When to use Expression

Question

I am trying to determine what to use ...

I have a requirement to pass an IEnumerable<TSource> to a function that writes the entity members' values to file. Here is what the IEnumerable looks like:

var persons = new[] {
    new Person {
        RecordID = 0,
        PersonFName = "Joe",
        PersonMName = "",
        PersonLName = "Smith",
        PersonZip="75227"
    }, 
    new Person {
        RecordID = 1,
        PersonFName = "Mary",
        PersonMName = "Hada",
        PersonLName = "Lamb",
        PersonZip="75217"
    }};

What is the best way to pass the IEnumerable to a function that reads each entity so I can read each field value?

I was thinking that I would use something like:

void WriteData<TSource>(Expression<IEnumerable<Person>> expression)
{
    // do stuff
}

I'm having trouble finding resources that explain how to determine when you should use an Expression versus just passing IEnumerable. And then, how do I create the Expression that reflects persons ?

Ideally, it seems like I would call WriteData like so:

WriteData(persons);

Am I even headed in the right direction?

if you are really interested in learning how to leverage expressions and the IQueryable interface, I highly recommend reading Matt Warren's LINQ: Building an IQueryable provider series. Warning: Metaprogramming can be complex and requires time and patience — smartcaveman
– smartcaveman, Commented Nov 13, 2011 at 2:55

Jon Skeet · Accepted Answer · 2011-10-11 16:36:46Z

5

I see no reason to use an expression tree there. If you just want to fetch all the properties by reflection, I suggest you do that - expression trees won't really help you do that though.

Note that the choice is rarely between Expression and IEnumerable<T> - it's usually either between an expression tree and a delegate, or between IEnumerable<T> and IQueryable<T>.

answered Oct 11, 2011 at 16:36

Jon Skeet

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3 Comments

IAbstract Over a year ago

Thank you ...just trying to figure out what I need to use. I didn't want to use reflection ...but I didn't really have a better solution either.

Jon Skeet Over a year ago

@IAbstract: Then it's unclear what you do want to do. Why do you think expression trees are adding anything? Expression<IEnumerable<Person>> isn't even a realistic type, as the T in Expression<T> should always really be a delegate type.

IAbstract Over a year ago

Okay, well, that makes sense - Expression<IEnumerable<Person>> looked really strange and didn't seem right. I'm venturing into something a little different without any other expertise to pull from.

Joey · Accepted Answer · 2011-10-11 16:58:02Z

0

I have no idea what you'd do with an expression but this is roughly what I'd do using reflection.

public void WriteData<T>(IEnumerable<T> objects) where T : class
{
  foreach (var obj in objects)
  {
    WriteObjectData(obj);
  }
}

private void WriteObjectData<T>(T obj) where T : class
{
    foreach (var propertyInfo in typeof(T).GetProperties())
    {
        // Use propertyInfo.GetValue to get the value from obj. 
    }
}

You could specify an interface instead of where T:class to limit the methods to just your data objects if you wanted to.

answered Oct 11, 2011 at 16:58

Joey

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1 Comment

IAbstract Over a year ago

Right, I think this is what @JonSkeet is saying I should do.

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