No, a nullable is a struct.  What is happening is that the nullable struct has two values:

1.  The value of the data type (`int` for `int?`, `DateTime` for `DateTime?`, etc.).
2.  A boolean value which tells if the data type value has been set.  (`HasValue` is the property.)

When you set the value of the data type, the struct changes `HasValue` to true.

[Nullable types (C# Programming Guide)][1]


  [1]: https://docs.microsoft.com/en-us/dotnet/csharp/programming-guide/nullable-types/

From [Nullable Types (C# Programming Guide)][1]:

&gt; Nullable types are instances of the System.Nullable struct. 

and

&gt; Nullable types represent value-type variables that can be assigned the value of null. You cannot create a nullable type based on a reference type. (Reference types already support the null value.)

So, no they&#39;re not reference types.

 [1]: https://docs.microsoft.com/en-us/dotnet/csharp/programming-guide/nullable-types/

Nullable types are neither value types nor reference types. They are more like value types, but have a few properties of reference types.

Naturally, nullable types may be set to `null`. Furthermore, a nullable type cannot satisfy a generic `struct` constraint. Also, when you box a nullable type with `HasValue` equal to `false`, you get a `null` pointer instead of a boxed nullable type (a similar situation exists with unboxing).

These properties make nullable types non-value types, but they sure aren&#39;t reference types either. They are their own special nullable-value type.

No, the Nullable type is in fact a struct.  The runtime will intelligently handle the setting of a null value for you, giving the appearance of a reference type, when it&#39;s not....

Nullable types cannot be reference types.

http://msdn.microsoft.com/en-us/library/2cf62fcy.aspx

You shouldn&#39;t need to make a reference type a nullable type as you can pass null in its place.

I&#39;ve implemented two REST services: Twitter and Netflix. Both times, I struggled to find the use and logic involved in the decision to expose these services as REST instead of SOAP. I hope somebody can clue me in to what I&#39;m missing and explain why REST was used as the service implementation for services such as these.

1. Implementing a REST service takes infinitely longer than implementing a SOAP service. Tools exist for all modern languages/frameworks/platforms to read in a WSDL and output proxy classes and clients. Implementing a REST service is done by hand and - get this - by reading documentation. Furthermore, while implementing these two services, you have to make &quot;guesses&quot; as to what will come back across the pipe as there is no real schema or reference document.

2. Why write a REST service that returns XML anyway? The only difference is that with REST you don&#39;t know the types each element/attribute represents - you are on your own to implement it and *hope* that one day a string doesn&#39;t come across in a field you thought was always an int. SOAP defines the data structure using the WSDL so this is a no-brainer.

3. I&#39;ve heard the complaint that with SOAP you have the &quot;overhead&quot; of the SOAP Envelope. In this day and age, do we really need to worry about a handful of bytes?

4. I&#39;ve heard the argument that with REST you can just pop the URL into the browser and see the data. Sure, if your REST service is using simple or no authentication. The Netflix service, for instance, uses OAuth which requires you to sign things and encode things before you can even submit your request.

5. Why do we need a &quot;readable&quot; URL for each resource? If we were using a tool to implement the service, do we really care about the actual URL?

Should a Netflix or Twitter-style web service use REST or SOAP?

I just read &lt;a href=&quot;https://stackoverflow.com/questions/493450/amending-a-single-file-in-a-past-commit-in-git&quot;&gt;amending a single file in a past commit in git&lt;/a&gt; but unfortunately the accepted solution &#39;reorders&#39; the commits, which is not what I want. So here&#39;s my question:

Every now and then, I notice a bug in my code while working on an (unrelated) feature. A quick `git blame` then reveals that the bug has been introduced a few commits ago (I commit quite a lot, so usually it&#39;s not the most recent commit which introduced the bug). At this point, I usually do this:

    git stash                      # temporarily put my work aside
    git rebase -i &lt;bad_commit&gt;~1   # rebase one step before the bad commit
                                   # mark broken commit for editing
    vim &lt;affected_sources&gt;         # fix the bug
    git add &lt;affected_sources&gt;     # stage fixes
    git commit -C &lt;bad_commit&gt;     # commit fixes using same log message as before
    git rebase --continue          # base all later changes onto this

However, this happens so often that the above sequence is getting annoying. Especially the &#39;interactive rebase&#39; is boring. Is there any shortcut to the above sequence, which lets me amend an arbitrary commit in the past with the staged changes? I&#39;m perfectly aware that this changes the history, but I&#39;m doing mistakes so often that I&#39;d really love to have something like

    vim &lt;affected_sources&gt;             # fix bug
    git add -p &lt;affected_sources&gt;      # Mark my &#39;fixup&#39; hungs for staging
    git fixup &lt;bad_commit&gt;             # amend the specified commit with staged changes,
                                       # rebase any successors of bad commit on rewritten 
                                       # commit.

