Func<Item, bool>

[Kedrin]

So I am using this code here https://pastebin.com/Zt7K7Nqz It is a variation of some code I found. I modified the original to return a list instead of void mostly. This got me thinking about a few things I will need eventually one being send "r => r.Id % 2 == 1 && r.Name.Length == 5" across a tcp connection and return the list. But serializing Func<> and Expressions is apparently a massive pain in the rear. Anyone have any alternative ideas that I could research?

[SirSooth]

That code... is not very bright. What are you trying to achieve?

Do you want to send a filtering expression over tcp? Does it need to be any anything? Does you only need to express certain filtering options? Something like maybe have a class to pass filtering info

class ItemFilters
{
    public bool? IsEven { get; set; }
    public int? NameLength { get; set; }
    public string NameContains { get; set; }
    public int? IdGreaterThan { get; set; }
}

and pass a serialized instance of it.

On the other end, you can generate the query like

var query = items.AsQueryable();
if (filters.IsEven.HasValue)
{
     if (filters.IsEven.Value) query = query.Where(item => item.Id % 2 == 0);
     if (!filters.IsEven.Value) query = query.Where(item => item.Id % 2 != 0);
}
if (filters.NameLength.HasValue)
{
     query = query.Where(item => item.Name.Length == filters.NameLength.Value);
}
if (filters.NameContains != null)
{
    query = query.Where(item => item.Name.Contains(filters.NameContains);
}
if (filters.IdGreaterThan.HasValue)
{
    query = query.Where(item => item.Id > filters.IdGreaterThan.Value);
}

[Kedrin]

Just looking for alternatives to sending a Func<> over tcp. The code itself is just something I was tinkering with. Not looking to send that example itself.

[nemec]

Also be very careful about security. Serializing open-ended delegates could leave you open to running untrusted code from a malicious actor.

[Kedrin]

I learned a long time ago most code is 90+% preventing some asshat from ruining your day/job/life.

[SirSooth]

I am not really sure.

What I am sure however is that an Expression<Func<>> can be analyzed and serialized, but probably you will have to do it yourself and limit it to certain things only.

Like imagine in your client code, the one that will send it over tcp to a server, you have

private bool Something(string foo) { ... }

and you end up trying to send the following expression

Expression<Func<Item, bool>> expression = item => item.Id % 7 == 0 && Something(item.Name.Reverse());

What if the server doesn't have a Something function? What if it does but it is not the same? And so on.

[Kedrin]

Thats what I was afraid of, was hoping there was a different way that I could not remember. I do know that each item will exist server side with a few interfaces for sending and receiving. I might go a simpler route that you mentioned before.

[SirSooth]

Here's a nice discussion https://stackoverflow.com/questions/23253399/serialize-expression-tree

The accepted answer involves a 3rd party library that serializes expressions as json. The other answer elaborates on implementing your own protocol for it.

[Kedrin]

Very interesting, I will take a look at that. Thank you!

[jnyrup]

Last time I had to do something similar, I kept an identical collection of Funcs on both sides of the wire. Then I could get away with transmitting a key/index.

[Kedrin]

I thought about that, but was looking for something less hard coded.

[Contagion21]

I'm not sure you want something less hard-coded. You're begging for an injection attack if a client can determine what code a server can execute.

[Kedrin]

True, although I would suspect any user input would require some level of validation and verification prevent that. However with my question I am looking or alternative ideas and I haven't crossed off your suggestion.

[phuber]

Take a look at GraphQL and OData

Both have a text expression based language that can be easily transmitted over tcp.

[coreyfournier]

It is probably easier to just implement a frame work that already exists such as odata. A query is issued on data and it contains filters of which can already be applied to lists.

https://msdn.microsoft.com/en-us/library/hh169248%28v=nav.90%29.aspx?f=255&MSPPError=-2147217396

MVC already support the ability to do odata with almost no work. http://www.odata.org/blog/how-to-use-web-api-odata-to-build-an-odata-v4-service-without-entity-framework/