What is the difference between **`const`** and **`readonly`** in C#? 

When would you use one over the other?

What is the difference between const and readonly in C#?

Is it possible to write something similar to the following?

    public const string[] Titles = { &quot;German&quot;, &quot;Spanish&quot;, &quot;Corrects&quot;, &quot;Wrongs&quot; };


Declare a const array

In a Django form, how do I make a field read-only (or disabled)?

When the form is being used to create a new entry, all fields should be enabled - but when the record is in update mode some fields need to be read-only.

For example, when creating a new `Item` model, all fields must be editable, but while updating the record, is there a way to disable the `sku` field so that it is visible, but cannot be edited?

    class Item(models.Model):
        sku = models.CharField(max_length=50)
        description = models.CharField(max_length=200)
        added_by = models.ForeignKey(User)


    class ItemForm(ModelForm):
        class Meta:
            model = Item
            exclude = (&#39;added_by&#39;)

    def new_item_view(request):
        if request.method == &#39;POST&#39;:
            form = ItemForm(request.POST)
            # Validate and save
        else:
                form = ItemForm()
        # Render the view

Can class `ItemForm` be reused? What changes would be required in the `ItemForm` or `Item` model class? Would I need to write another class, &quot;`ItemUpdateForm`&quot;, for updating the item?

    def update_item_view(request):
        if request.method == &#39;POST&#39;:
            form = ItemUpdateForm(request.POST)
            # Validate and save
        else:
            form = ItemUpdateForm()


In a Django form, how do I make a field readonly (or disabled) so that it cannot be edited?

What are the benefits of having a member variable declared as read only?  Is it just protecting against someone changing its value during the lifecycle of the class or does using this keyword result in any speed or efficiency improvements?

What are the benefits to marking a field as `readonly` in C#?

I would like to show a radio button, have its value submitted, but depending on the circumstances, have it not editable. Disabled doesn&#39;t work, because it doesn&#39;t submit the value (or does it?), and it grays out the radio button. Read-only is really what I&#39;m looking for, but for some mysterious reason it doesn&#39;t work.

Is there some weird trick I need to pull to get read-only to work as expected? Should I just do it in JavaScript instead?

Incidentally, does anyone know why read-only doesn&#39;t work in radio buttons, while it does work in other input tags? Is this one of those incomprehensible omissions in the HTML specs?

Why can&#39;t radio buttons be &quot;readonly&quot;?

In .NET why is `String.Empty` read only instead of a constant? I&#39;m just wondering if anyone knows what the reasoning was behind that decision.

Why isn&#39;t String.Empty a constant?

I&#39;m returning a reference to a dictionary in my read only property.  How do I prevent consumers from changing my data?  If this were an `IList` I could simply return it `AsReadOnly`.  Is there something similar I can do with a dictionary?

    Private _mydictionary As Dictionary(Of String, String)
    Public ReadOnly Property MyDictionary() As Dictionary(Of String, String)
        Get
            Return _mydictionary
        End Get
    End Property

Is there a read-only generic dictionary available in .NET?

I am developing some private projects on Github, and I would like to add nightly cronjobs to my deployments servers to pull the latest version from github. I am currently doing this by generating keypairs on every deployment server and adding the public key to the github project as &#39;Deployment key&#39;. 

However, I recently found out that these deployment keys actually do have write access to the project. Hence, every of the server administrators could potentially start editing. Furthermore I can add every deployment key to only one repository, whereas I would like to be able to deploy multiple repositories on one and the same deployment server.

Is there a way to provide read-only access for private repositories to selected users on Github?

Github: readonly access to a private repo

I am working with Docker, and I want to mount a dynamic folder that changes a lot (so I would not have to make a Docker image for each execution, which would be too costly), but I want that folder to be read-only. Changing the folder owner to someone else works. However, `chown` requires `root` access, which I would prefer not to expose to an application. 

When I use `-v` flag to mount, it gives whatever the username I give, I created a non-root user inside the docker image, however, all the files in the volume with the owner as the user that ran docker, changes into the user I give from the command line, so I cannot make read-only files and folders. How can I prevent this? 

I also added `mustafa ALL=(docker) NOPASSWD: /usr/bin/docker`, so I could change to another user via terminal, but still, the files have permissions for my user.

Docker, mount volumes as readonly

I want to write a ViewModel that always knows the current state of some read-only dependency properties from the View.

Specifically, my GUI contains a FlowDocumentPageViewer, which displays one page at a time from a FlowDocument. FlowDocumentPageViewer exposes two read-only dependency properties called CanGoToPreviousPage and CanGoToNextPage. I want my ViewModel to always know the values of these two View properties.

