Unecessary assignment of a value c# when using out

Question

I recently came up to the following Message with the code below

string errorMessage = String.Empty;

I've been getting:

Message IDE0059 Unnecessary assignment of a value to 'errorMessage'

The only place this is used was

var valid = IsValid(out errorMessage);

Why am I getting this Message?

@SeM I changed the Warning to Message. @Panagiotis Kanavos - I was wondering why `string errorMessage` works. Since I figured it out, I thought to share with the community. — Athanasios Kataras, Feb 24 '20 at 10:21
@AthanasiosKataras what you posted doesn't even compile though. It's not mandatory to assign a value to the *variable*, it's mandatory to assign a value to the parameter — Panagiotis Kanavos, Feb 24 '20 at 10:24

score 3 · Accepted Answer · answered Feb 24 '20 at 10:13

Just don't set the initial value. This is perfectly valid code :

string errorMessage;
var valid = IsValid(out errorMessage);

Or use

var valid=IsValid(out var errorMessage);

The compiler knows that the variable is used as an out parameter and will get a value unless an exception is thrown.

On the other hand, IsValid has to store a value in the out parameter, overwriting any initial value. The original value is never used, and so doesn't need to be assigned.

This won't compile :

public bool IsValid(out string errorMessage) {
   return true;
}

and return :

CS0177 The out parameter 'errorMessage' must be assigned to before control leaves the current method

This will work :

public bool IsValid(out string errorMessage) {
   errorMessage="";
   return true;
}

SᴇM · Answer 2 · 2020-02-24T10:35:04.817

2

I don't think that message needs explanation, it basically says that the value that you've assigned to errorMessage is unnecessary, because you have never used that (errorMessage) variable.

In that case, if you never planning to use errorMessage variable, you can use one handy tool, called Discards, so you can write:

bool value = IsValid(out _);

edited Feb 24 '20 at 10:35

answered Feb 24 '20 at 10:20

SᴇM

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I am using the variable. This is not the reason why. Check mine and Panagiotis explanations below. – Athanasios Kataras Feb 24 '20 at 10:24
1

@AthanasiosKataras the question says that the variable is never used : `The only place this is used was ...`. This is a very good answer that **should** be upvoted. – Panagiotis Kanavos Feb 24 '20 at 10:27
1

Why was this dowvnoted? I wish I had thought of this first – Panagiotis Kanavos Feb 24 '20 at 10:28
@PanagiotisKanavos I'm not sure why, any explanation guys? – SᴇM Feb 24 '20 at 10:33
@AthanasiosKataras Assigning variable and actually using it is not the same thing. – SᴇM Feb 24 '20 at 10:34
1

Sorry m8, tunnel vision. I had something specific in mind. It is indeed a very worthy answer! My bad for not phrasing the question in a better way to convey the initial intent. – Athanasios Kataras Feb 24 '20 at 14:17

score 0 · Answer 3 · answered Feb 24 '20 at 09:50

0

The .net compiler forces us to assign a value to any out parameter in the function getting the variable with the out modifier.

So in the example below:

public bool IsValid(out string errorMessage) {
   return true;
}

We would get the error

Error CS0177 The out parameter 'errorMessage' must be assigned to before control leaves the current method ServicesUtils

Since it's mandatory to assign a value, the compiler is clever enough to warn us of a redundant assignment beforehand, which will be overriden in a mandatory way.

This way we are protected from being misled by the initial assignment.

answered Feb 24 '20 at 09:50

Athanasios Kataras

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Or you could use `out var`. Or *not* assign anything - `string errorMessage;` works – Panagiotis Kanavos Feb 24 '20 at 10:03
In the example `IsValid` won't even compile because `errorMessage` is never assigned. `IsValid` *has* to put something in there, even if it's a `null` – Panagiotis Kanavos Feb 24 '20 at 10:06
This is what I'm saying. The fact that it will provide an error is why I'm getting the Message in the first place. – Athanasios Kataras Feb 24 '20 at 10:25

Unecessary assignment of a value c# when using out

3 Answers3