How to parse a simple captured closure expression and get the target object and selector

Question

If I have simple class

class Foo {
    int Value { get; set; } = 20
}

class Bar {
    Foo foo { get; set;} = new Foo();
}

and then I write a selector expression

var bar = new Bar();
Expression<Func<int>> fn = () => bar.foo.Value;

I am aware that if I type

var v = fn.Compile()()

v will equal 20 demonstrating that the closure is indeed captured. I infer that inside the expression a reference to the captured target of the expression is stored. I would like to get this back out of the expression.

For example

Bar capturedBar = GetTarget(fn);
Assert(capturedBar == bar);

and another function

string selector = GetSelector(fn)

which should return

"bar.foo.A"

Answer 1

You can accomplish this tasks with using the Expression Visitor technique.

Here is the simple and dirty demonstration of the main idea:

var bar = new Bar();
Expression<Func<int>> fn = () => bar.foo.Value;

BarExtractor be = new BarExtractor();
be.Visit(fn);
Bar capturedBar = be.bar;

The BarExtractor class is declared as follows:

class BarExtractor : ExpressionVisitor {
    public Bar bar;
    protected override Expression VisitConstant(ConstantExpression node) {
        object closureObj = node.Value;
        if(closureObj != null && closureObj.GetType().Name.StartsWith("<>c__DisplayClass")) {
            var barField = closureObj.GetType().GetField("bar");
            if(barField != null && barField.FieldType == typeof(Bar))
                bar = barField.GetValue(closureObj) as Bar;
        }
        return base.VisitConstant(node);
    }
}