I figured I could do this with a OneWayToSource databinding:

    &lt;FlowDocumentPageViewer
        CanGoToNextPage=&quot;{Binding NextPageAvailable, Mode=OneWayToSource}&quot; ...&gt;

If this was allowed, it would be perfect: whenever the FlowDocumentPageViewer&#39;s CanGoToNextPage property changed, the new value would get pushed down into the ViewModel&#39;s NextPageAvailable property, which is exactly what I want. 

Unfortunately, this doesn&#39;t compile: I get an error saying **&#39;CanGoToPreviousPage&#39; property is read-only and cannot be set from markup.** Apparently read-only properties don&#39;t support *any* kind of databinding, not even databinding that&#39;s read-only with respect to that property.

I could make my ViewModel&#39;s properties be DependencyProperties, and make a OneWay binding going the other way, but I&#39;m not crazy about the separation-of-concerns violation (ViewModel would need a reference to the View, which MVVM databinding is supposed to avoid).

FlowDocumentPageViewer doesn&#39;t expose a CanGoToNextPageChanged event, and I don&#39;t know of any good way to get change notifications from a DependencyProperty, short of creating another DependencyProperty to bind it to, which seems like overkill here.

How can I keep my ViewModel informed of changes to the view&#39;s read-only properties?

Pushing read-only GUI properties back into ViewModel

I need to expose an Entity Framework Data Context to 3rd party plugins. The purpose is to allow these plugins to fetch data only and not to let them issue inserts, updates or deletes or any other database modification commands. Hence how can I make a data context or entity readonly.

How to make Entity Framework Data Context Readonly

Having a friendly debate with a co-worker about this.  We have some thoughts about this, but wondering what the SO crowd thinks about this?

Why does C# disallow readonly local variables?

I am wondering, since a lot of things can be done using reflection, can I change a private readonly field after the constructor completed its execution?  
(note: just curiosity)

    public class Foo
    {
     private readonly int bar;
     
     public Foo(int num)
     {
      bar = num;
     }
     
     public int GetBar()
     {
      return bar;
     }
    }
    
    Foo foo = new Foo(123);
    Console.WriteLine(foo.GetBar()); // display 123
    // reflection code here...
    Console.WriteLine(foo.GetBar()); // display 456



Can I change a private readonly field in C# using reflection?

**I don&#39;t know when an attribute should be private and whether I should use `property`.**

I read recently that setters and getters are not pythonic but using the `property` decorator is OK. 

But what if I have attribute, that mustn&#39;t be set from outside of class but can be read (read-only attribute). Should this attribute be private, and by private I mean with underscore, like that `self._x`?
If yes, then how can I read it without using getter?
Only method I know right now is to write

    @property
    def x(self):
        return self._x

That way I can read attribute by `obj.x` but I can&#39;t set it `obj.x = 1` so it&#39;s fine.

But should I really care about setting object that mustn&#39;t be set? Maybe I should just leave it. But then again I can&#39;t use underscore because reading `obj._x` is odd for user, so I should use `obj.x` and then again user doesn&#39;t know that he mustn&#39;t set this attribute.

What&#39;s your opinion and practices? 


When should an attribute be private and made a read-only property?

I need to implement a *read only* property on my type. Moreover the value of this property is going to be set in the constructor and it is not going to be changed (I am writing a class that exposes custom routed UI commands for WPF but it does not matter).

I see two ways to do it:

1. 
        class MyClass
        {
            public readonly object MyProperty = new object();
        }

2.  
        class MyClass
        {
            private readonly object my_property = new object();
            public object MyProperty { get { return my_property; } }
        }

With all these FxCop errors saying that I should not have public member variables, it seems that the second one is the right way to do it. Correct?

Is there any difference between a get only property and a read only member in this case?

I would appreciate any comments/advice/etc.

How to implement a read only property

I have declared a readonly property in my interface as such:

     @property (readonly, nonatomic, copy) NSString* eventDomain;

Maybe I&#39;m misunderstanding properties, but I thought that when you declare it as `readonly`, you can use the generated setter inside of the implementation (`.m`) file, but external entities cannot change the value. [This SO question][1] says that&#39;s what should happen. That is the behavior I&#39;m after. However, when attempting to use the standard setter or dot syntax to set `eventDomain` inside of my init method, it gives me an `unrecognized selector sent to instance.` error. Of course I&#39;m `@synthesize`ing the property. Trying to use it like this:

     // inside one of my init methods
     [self setEventDomain:@&quot;someString&quot;]; // unrecognized selector sent to instance error
 
So am I misunderstanding the `readonly` declaration on a property? Or is something else going on? 


