How do I build this c# "Expression" at runtime using reflection?

Question

To this day, I have not found a great article about expressions - and how to look at a C# lambda statement and say "oh, that's a blah blah"... so, if you know of a good article, I'd appreciate that as an answer too.

Code sample to explain the question

So... given the following c# code:

public class SomeClass<T>
{
    public TResult SomeMethod<TResult>(Expression<Func<T, TResult>> expression)
    {
        // This is just an example... don't get hung up on this :)
        return default(TResult);
    }
}

public class Person
{
    public string FirstName { get; set; }

    public string LastName { get; set; }
}

How do I do this...

var blah = new SomeClass<Person>();

blah.SomeMethod(p => p.FirstName);

at runtime (using reflection)?

What I expect as an answer

I kinda expect something like this... but I'm sure I'm way off with my choice of expressions.

// By the way, these values are being passed to me... so you
// can't change this part of the question :)
Type personType = typeof(Person);
string propertyName = "FirstName";

// THIS CODE BELOW IS OBVIOUSLY WRONG, BUT YOU GET THE IDEA OF
// WHAT I HOPE TO DO... THIS LINE OF CODE BELOW IS **ALL** I'M
// ASKING HOW TO DO :)
var expression = Expression.MakeUnary(ExpressionType.Lambda,
    Expression.Property(Expression.Parameter(personType, "p"),
    propertyName), typeof(string));

blah.SomeMethod(expression);

Sergey Kalinichenko · Accepted Answer · 2011-12-16 22:06:20Z

6

Try this:

var functorParam = Expression.Parameter(typeof(Person));
var lambda = Expression.Lambda(
    Expression.Property(functorParam, typeof(Person).GetProperty("FirstName"))
,   functorParam /* <<= EDIT #1 */
);
blah.SomeMethod((Expression<Func<Person,string>>)lambda); /* EDIT #2 */

edited Dec 16, 2011 at 22:06

answered Dec 16, 2011 at 20:07

Sergey Kalinichenko

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4 Comments

Timothy Khouri Over a year ago

blah.SomeMethod(f); does not compile... but I feel that this is very close.

Sergey Kalinichenko Over a year ago

@TimothyKhouri If I understand your intent correctly, SomeMethod should be taking Func<T,TResult> functor rather than Expression<Func<T,TResult>> expression. This way you'd be able to write things like var personsProp = functor(myPerson); and get the last name.

Timothy Khouri Over a year ago

The method that I'm calling into cannot change... it's part of an assembly I'm referencing, and therefor don't have the option to change it... that's why I'm asking this question :)

Sergey Kalinichenko Over a year ago

@TimothyKhouri Remove lambda.Compile and pass the lambda then.

Jahan Zinedine · Accepted Answer · 2011-12-16 20:09:53Z

1

ExpressionBuilder is the way to go.

answered Dec 16, 2011 at 20:09

Jahan Zinedine

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How do I build this c# "Expression" at runtime using reflection?

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