Get the name of the lambda expression with the index from the expression's array

Question

I need to get expression name for query alias.

I need it:

Customer customer = null;

string expressionAdresses2Number = ExpressionUtils.PathString(() => customer.Adresses[2].Number);

And the result I would like was "Adresses[2].Number", but I could only get"Adresses.Number", I couldn't get the array part of the expression.

I tried this example: https://dotnetfiddle.net/eJ5nvl (based on this example: https://stackoverflow.com/a/22135756/2290538)

Does anyone have any suggestions how to get the expression part of the array with index selected "[2]"?

Model example:

public class Customer
{
    public int Id { get; set; }
    public Adress Adress { get; set; }
    public IList<Adress> Adresses { get; set; }
}

public class Adress
{
    public int Id { get; set; }
    public int Number { get; set; }
}

Does your expression _always_ contains only property access + array access and nothing else? — Evk, Oct 06 '21 at 15:07
No, (property access) = "Adresses" + (array access) = "[2]" + (array property access) = "Number", if I understood you, hehe — Fernando Leal, Oct 06 '21 at 18:02

Sweeper · Accepted Answer · 2021-10-06T15:19:21.110

The indexer is translated to a MethodCallExpression, so you need to override VisitMethodCall.

You can do a visitor like this:

public class PathVisitor : ExpressionVisitor
{

    // You should not use MemberInfo for the list type anymore, since the path can have indexer expressions now!
    // Here I convert both the indexer expressions and the member accesses to string in the visitor
    // You can do something more fancy, by creating your own class that represents either of these two cases
    // and then formatting them afterwards.
    internal readonly List<string> Path = new List<string>();

    protected override Expression VisitMember(MemberExpression node)
    {
        if (node.Member is PropertyInfo)
        {
            Path.Add(node.Member.Name);
        }
        return base.VisitMember(node);
    }
 
    protected override Expression VisitMethodCall(MethodCallExpression node) {
        if (node.Method.Name == "get_Item") { // name of the indexer

            // you can format this in a better way on your own. This is just an example
            Path.Add(node.Arguments.First().ToString());
        }
        return base.VisitMethodCall(node);
    }
}

Usage:

public static IReadOnlyList<string> Get(LambdaExpression expression)
{
    var visitor = new PathVisitor();
    visitor.Visit(expression.Body);
    visitor.Path.Reverse();
    return visitor.Path;
}

public static string PathString(Expression<Func<object>> expression)
{
    return string.Join(".", Get(expression));
}

This produces an output of Adresses.2.Number for your lambda.

Note that this assumes the lambdas are only with property accesses and indexers with constant arguments.

Thanks for the help, saved the day! I made some changes to other types of lists like IEnumerable, ICollection and primitive array, and I would like to [share the final version](https://dotnetfiddle.net/WoRVgY). — Fernando Leal, Oct 06 '21 at 18:00

Get the name of the lambda expression with the index from the expression's array

1 Answers1