  [1]: https://stackoverflow.com/questions/1876495/objective-c-accessor-declarations-readonly-readwrite-etc

Readonly Properties in Objective-C?

How can I make a model completely read-only in the admin interface? It&#39;s for a kind of log table, where I&#39;m using the admin features to search, sort, filter etc, but there is no need to modify the log.

In case this looks like a duplicate, here&#39;s *not* what I&#39;m trying to do:

 * I&#39;m not looking for readonly *fields* (even making every field readonly would still let you create new records)
 * I&#39;m not looking to create a readonly *user*: every user should be readonly.



Readonly models in Django admin interface?

**I&#39;m looking for the C# equivalent of Java&#39;s `final`. Does it exist?**

Does C# have anything like the following:

    public Foo(final int bar);

In the above example, `bar` is a read only variable and cannot be changed by `Foo()`. Is there any way to do this in C#?

For instance, maybe I have a long method that will be working with `x`, `y`, and `z` coordinates of some object (ints). I want to be absolutely certain that the function doesn&#39;t alter these values in any way, thereby corrupting the data. Thus, I would like to declare them readonly.

    public Foo(int x, int y, int z) {
         // do stuff
         x++; // oops. This corrupts the data. Can this be caught at compile time?
         // do more stuff, assuming x is still the original value.
    }

Can parameters be constant?

I&#39;m trying to bind to a `Readonly` property with `OneWayToSource` as mode, but it seems this cannot be done in XAML:

    &lt;controls:FlagThingy IsModified=&quot;{Binding FlagIsModified, 
                                              ElementName=container, 
                                              Mode=OneWayToSource}&quot; /&gt;

I get: 

&gt; The property &#39;FlagThingy.IsModified&#39; cannot be set because it does not have an accessible set accessor.

`IsModified` is a readonly `DependencyProperty` on `FlagThingy`. I want to bind that value to the `FlagIsModified` property on the container. 

To be clear: 

    FlagThingy.IsModified --&gt; container.FlagIsModified
    ------ READONLY -----     ----- READWRITE --------

Is this possible using just XAML?

---
**Update:** Well, I fixed this case by setting the binding on the container and not on the `FlagThingy`. But I&#39;d still like to know if this is possible. 

OneWayToSource binding from readonly property in XAML

Is there a simple way to create an immutable NumPy array?

If one has to derive a class from `ndarray` to do this, what&#39;s the minimum set of methods that one has to override to achieve immutability?

Immutable numpy array?

I&#39;m aware of opening files in readonly mode from shell using `vim -R`, but how to open a file from inside vim in a separate tab (`:tabe &lt;filename&gt;`) in readonly mode?

Thanks for your time.

Open a file in a tab in vim in readonly mode

I&#39;ve been working on creating a class and suddenly a thought came to my mind of what is the difference between the two codes:

    public readonly string ProductLocation;

AND

    public string ProductLocation
    {
         get;
         private set;
    }

Can you guys give me idea when to use the following better. thanks.

Which is better between a readonly modifier and a private setter?

I want to make readOnly with EditorFor in edit page.

I tried to put readonly and disabled as:

    &lt;div class=&quot;editor-field&quot;&gt;
            @Html.EditorFor(model =&gt; model.userName, new { disabled = &quot;disabled&quot;, @readonly = &quot;readonly&quot; })
        &lt;/div&gt;

However, it does not work. How can I make to disable edit this field?

Thank you.

MVC3 EditorFor readOnly

As Most, I am familiar with the `readonly` attribute for `text input`, But while reading code from other websites (a nasty habit of mine ) I saw more than one implementation for this attribute:

    &lt;input type=&quot;text&quot; value=&quot;myvalue&quot; class=&quot;class anotherclass&quot; readonly &gt;
and 

    &lt;input type=&quot;text&quot; value=&quot;myvalue&quot; class=&quot;class anotherclass&quot; readonly=&quot;readonly&quot; &gt;

and I have even seen 

    &lt;input type=&quot;text&quot; value=&quot;myvalue&quot; class=&quot;class anotherclass&quot; readonly=&quot;true&quot; &gt;

.. And I believe I saw even more, but can not recall the exact syntax now..

So, which one is the correct one that I should use?

What is the correct readonly attribute syntax for input text elements?

