Dynamic Linq Expression with return value

c# expression-trees

Question

I need to create a dynamic linq expression an i started work with many examples. I tested some and some work and some not. In this case i want to create a method that looks like :

public bool Check(int intvar)
{
   if ( i > 2 )
     return true;
   else
     return false;
}

Now i have written the following :

LabelTarget returnTarget = Expression.Label("label");
ParameterExpression para = Expression.Parameter(typeof(int), "intvalue");
Expression test = Expression.GreaterThan(para, Expression.Constant(5));
Expression iftrue = Expression.Return(returnTarget, Expression.Constant(true));
Expression iffalse = Expression.Return(returnTarget,                   Expression.Constant(false));
Expression.IfThenElse(test, iftrue, iffalse);

this.TheExpression = Expression.IfThenElse(test, iftrue, iffalse);
Expression.Lambda<Action<int>>(
this.TheExpression,
new ParameterExpression[] { para }
).Compile()(5);

Now it throws InvalidOperationException:

Cannot jump to the label "label"`

What is wrong ? I only need a return true or false.

Accepted Answer

You need to change a few things:

  • Put the return label at the bottom of your function in a block expression, as René suggested. This is where your return statement will jump.

  • Declare the Lambda as type Func<int, bool>. Since you want a return value, this needs to be a function, not an action.

  • Declare the returnTarget label as type bool. Since the return value of a block expression is the value of its last statement, the label must be of the correct type.

  • Provide a default value for the final label (= the return value of your function if the label is reached by normal control flow instead of a return statement).

    LabelTarget returnTarget = Expression.Label(typeof(bool));
    ParameterExpression para = Expression.Parameter(typeof(int), "intvalue");
    Expression test = Expression.GreaterThan(para, Expression.Constant(5));
    Expression iftrue = Expression.Return(returnTarget, Expression.Constant(true));
    Expression iffalse = Expression.Return(returnTarget, Expression.Constant(false));
    
    var ex = Expression.Block(
        Expression.IfThenElse(test, iftrue, iffalse),
        Expression.Label(returnTarget, Expression.Constant(false)));
    
    var compiled = Expression.Lambda<Func<int, bool>>(
        ex,
        new ParameterExpression[] { para }
    ).Compile();
    
    Console.WriteLine(compiled(5));     // prints "False"
    Console.WriteLine(compiled(6));     // prints "True"
    

Popular Answer

returnTarget currently is only referenced by your if/then/else statement. The label is not placed in the statement anywhere. So it does not know where to jump to. The label is only defined and referenced, but not placed.

Try using Expression.Block to combine your lambda and your label.

Expression.Lambda<Action<int>>(
    Expression.Block(
        this.TheExpression,
        Expression.Label(returnTarget)
    ),
    new ParameterExpression[] { para }
    ).Compile()(5);

Haven't tested it, but this is the general direction in which you can find your answer.

-update- tested it, the lambda above compiles and runs just fine as it stands now.

-update2- apparantly, you want to return a value as well, let me have a look, at the least, it should be a Func rather than an Action.



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Licensed under: CC-BY-SA with attribution
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Is this KB legal? Yes, learn why