How to return value in ExpressionTree

Question

i'm trying to do for an expression tree and try to let it return a simple int value. but it not working anymore

        var method = typeof(Console).GetMethod("WriteLine", new Type[] {typeof(string)});

        var result = Expression.Variable(typeof(int));



        var block = Expression.Block(
            result,
          Expression.Assign(result,Expression.Constant(0)),
            Expression.IfThenElse(Expression.Constant(true),
                Expression.Block(Expression.Call(null, method, Expression.Constant("True")),
                    Expression.Assign(result, Expression.Constant(1))),
                Expression.Block(Expression.Call(null, method, Expression.Constant("False")), Expression.Assign(
                    result, Expression.Constant(0)
                ))),
            result
        );


        Expression.Lambda<Func<int>>(block).Compile()();

DavidG · Accepted Answer · 2019-01-08 10:35:24Z

7

The problem is not with returning a vaue from the block (you are doing it correctly), but the out of scope variable due to the used wrong Expression.Block method overload.

Variable expressions like your result must be passed to the block expression using some of the overloads with IEnumerable<ParameterExpression> variables argument, for instance

    var block = Expression.Block(
        new ParameterExpression[] { result },
        //... (the rest of the sample code unchanged)
    );

edited Jan 8, 2019 at 10:35

DavidG

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answered Jan 8, 2019 at 10:30

Ivan Stoev

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5 Comments

DavidG Over a year ago

Fixed a typo. Also, congrats on 100k, enjoy your swag when it arrives in 6-8 weeks!

Ivan Stoev Over a year ago

@DavidG Thank you, HNY and coding!

Mrinal Kamboj Over a year ago

Congrats Ivan, indeed an inspiration, keep it up

Ivan Stoev Over a year ago

Thanks @MrinalKamboj, wish you the same.

Linkanyway Over a year ago

thanks @Ivan Stoev. its working now! i forget setup the return parameter at begin of block definition : )

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