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Const function parameter in C# [duplicate]

Possible Duplicate:
Read-only (“const”-like) function parameters of C#
Why is there no const member method in C# and const parameter?

Having programmed in C++ in the past, I recall we could make a constant reference/pointer parameter in a method.

If my memory is correct, the below means, that the method cannot alter the reference and the reference itself is a constant reference.

C++ example

void DisplayData(const string &value) const
{
   std::count << value << endl;
}

Is there an equivalent in C# for methods in a class?

The reason why I'm asking is, I'm trying to pass a object by reference (for speed) and at the same time don't want anyone to alter it.

Answer*

***Update 16/09/2020***

There now appears to be the [`in` parameter modifier][1] that exhibits this behaviour (in essence, a `ref readonly`). A brief search on when you would ever use this yields the following answer:

https://stackoverflow.com/questions/52820372/why-would-one-ever-use-the-in-parameter-modifier-in-c

***Original Answer***

There is no equivalent for C# and it has been asked [many][2], [many][3], [many][4], [many][5] times before.

If you don't want anyone to alter the "reference", or perhaps you mean the content of the object, make sure the class doesn't expose any public setters or methods of mutating the class. If you cannot change the class, have it implement an interface that only publicly exposes the members in a read-only fashion and pass the interface reference instead.

If you mean you want to stop the method from changing the reference, then by default if you pass it "by reference", you are actually passing the reference by value. Any attempt from the method to change what the reference points to will only affect the local method copy, not the caller's copy. This can be changed by using the `ref` keyword on a reference type, at which point the method *can* point the reference at a new underlying object and it *will* affect the caller.


  [1]: https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/keywords/in-parameter-modifier
  [2]: https://stackoverflow.com/questions/4150478/why-there-is-no-const-member-method-in-c-sharp-and-const-parameter
  [3]: https://stackoverflow.com/questions/8236358/const-objects-in-c-sharp-as-parameter
  [4]: https://stackoverflow.com/questions/3826542/read-only-const-like-function-parameters-of-c-sharp
  [5]: https://stackoverflow.com/questions/3263001/why-const-parameters-are-not-allowed-in-c-sharp

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2

in C# now new you can use in keyword to pass value type as constant

divyang4481
– divyang4481

2020-03-20 07:12:08 +00:00
Commented Mar 20, 2020 at 7:12
@divyang4481 I had a quick look at the docs and couldn't see any examples of what you mean, can you post something?

Adam Houldsworth
– Adam Houldsworth

2020-03-20 07:56:22 +00:00
Commented Mar 20, 2020 at 7:56
please refer this link learn.microsoft.com/en-us/dotnet/csharp/language-reference/…

divyang4481
– divyang4481

2020-03-20 08:28:27 +00:00
Commented Mar 20, 2020 at 8:28
Cannot believe that the answer that suggests ref which let change the value is accepted and has highest number of bids so quickly adding quotation from official C# documentation: - in - specifies that this parameter is passed by reference but is only read by the called method. - ref - specifies that this parameter is passed by reference and may be read or written by the called method. - out - specifies that this parameter is passed by reference and is written by the called method.

XWiśniowiecki
– XWiśniowiecki

2020-09-15 22:57:58 +00:00
Commented Sep 15, 2020 at 22:57

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Const function parameter in C# [duplicate]

C++ example

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