Expressions... What am I doing wrong?

Question

I've decided to take some golden time to learn a bit more about Expressions. I'm trying a very simple exercise, namely adding two numbers. I've hit an exception that's proving to be tricky to search for.

Here's my code

Expression<Func<int,int,int>> addExpr = (x, y) => x + y;
var p1 = Expression.Parameter(typeof(int), "p1");
var p2 = Expression.Parameter(typeof(int), "p2");
var lambda = Expression.Lambda<Func<int,int,int>>(addExpr, p1, p2); //<-here
var del = lambda.Compile();
var result = del(2,3); //expect 5

but this is throwing an ArgumentException : Expression of type 'System.Func`3[System.Int32,System.Int32,System.Int32]' cannot be used for return type 'System.Int32'

at the line indicated above. What have I done wrong?

What are you trying to do in this code? Are you trying to build (x,y) => x + y lambda using expressions only? — petro.sidlovskyy, Sep 05 '12 at 12:35
Just poking around. Ultimate goal is an exercise to write a Countdown numbers game solver. http://en.wikipedia.org/wiki/Countdown_%28game_show%29#Numbers_round Seems to me that expression trees would be a good means of tackling this, but it's all very new and shiny to me ATM. — spender, Sep 05 '12 at 12:46

Bob Vale · Accepted Answer · 2012-09-05T12:43:00.220

4

You need to wrap the addExpr in an invoke using the expression parameters

Expression<Func<int,int,int>> addExpr = (x, y) => x + y;
var p1 = Expression.Parameter(typeof(int), "p1");
var p2 = Expression.Parameter(typeof(int), "p2");
var invokeExpression=Expression.Invoke(addExpr,p1,p2);
var lambda = Expression.Lambda<Func<int,int,int>>(invokeExpression,p1,p2);
var del = lambda.Compile();
var result=del(2,3);

The invoke is how you type p1 to x and p2 to y, alternatively you could just write the above as

var p1 = Expression.Parameter(typeof(int), "p1");
var p2 = Expression.Parameter(typeof(int), "p2");
var lambda=Expresion.Lambda<Func<int,int,int>>(Expression.Add(p1,p2),p1,p2);
var del = lambda.Compile();
var result=del(2,3);

Otherwise you need to grab the expression body into the lambda and pass the expression parameters.

var lambda=Expresion.Lambda<Func<int,int,int>>(addExpr.Body,addExpr.Parameters);

edited Sep 05 '12 at 12:43

answered Sep 05 '12 at 12:36

Bob Vale

18,094
1
42
49

Gotcha. All falling into place now. – spender Sep 05 '12 at 12:42
@Spender and after all that I've added a final update as the third option. This option will work if you do not need to use the expression parameters from elsewhere. – Bob Vale Sep 05 '12 at 12:46
Thanks for taking time to present some alternatives. – spender Sep 05 '12 at 13:02

James · Answer 2 · 2012-09-05T12:37:16.710

2

Your code should be:

var lambda = Expression.Lambda<Func<Expression<Func<int, int, int>>, int, int>(addExpr, p1, p2);

Your current code expects an int and your passing in Expression<Func<int, int, int>>.

Update

Actually the above won't compile, you would need to do:

var lambda = Expression.Lambda<Func<int, int, int>>(Expression.Add(p1, p2), p1, p2);

edited Sep 05 '12 at 12:37

answered Sep 05 '12 at 12:29

James

80,725
18
167
237

Yeah, that is not going to work, unless you compile and call the passed expression. – leppie Sep 05 '12 at 12:33
@leppie yeah I just tested the code there, I have updated my answer to use `Expression.Add` instead. – James Sep 05 '12 at 12:37

score 1 · Answer 3 · answered Sep 05 '12 at 12:32

1

You need to decompose addExpr's body or preferably just write it from scratch ie Expression.Add(p1,p2) instead of addExpr.

answered Sep 05 '12 at 12:32

leppie

115,091
17
196
297

What does "decompose" even mean? Pretend I have rep 0! – spender Sep 05 '12 at 12:35
@spender: decompose == take apart ie `addExpr.Body` – leppie Sep 05 '12 at 13:09
Thanks. I'm getting there now. – spender Sep 05 '12 at 13:10

Expressions... What am I doing wrong?

3 Answers3