I have a page with many inputs, and I want to make it &#39;`readOnly`&#39;
I find this solution: https://stackoverflow.com/questions/35212475/how-to-change-html-element-readonly-and-required-attribute-in-angular2-typescrip

But I don&#39;t want to do it for every input separately.

How can I add `readOnly` property to all inputs in some div.

How to make readonly all inputs in some div in Angular2?

What&#39;s the difference between the Enabled and the ReadOnly-properties of an asp:TextBox control?

asp:TextBox ReadOnly=true or Enabled=false?

I have a server side class which I make available on the client side through a [DataContract]. This class has a readonly field which I&#39;d like to make available through a property. However, I&#39;m unable to do so because it doesn&#39;t seem that I&#39;m allowed to add a [DataMember] property without having both get and set. 

So - is there a way to have a [DataMember] property without setter? 

    [DataContract]
    class SomeClass
    {
        private readonly int _id; 
        
        public SomeClass() { .. }

        [DataMember]
        public int Id { get { return _id; } }        

        [DataMember]
        public string SomeString { get; set; }
    }

Or will the solution be use the [DataMember] as the field - (like e.g. shown [here][1])? Tried doing this too, but it doesn&#39;t seem to care the field is readonly..? 

*Edit*: Is the only way to make a readonly property by hacking it like this? (no - I don&#39;t want to do this...) 

    [DataMember]
    public int Id
    {
        get { return _id; }
        private set { /* NOOP */ }
    }

  [1]: https://stackoverflow.com/questions/556775/wcf-datamember-attribute-on-property-vs-member/556886#556886

WCF: Exposing readonly DataMember properties without set?

I found a topic on [MSDN][1] that talks that yes, this is possible.

I did a test that seems to break this statement:

    using System;

    namespace Test
    {
        class Program
        {
            static void Main(string[] args)
            {
                Foo f = new Foo(&quot;1&quot;);
                Console.WriteLine(f.Bar); // prints 1
                f.Test(&quot;2&quot;);
                Console.WriteLine(f.Bar);// successfully prints 2
            }
        }
    
        class Foo
        {
            public Foo(string b)
            {
                this.Bar = b;
            }
    
            public string Bar { get; private set; }
    
            public void Test(string b)
            {
                // this would be impossible for readonly field!
                // next error would be occur: CS0191 or CS0191
                // A readonly field cannot be assigned to (except in a constructor or a variable initializer)
                this.Bar = b; 
            }
        }
    }


Where am I wrong?

  [1]: http://msdn.microsoft.com/en-us/library/bb383979.aspx


Is read-only auto-implemented property possible?

I need to delete a directory that contains read-only files.  Which approach is better:

 - Using `DirectoryInfo.Delete()`, or,

 - `ManagementObject.InvokeMethod(&quot;Delete&quot;)`? 

With `DirectoryInfo.Delete()`, I have to manually turn off the read-only attribute for each file, but `ManagementObject.InvokeMethod(&quot;Delete&quot;)` doesn&#39;t appear to need to.  Is there any situation where one is more preferable to the other?

Sample code (test.txt is read only).

###First way:

    DirectoryInfo dir = new DirectoryInfo(@&quot;C:\Users\David\Desktop\&quot;);
    dir.CreateSubdirectory(&quot;Test&quot;);

    DirectoryInfo test = new DirectoryInfo(@&quot;C:\Users\David\Desktop\Test\&quot;);
    File.Copy(@&quot;C:\Users\David\Desktop\test.txt&quot;, @&quot;C:\Users\David\Desktop\Test\test.txt&quot;);
    File.SetAttributes(@&quot;C:\Users\David\Desktop\Test\test.txt&quot;, FileAttributes.Archive);
    test.Delete(true);

###Second way:

    DirectoryInfo dir = new DirectoryInfo(@&quot;C:\Users\David\Desktop\&quot;);
    dir.CreateSubdirectory(&quot;Test&quot;);

    DirectoryInfo test = new DirectoryInfo(@&quot;C:\Users\David\Desktop\Test\&quot;);
    File.Copy(@&quot;C:\Users\David\Desktop\test.txt&quot;, @&quot;C:\Users\David\Desktop\Test\test.txt&quot;);

    string folder = @&quot;C:\Users\David\Desktop\Test&quot;;
    string dirObject = &quot;Win32_Directory.Name=&#39;&quot; + folder + &quot;&#39;&quot;;
    using (ManagementObject managementObject = new ManagementObject(dirObject))
    {
        managementObject.Get();
        ManagementBaseObject outParams = managementObject.InvokeMethod(&quot;Delete&quot;, null,
        null);
        // ReturnValue should be 0, else failure
        if (Convert.ToInt32(outParams.Properties[&quot;ReturnValue&quot;].Value) != 0)
        {
        }
    }

How do I delete a directory with read-only files in C#?

