Getting the values of method parameters inside expression trees

Question

I'm messing around with expression trees, but I'm little stuck.

I have this expression:

Expression<Func<IX, int>> expr = i => i.GetAll(1, b, method());

Where :

int b = 2;

public static int method()
{
    return 3;
}

public interface IX
{
    int GetAll(int a, int b, int c);
}

Now I want to get name of the method and values of parameters for this method. Name of the method is easy, but parameter values are harder part. I know I can parse them myself, but I would need to handle all cases (ConstantExpression, MemberExpression, MethodCallExpression and maybe more I'm not aware of). So I was thinking if there was "general" way to get their values. eg 1, 2, 3.

Answer 1

You can get the arguments of the MethodCallExpression in question and create compiled Func<object>s from them (boxing value-types if necessary), which can then be evaluated.

E.g.:

var args = from arg in ((MethodCallExpression)expr.Body).Arguments
           let argAsObj = Expression.Convert(arg, typeof(object))
           select Expression.Lambda<Func<object>>(argAsObj, null)
                            .Compile()();

This will obviously blow up if the expression's body is not a method-call expression or if any of the arguments to the method cannot be evaluated as is (e.g. if they depend on the argument to the expression).

Obviously, you can do a better job if you know the types of the arguments to the method beforehand. For your specific example, this should work:

var args = from arg in ((MethodCallExpression)expr.Body).Arguments               
           select Expression.Lambda<Func<int>>(arg, null)
                            .Compile()();