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.

Answer 1

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);
}

Answer 2

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] + ".", ""));
    }
}