Creating a property selector Expression from a string

Question

I'm trying to generate a "Property Selector" from a string.

Let me explain myself a bit with a Real life example:

We have a Person class with a Name (string) property.

I could manually create a "property selector" like this propertySelector writing:

Expression<Func<Person, string>> propertySelector = x => x.Name;

But I would like to get the same property selector with my method.

var propertySelector = CreatePropertySelectorExpression<Person, string>("Name");

What I have so far is this:

public static Expression<Func<TIn, TOut>> CreatePropertySelectorExpression<TIn, TOut>(string path)
{
    Expression exp = Expression.Parameter(typeof(TIn), "x");
    foreach (var property in path.Split('.'))
    {
        exp = Expression.PropertyOrField(exp, property);
    }
    return exp;
}

But... I've got and invalid cast error!

Cannot implicitly convert type 'System.Linq.Expressions.Expression' to 'System.Linq.Expressions.Expression>'. An explicit conversion exists (are you missing a cast?)

I'm very new to Expressions and I don't know how to continue :(

Answer 1

Your exp only contains the body of the lambda. But you want an actual lambda function that takes a parameter of type TIn there. So you need to create a lambda using Expression.Lambda:

var param = Expression.Parameter(typeof(TIn));
var body = Expression.PropertyOrField(param, propertyName);
return Expression.Lambda<Func<TIn, TOut>>(body, param);

Note though that the expression does not really help you much. You probably want a compiled function instead:

private static Func<TIn, TOut> CreatePropertyAccessor<TIn, TOut> (string propertyName)
{
    var param = Expression.Parameter(typeof(TIn));
    var body = Expression.PropertyOrField(param, propertyName);
    return Expression.Lambda<Func<TIn, TOut>>(body, param).Compile();
}

You can then use it like this:

var name1 = CreatePropertyAccessor<Obj, string>("Name");
var name2 = CreatePropertyAccessor<Obj, string>("Name2");
var name3 = CreatePropertyAccessor<Obj, string>("Name3");

var o = new Obj() // Obj is a type with those three properties
{
    Name = "foo",
    Name2 = "bar",
    Name3 = "baz"
};

Console.WriteLine(name1(o)); // "foo"
Console.WriteLine(name2(o)); // "bar"
Console.WriteLine(name3(o)); // "baz"