I have some git repositories accessed remotely through SSH and I want to make some of them read-only to prevent more pushes. Some people have remotes pointing to these repositories.

These bare repositories were initialised `--shared=group`, so is setting file permissions to 660 for all files good enough to still allow SSH access, but disallow writes? Or is there an easier way?

Cheers.

How to make a git repository read-only?

I&#39;ve got the extremely unlikely and original situation of wanting to return a readonly array from my property. So far I&#39;m only aware of one way of doing it - through the `System.Collections.ObjectModel.ReadOnlyCollection&lt;T&gt;`. But that seems somehow awkward to me, &lt;s&gt;not to mention that this class loses the ability to access array elements by their index&lt;/s&gt; (**added: whoops, I missed the indexer**). Is there no better way? Something that could make the array itself immutable?

What&#39;s the best way of creating a readonly array in C#?

I was thinking to use `Tuple` class to store 2 integer information (StartAddress, EndAddress) I need in my program.

But I discover that `Tuple` items are ReadOnly, so if I need to set a value for an item, I need to re-instantiate a Tuple.

What is the reason behind this design decision?

Why Tuple&#39;s items are ReadOnly?

I have a form that pop up inside a layer, and I need to make everything inside that form read only regarding what type of input it is. Anyway to do so?

how can I disable everything inside a form using javascript/jquery?

I have some .csv files which I&#39;m using as part of a test bench. I can open them and read them without any problems *unless* I&#39;ve already got the file open in Excel in which case I get an `IOException`:

&gt; System.IO.IOException : The process cannot access the file &#39;TestData.csv&#39; because it is being used by another process.

This is a snippet from the test bench:

    using (CsvReader csv = new CsvReader(new StreamReader(new FileStream(fullFilePath, FileMode.Open, FileAccess.Read)), false))
    {
        // Process the file
    }

Is this a limitation of StreamReader? I can open the file in other applications (Notepad++ for example) so it can&#39;t be an O/S problem. Maybe I need to use some other class? If anyone knows how I can get round this (aside from closing excel!) I&#39;d be very grateful.

How do I open an already opened file with a .net StreamReader?

I&#39;m making a form. And on one `input` tag is an `OnClick` event handler, which is opening a popup, where you can choose some stuff, and then it autofills the `input` tag.

That input tag is also `readonly`, so only right data will be entered.

This is the code of the `input` tag:

    &lt;input type=&quot;text&quot; name=&quot;formAfterRederict&quot; id=&quot;formAfterRederict&quot; size=&quot;50&quot; required readonly=&quot;readonly&quot; OnClick=&quot;choose_le_page();&quot;  /&gt;

But the `required` attribute isn&#39;t working in Chrome. But the field is required.

Does anybody know how I can make it work?

HTML required readonly input in form

I&#39;m writing software for an embedded system.  

We are using pointers to access registers of an FPGA device.  
Some of the registers are read-only, while others are write-only.  

The write-only registers will produce undefined values when read.  

I want to define a pointer type that will allow the compiler to detect when reading values from a write-only registers (a.k.a. dereferencing).

Can a write-only pointer be created using only C language syntax?  
(We are developing first prototype using C, but moving to C++ on 2nd generation.)

How can an efficient write-only pointer be created in C++?
(Remember, this is not tracking items in dynamic memory, but accessing hardware addresses.)

This code is used on an embedded system where safety and quality are highest concerns.  


Write-Only pointer type

I know you can add `readonly=&quot;readonly&quot;` to an input field so its not editable. But I need to use javascript to target the id of the input and make it readonly as I do not have access to the form code (it&#39;s generated via marketing software)

I don&#39;t want to disable the input as the data should be collected on submit. 

Here is the page I have added in the below suggestion with no luck so far: 

&gt; https://www.pages05.net/engagedigital/inputReadOnly/test?spMailingID=6608614&amp;spUserID=MTI5MDk4NjkzMTMS1&amp;spJobID=Nzk4MTY3MDMS1&amp;spReportId=Nzk4MTY3MDMS1

Make sure you use `&lt;body onload=&quot;onLoadBody();&quot;&gt;` for anyone using this in the future.

All Readonly Solutions on Gang of Coders

Total of 37 Readonly Solutions