Is there some trick to use 'out' parameters inside lambda function?

Question

if ( (new Func</*out*/ string, bool>( (/*out*/ string uname) => ....

more details : that is a part of login function and I just want that my lambda function to changes login-name user with a out parameter and said me that user logined with it's bool return.

I really understand that I can return the Tuple and then get my string value but I want exactly out parameter for some personal clarity. I better return only string with null if user is not login, just want to know if I can use out parameters inside lambda functions.

And I really get that the code with expressions on the statement places is not so clean But none said me if that is really bad for compiler.

Note, as such lambda expressions can have `ref/out` parameters. For e.g. this works: `delegate bool Logger(out string name); Logger f = (out string bar) => { bar = ""; //assign return ... };` ..Just that with `Action/Func` you cant do it. You need your own delegate. — nawfal, Dec 22 '13 at 03:00

score 6 · Accepted Answer · answered Apr 04 '11 at 14:12

6

Lambda expressions won't work, but for delegates you should be fine using a statement body:

bool outval = false; // definite assignment
Func<bool> func = () => {
    return SomeMethod(out foo);
};
bool returned = func();
// check both outval and returned

For delegates... You will need to define your own:

public delegate bool MyType(out string value);

answered Apr 04 '11 at 14:12

Marc Gravell

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So... I really don't wanted to use delegates here but I even didn't knew if they can works fine here. – cnd Apr 04 '11 at 14:16
1

No, you *do* want delegates here, just probably not lambda expressions. – Adam Rackis Apr 04 '11 at 14:19
Ok , Yes, I do :) if ((new ChUname((out string uname) => ... works well. – cnd Apr 04 '11 at 14:23
1

that's still a lambda. You can't have (out string usname) => ... Forget about lambdas and just read about basic delegate usage. – Adam Rackis Apr 04 '11 at 14:26
But it works just fine, ChUname is my delegate it can changes my user's name. – cnd Apr 05 '11 at 05:08
@AdamRackis Why yes, you _can_ have `(out string usname) => ...`, you just have to have a delegate type whose signature matches. (None of the generic `Func` and `Action` types in the framework have `out` parameters, so they don't match.) For example with `MyType` as above, you can say `MyType f = (out string u) => { u = "assigned"; return true; };`. When `out` or `ref` is applied to some of the parameters, the parameter types (like here `string`) must be present. Then this variable `f` is used as exprected: `string uname; bool result = f(out uname);`. – Jeppe Stig Nielsen Feb 26 '13 at 19:53
@AdamRackis So you can assign an `(out string usname) => ...` expression to a delegate variable of type `MyType`. What you cannot do is assign to an expression tree, like a variable of type `Expression`. – Jeppe Stig Nielsen Feb 26 '13 at 19:57

score 2 · Answer 2 · edited Nov 23 '20 at 09:30

2

You cannot use out parameters with a lambda expression. See this Stack Overflow question.

edited Nov 23 '20 at 09:30

Pang

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answered Apr 04 '11 at 14:13

Adam Rackis

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1

A little confusing. You can have a lambda like `(int i, out int j) => ...`, with the `out` keyword in it, and that lamda can be assigned to a variable of delegate type. It can't be assigned to an expression tree, though. The question you are referring, talks about capturing an `out` parameter of a regular method inside a lambda. That's like `public static bool UsualMethod(out int i) { /* body of normal method */ Action f = () => { Console.WriteLine(i); }; ... ... }`. ***This*** is illegal. The lambda tries to capture an `out` parameter of the containing method. – Jeppe Stig Nielsen Feb 26 '13 at 20:04

score 2 · Answer 3 · answered Apr 04 '11 at 14:26

While you can't use the out keyword I did find a solution that lets you basically achieve C++ style memory pointers in .NET. I found this class due to the very reason you opened this SO question not being able to use an out parameter where I wanted it.

public class Ptr<T>
{
    Func<T> getter;
    Action<T> setter;

    public Ptr(Func<T> g, Action<T> s)
    {
        getter = g;
        setter = s;
    }

    public T Deref
    {
        get { return getter(); }
        set { setter(value); }
    }
}

Usage example

private IDocumentSession _session = DocumentStore.OpenSession()

var ptr = new Ptr<IDocumentSession>(
                () => _session, 
                newValue => _session = newValue))

session.Deref.SaveChanges();
session.Deref = DocumentStore.OpenSession();

I use this in a batch program that allows batch operations to control session flushing with RavenDB when I need fine grained session control while also leaving an ambient session context. Word of warning I have no idea what implications this type of code would have in a long running production app since I'm not sure if this would confuse the GC and cause memory to never be reclaimed.

Is there some trick to use 'out' parameters inside lambda function?

3 Answers3