How to get the value of a lambda expression

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Say we have the following :

C#

        private static string Query<T>(Expression<Func<T, bool>> pr)
        {
            return pr.ToString();
        }
...
int ii = 1000;
string str = Query((SalesInvoiceRow r) => r.Serial > ii && r.CustomerName.Contains( "23"));

When you run the above we get:

r => ((r.Serial > value(testing.program+<>c__DisplayClass0).ii) AndAlso r.CustomerName.Contains("23"))

How can we get the value of ii instead of the ugly statement like below?

r => ((r.Serial > 1000) AndAlso r.CustomerName.Contains("23"))

Posted 1-Feb-12 4:50am

Mehdi Gholam

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2 solutions

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Andreas Gieriet · Answer 1 · 2012-02-01T06:44:00

Hello Mehdi,

This is some experimental code to tweak with the ExpressionVisitor:

C#

static void Main(string[] args)
{
    int a = 5;
    int b = 100;
    Expression<Func<int, bool>> expr = v => v > (a + b);
    WalkVisitor w = new WalkVisitor();
    w.Visit(expr);
}
public class WalkVisitor : ExpressionVisitor
{
    protected override Expression VisitMember(MemberExpression node)
    {
        var e = base.VisitMember(node);
        var c = node.Expression as ConstantExpression;
        if (c != null)
        {
            Type t = c.Value.GetType();
            var x = t.InvokeMember(node.Member.Name, BindingFlags.GetField,
                                   null, c.Value, null);
            Console.WriteLine("{0} = {1}", node.ToString(), x);
        }
        return e;
    }
}

The output is:

value(ExprVisitor.Program+<>c__DisplayClass0).a = 5
value(ExprVisitor.Program+<>c__DisplayClass0).b = 100

You might need to implement a print visitor around this snippet...

Cheers

Andi

N_tro_P · Answer 2 · 2012-02-01T06:07:00

Solution 1

You should be able to compile the expression and then ToString it.

C#

pr.Compile().ToString();

[Edit]
I just realized you want ii. I am totally confused now. Aren't you setting ii to 1000? Why do you now want the value?
Maybe you meant you want the query resolved. Well you should do what I posted then, Compile :)

Posted 1-Feb-12 6:07am

N_tro_P

Updated 1-Feb-12 6:14am

v3

Comments

Pete O'Hanlon 1-Feb-12 12:14pm

This was the problem I had. I couldn't quite see what he was after. Mehdi's a smart guy so I reckon it's something really esoteric.

Mehdi Gholam 1-Feb-12 14:30pm

Sorry Compile().ToString() gives : System.Func`2[testing.program+SalesInvoiceRow,System.Boolean]

What I am after is : ((r.Serial > 1000) AndAlso r.CustomerName.Contains("23")) where the ii is replaced with it's value not the ref to the variable.

[no name] 1-Feb-12 14:49pm

OK I think I am starting to see what you are trying to do now. Never mind about the compile as it will execute it (I think we needed one more set of () though).
Yoru wording through me off. It sounded like you did not want the "Ugly statement bellow"... So I thought you were trying to execute the expression.

How to get the value of a lambda expression

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