate { };

    public int Value { get; set; }

    public void RunAndRaise(MemberAssignment Exp) {
        Expression.Invoke(Exp.Expression);
        PropertyChanged(Exp.Member.Name);
    }
}

The problem is I'm not sure how to call this. This naive attempt was rejected by the compiler for reasons that I'm sure will be obvious to anyone who can answer this:

        ViewModelBase vm = new ViewModelBase();

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

EDIT

Thank you @svick for the perfect answer. I moved one little thing around and made it into an extension method. Here's the complete code sample with unit test:

[TestClass]
public class UnitTest1 {
    [TestMethod]
    public void TestMethod1() {
        MyViewModel vm = new MyViewModel();
        bool ValuePropertyRaised = false;
        vm.PropertyChanged += (s, e) => ValuePropertyRaised = e.PropertyName == "Value";

        vm.SetValue(v => v.Value, 1);

        Assert.AreEqual(1, vm.Value);
        Assert.IsTrue(ValuePropertyRaised);
    }
}


public class ViewModelBase : INotifyPropertyChanged {
    public event PropertyChangedEventHandler PropertyChanged = delegate { };

    public void OnPropertyChanged(string propertyName) {
        PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
    }
}

public class MyViewModel : ViewModelBase {
    public int Value { get; set; }
}

public static class ViewModelBaseExtension {
    public static void SetValue<TViewModel, TProperty>(this 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.OnPropertyChanged(propertyInfo.Name);
    }
}

回答1:

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));
}

回答2:

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?

回答3:

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.

回答4:

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