Question

Update: This does work, I was being stupid 🙁

i have the following extension method

public static string ExtMethod(this object self, object myparameter);

at runtime this is called in any number of ways ways, i think these are all possibilities:

Expression<Func<T, string>> expr = x => x.property.ExtMethod(5);
Expression<Func<T, string>> expr = x => x.property.ExtMethod(new object());
Expression<Func<T, string>> expr = x => x.property.ExtMethod(someMethod());
Expression<Func<T, string>> expr = x => x.property.ExtMethod(x.someMethod());
Expression<Func<T, string>> expr = x => x.property.ExtMethod(x.OtherProperty);

what i need to do is evaluate the “myparameter“, given “expr” and a “T“

because of the two cases where x is used in myparameter, i thought i needed to create a delegate of the form:

Expression<Func<T, object>> expr = x => [myparameter expression here]

i thought this would work:

var extMethodExpr = expr.Body as MethodCallExpression;
var myparameterExpr = extMethodExpr.Arguments[1];

var myparam = Expression.Lambda(myparameterExpr, expr.Parameters).Compile().Invoke(someT)

but for the expressions that do not involve x, i get TargetParameterCountException 🙁

in these cases, if i do:

var myparam = Expression.Lambda(myparameterExpr).Compile().Invoke(someT)

it works fine.

How do I solve this?

thanks

Answer 1

OK; got to the bottom of it; in the line:

var myparam = Expression.Lambda(myparameterExpr).Compile().Invoke(someT);

If you weren’t trying to pass in a someT, this would work for those expressions that don’t involve x in the argument; for those that do, you need to tell the lambda to include the parameter (the same one from the original lambda) – simply by:

var myparam = Expression.Lambda(myparameterExpr,
             outerLambda.Parameters[0]).Compile().Invoke(someT);

Here’s some working code that evaluates the inner parameter (given an instance of the argument type); note that I use the parameter even if it doesn’t involve an x – otherwise, what would it do with the instance?

using System;
using System.Linq.Expressions;
using System.Reflection;
class Foo {
    public string Bar {get;set;}
    public int someMethod() { return 4; }
    public int OtherProperty { get { return 3; } }
}
static class Program
{
    static int someMethod() { return 3; }
    static void Main()
    {
        Foo foo = new Foo();
        Test<Foo>(x => x.Bar.ExtMethod(5), foo);
        Test<Foo>(x => x.Bar.ExtMethod(new object()), foo);
        Test<Foo>(x => x.Bar.ExtMethod(someMethod()), foo);
        Test<Foo>(x => x.Bar.ExtMethod(x.someMethod()), foo);
        Test<Foo>(x => x.Bar.ExtMethod(x.OtherProperty), foo);
    }
    static void Test<T>(Expression<Func<T, string>> expr, T instance)
    {
        if (expr.Body.NodeType != ExpressionType.Call)
        {
            throw new InvalidOperationException("Call expected");
        }
        var call = ((MethodCallExpression)expr.Body);
        if (call.Method != typeof(Program).GetMethod(
            "ExtMethod", BindingFlags.Static | BindingFlags.NonPublic))
        {
            throw new InvalidOperationException("ExtMethod expected");
        }
        // we know that ExtMethod has 2 args; pick myParameter (the 2nd);
        // then build an expression over arg, re-using our outer param
        var newLambda = Expression.Lambda<Func<T, object>>(
            call.Arguments[1], expr.Parameters[0]);

        // evaluate it and show the argument value
        object value = newLambda.Compile()(instance);
        Console.WriteLine(value);
    }
    static string ExtMethod(this object self, object myParameter) {
        return self.ToString();
    }
}