Building Expression Tree Using a Parameter's Indexer

c# expression expression-trees lambda linq

Question

Given a class that has a property that is a Dictionary

public class Product
{
    public Dictionary<string, string> Attributes { get { return attributes; } }

    private Dictionary<string, string> attributes = new Dictionary<string, string>();
}

I want to be able to match products in a list of products based on criteria that are retrieved from a data store that are in the format of

Brand == Tyco
Color != Blue

My current approach is to construct an expression from the filter, and then pass that expression as the parameter to a LINQ Where method call like so

products = products.Where(myConstructedExpression);

where myConstructedExpression would normally be a lamda expression that looks like

p => p.Attributes[attribute] == value

I have assembled the following code for testing purposes, but it always fails the call to lambda.Compile() regardless of what I have tried for he left expression.

Dictionary<string, ExpressionType> expressionType = new Dictionary<string, ExpressionType>();
expressionType.Add("==", ExpressionType.Equal);
expressionType.Add("!=", ExpressionType.NotEqual);

string filter = "Brand == Tyco";
string[] fields = filter.Split(' ');
string attribute = fields[0];
string op = fields[1];
string value = fields[2];

Product product = new Product();
product.Attributes.Add("Brand", "Tyco"); 

var parameter = Expression.Parameter(typeof(Product), "p");
var left = /***** THIS IS WHAT I AM FAILING TO CONSTRUCT PROPERLY ********/
var right = Expression.Constant(value);
var operation = Expression.MakeBinary(expressionType[op], left, right);
var lambda = Expression.Lambda<Func<Product, bool>>(operation, parameter);

var result = lambda.Compile()(product);

Questions

  1. Is this even a reasonable approach, and, if so,
  2. How do I construct the left expression?

Accepted Answer

So to get p => p.Attributes["Brand"] <someoperator> "Tyco", you can do this.

The "trick", to work with indexed types, is to use their Item property (you could also work with the get_item method)

var parameter = Expression.Parameter(typeof(Product), "p");
Expression left = Expression.Property(parameter, "Attributes");
left = Expression.Property(left, "Item", new Expression[] { Expression.Constant(attribute) });

EDIT

the version with the IDictionary.ContainsKey(<value>) test

really step by step, but I think this makes things clearer at first.

//left part of lambda, p
var parameter = Expression.Parameter(typeof(Product), "p");
//right part
//p.Attributes
Expression left = Expression.Property(parameter, "Attributes");

var method = typeof(IDictionary<string, string>).GetMethod("ContainsKey");
//p.Attributes.ContainsKey("Brand");
Expression containsExpression = Expression.Call(left, method, Expression.Constant(attribute));
//p.Attributes.Item["Brand"]
Expression keyExpression= Expression.Property(left, "Item", new Expression[] { Expression.Constant(attribute) });
//"Tyco"
var right = Expression.Constant(value);
//{p => IIF(p.Attributes.ContainsKey("Brand"), (p.Attributes.Item["Brand"] == "Tyco"), False)}
Expression operation = Expression.Condition(
                           containsExpression,
                           Expression.MakeBinary(expressionType[op], keyExpression, right), 
                           Expression.Constant(false));
var lambda = Expression.Lambda<Func<Product, bool>>(operation, parameter);


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