How do I get a value of a reference type in an Expression?

Question

I have this method:

public void DoSomething<T>(Expression<Func<T, object>> method)
{
}

If this method is called like this:

DoSomething(c => c.SomeMethod(new TestObject()));

... how do I get the value of the parameter that was passed into SomeMethod()?

If the parameter is a value type, this works:

var methodCall = (MethodCallExpression)method.Body;
var parameterValue = ((ConstantExpression)methodCall.Arguments[0]).Value;

However, when I pass in a reference type, methodCall.Arguments[0] is a MemberExpression, and I can't seem to figure out how to write code to get the value out of it.

Answer 1

This isn't really a matter of value type or reference type - it's a matter of a constant expression or not-constant expression. I'm sure your existing code would fail if you called

DoSomething(c => c.SomeMethod(DateTime.Now));

as well.

Basically, the argument to the method is just an expression. It's not a value. You could potentially compile that expression and then execute it to get the value at that point in time, but it's important to understand that an expression isn't a value in itself. In the case of a constant expression it's easy, but taking the DateTime.Now example, by definition the evaluated value of the expression changes over time :)

What are you trying to do with the argument? What's the bigger picture here?

Answer 2

Firstly, what Jon said.

There are no values to get ahold of, it's all just expressions. What might be of interest is the NewExpression, which has a Constructor property; this property contains the reflected constructor that would be called if you compiled expression and ran the resulting delegate. You could manually invoke that constructor and get an instance of what the user was intending on instantiating.

Answer 3

You will have to evaluate member expression manually, MemberExpression contains "Expression" which is the container object of Member specified, to do this you can crete Lamda of your arguement and compile and execute it.

LamdaExpression l = Expression.Lamda(methodCall.Arguments[0]);
var c = l.Compile();
var v = l.Invoke();

So whatever you pass, you will get it in "v" variable.

Answer 4

Here is the answer (inspired by Akash's answer):

LambdaExpression lambda = Expression.Lambda(methodCall.Arguments[0]);
var compiledExpression = lambda.Compile();
return compiledExpression.DynamicInvoke();