Question

I’m playing around with Expression trees in C# and would like to modify an expression.

I used this example however in the example they reuse the left and right node. How would I go about modifying either of the nodes?

I got the following code:

    [TestMethod]
    public void ExpressionDemo_ModifiesExpression()
    {
        var demo = new ExpressionDemo();

        var expression = demo.ModifyAddition((Expression<Func<int, int>>)(x => x + 1));
    }

I call ModifyAddition using a simple + 1 addition Func. I would like to modify the right operand (1) to a different integer.

public class ExpressionDemo : ExpressionVisitor
{
    public Expression<Func<int, int>> ModifyAddition(Expression func)
    {
        return (Expression<Func<int, int>>) Visit(func);
    }

    protected override Expression VisitBinary(BinaryExpression node)
    {
        if (node.NodeType == ExpressionType.Add)
        {
            Expression left = node.Left;

            Expression right = ???

            return Expression.MakeBinary(ExpressionType.Add, left, right);
        }

        return base.VisitBinary(node);
    }
}

I’m confused as how to construct the proper right operand so I can return a new BinaryExpression.

Answer 1

Do you just want to turn any x + y into x + 1? If so, I suspect you just want:

Expression right = Expression.Constant(1);

(You should validate that the addition type is for integers, of course.)

If that’s not what you’re trying to do, please be more specific.