Dynamically evaluating a property string with Expressions

Question

How do I build an expression that will fulfill the following goal:

public object Eval(object rootObj, string propertyString)

eg: Eval(person, "Address.ZipCode") => return person.Address.ZipCode

Expression.PropertyOrField doesn't work because I don't have the type of each intermediate property. I'd like to avoid creating a dependency on a scripting library.

I want to try to use expressions because it would allow me to store a cache of these expression trees as they would be executed several times. I'm aware that it's possible to do this iteratively or recursively with reflection.

I'm assuming Roslyn is off the table? – Benjamin Gruenbaum May 14 '13 at 13:37 — Benjamin Gruenbaum, May 14 '13 at 13:37

p.s.w.g · Accepted Answer · 2013-05-14T13:52:43.370

10

It sounds like you're looking for something like this:

public object Eval(object root, string propertyString)
{
    var propertyNames = propertyString.Split('.');
    foreach(var prop in propertyNames)
    {
        var property = root.GetType().GetProperty(prop);
        if (property == null)
        {
            throw new Exception(...);
        }

        root = property.GetValue(root, null);
    }

    return root;
}

To create an Expression use this:

public Expression Eval(object root, string propertyString)
{
    var propertyNames = propertyString.Split('.');
    ParameterExpression param = Expression.Parameter(root.GetType, "_");
    Expression property = param;
    foreach(var prop in propertyName)
    {
        property = Expression.PropertyOrField(property, prop);
    }

    return Expression.Lambda(property, param);
}

edited May 14 '13 at 13:52

answered May 14 '13 at 13:43

p.s.w.g

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+1 - I was working on something just like this, but you beat me to it. – Bobson May 14 '13 at 13:44
This would not work, since in the example `Person.Address.Zipcode` has root object `Person`, which does not have `Zipcode` as a property -- that is a property of the class/struct `Address`. So I think recursive evaluation is necessary. – John Willemse May 14 '13 at 13:46
2

@JohnWillemse - `root` changes on each iteration, so on the second pass, it would be looking at `Address` for `Zipcode`. – Bobson May 14 '13 at 13:48
I inquired about expressions specifically. I clarified why in my edit. Thanks. – Kir May 14 '13 at 13:49
@Bobson I stand corrected! I failed to see that. `Address` must be a property though, not a field, then it will fail. – John Willemse May 14 '13 at 13:54

score 3 · Answer 2 · answered May 14 '13 at 13:55

3

Here's a recursive version of p.s.w.g's code, working with Expressions.

public Expression Eval(Expression expression, string property)
{
    var split = property.Split('.');
    if (split.Length == 1)
    {
        return Expression.PropertyOrField(expression, property);
    }
    else
    {
        return Eval(Expression.PropertyOrField(expression, split[0]), property.Replace(split[0] + ".", ""));
    }
}

answered May 14 '13 at 13:55

Bobson

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+1 But I don't think you need to split the string every time if you use recursion. You can just use `property.IndexOf('.')` and `property.Substring(...)` – p.s.w.g May 14 '13 at 13:58
@p.s.w.g - Entirely true, and a better way to do it. I was just being lazy. – Bobson May 14 '13 at 16:10

Dynamically evaluating a property string with Expressions

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