How do I extract values from an Expression?

Question

Hello. It is possible to extract from an Expression which filters, arrays, etc were used in a query?

Example:

public class Program
{
    class Produto
    {
        public int Id;
        public string Nome;

        public Produto(int id, string nome)
        {
            this.Id = id;
            this.Nome = nome;
        }
    }

    public static void Main()
    {
        List<Produto> produtos = new List<Produto>();
        produtos.Add(new Produto(1, "Arroz"));
        produtos.Add(new Produto(2, "Feijão"));
        produtos.Add(new Produto(3, "Trigo"));
        produtos.Add(new Produto(4, "Batata"));

        var expression = (from produto in produtos
                          where produto.Id < 3
                          select produto).AsQueryable().Expression;
    }
}

In this case, what I need is to extract from Expression that the filter used was Id < 3

Answer 1

As I told you in the previous question, there are various types of Expressions, which requires you to put additional logic to recover your filter values.

Normally, this parameter Expression is a BinaryExpression. Its components can be obtained as follows::

var expression = meuQueryable.Expression as BinaryExpression;

BinaryExpressions usually possess Left and Right which, in turn, are also Expressions. A code I use to test what kind of expression is Left and Right (not very good, I’m still perfecting) is the following:

            switch (expression.Left.GetType().ToString())
            {
                case "System.Linq.Expressions.LogicalBinaryExpression":
                case "System.Linq.Expressions.MethodBinaryExpression":
                    valorFinalEsquerdo = Condicao(expression.Left);
                    break;
                case "System.Linq.Expressions.UnaryExpression":
                    var body2 = expression.Left as UnaryExpression;

                    if (body2 != null)
                    {
                        var left2 = body2.Operand as MemberExpression;
                        if (left2 != null)
                        {
                            var teste = ReflectionUtils.ExtrairAtributoColumnDeMember(left2.Member);
                            valorFinalEsquerdo = teste.Name;
                        }
                    }
                    break;
                default:
                    var left = expression.Left as MemberExpression;
                    if (left != null)
                    {
                        var teste = ReflectionUtils.ExtrairAtributoColumnDeMember(left.Member);
                        valorFinalEsquerdo = teste.Name;
                    }
                    break;
            }

            switch (expression.Right.GetType().ToString())
            {
                case "System.Linq.Expressions.MethodBinaryExpression":
                    valorFinalDireito = Condicao(expression.Right);
                    break;
                case "System.Linq.Expressions.PropertyExpression":
                        var right2 = expression.Right as MemberExpression;
                        if (right2 != null)
                        {
                            valorFinalDireito = AvaliarExpressao(right2);
                        }

                    break;
                default:
                    var right = expression.Right as ConstantExpression;
                    if (right != null)
                    {
                        valorFinalDireito = right.Value;
                    }
                    break;
            }

The code is part of a function called Condicao, I call recursively. Values are obtained within valorFinalDireito and valorFinalEsquerdo.