Maybe a smart script which can rewrite commits using plumbing tools or so?


How can I easily fixup a past commit?

When I declare an `int` as nullable

    int? i=null;

Does `i` here become a reference type?

Are nullable types reference types?

When I declare an <code>int</code> as nullable
<pre><code class="hljs language-ini">int? i=null;
</code></pre>
Does <code>i</code> here become a reference type?

[Note: This question had the original title &quot;__C (ish) style union in C#__&quot;
but as Jeff&#39;s comment informed me, apparently this structure is called a &#39;discriminated union&#39;]


Excuse the verbosity of this question.

There are a couple of similar sounding questions to mine already in SO but they seem to concentrate on the memory saving benefits of the union or using it for interop.
[Here is an example of such a question][1].

My desire to have a union type thing is somewhat different.

I am writing some code at the moment which generates objects that look a bit like this

    public class ValueWrapper
    {
        public DateTime ValueCreationDate;
        // ... other meta data about the value

        public object ValueA;
        public object ValueB;
    }

Pretty complicated stuff I think you will agree.  The thing is that `ValueA` can only be of a few certain types (let&#39;s say `string`, `int` and `Foo` (which is a class) and `ValueB` can be another small set of types.  I don&#39;t like treating these values as objects (I want the warm snugly feeling of coding with a bit of type safety).  

So I thought about writing a trivial little wrapper class to express the fact that ValueA logically is a reference to a particular type.  I called the class `Union` because what I am trying to achieve reminded me of the union concept in C.


    public class Union&lt;A, B, C&gt;
    {
        private readonly Type type; 
        public readonly A a;
        public readonly B b;
        public readonly C c;

        public A A{get {return a;}}
        public B B{get {return b;}}
        public C C{get {return c;}}

        public Union(A a)
        {
            type = typeof(A);
            this.a = a;
        }

        public Union(B b)
        {
            type = typeof(B);
            this.b = b;
        }

        public Union(C c)
        {
            type = typeof(C);
            this.c = c;
        }

        /// &lt;summary&gt;
        /// Returns true if the union contains a value of type T
        /// &lt;/summary&gt;
        /// &lt;remarks&gt;The type of T must exactly match the type&lt;/remarks&gt;
        public bool Is&lt;T&gt;()
        {
            return typeof(T) == type;
        }

        /// &lt;summary&gt;
        /// Returns the union value cast to the given type.
        /// &lt;/summary&gt;
        /// &lt;remarks&gt;If the type of T does not exactly match either X or Y, then the value &lt;c&gt;default(T)&lt;/c&gt; is returned.&lt;/remarks&gt;
        public T As&lt;T&gt;()
        {
            if(Is&lt;A&gt;())
            {
                return (T)(object)a;    // Is this boxing and unboxing unavoidable if I want the union to hold value types and reference types? 
                //return (T)x;          // This will not compile: Error = &quot;Cannot cast expression of type &#39;X&#39; to &#39;T&#39;.&quot;
            }

            if(Is&lt;B&gt;())
            {
                return (T)(object)b; 
            }

            if(Is&lt;C&gt;())
            {
                return (T)(object)c; 
            }

            return default(T);
        }
    }


Using this class ValueWrapper now looks like this

    public class ValueWrapper2
    {
        public DateTime ValueCreationDate;
        public  Union&lt;int, string, Foo&gt; ValueA;
        public  Union&lt;double, Bar, Foo&gt; ValueB;
    }

which is something like what I wanted to achieve but I am missing one fairly crucial element - that is compiler enforced type checking when calling the Is and As functions as the following code demonstrates

        public void DoSomething()
        {
            if(ValueA.Is&lt;string&gt;())
            {
                var s = ValueA.As&lt;string&gt;();
                // .... do somethng
            }

            if(ValueA.Is&lt;char&gt;()) // I would really like this to be a compile error
            {
                char c = ValueA.As&lt;char&gt;();
            }
        }


IMO It is not valid to ask ValueA if it is a `char` since its definition clearly says it is not - this is a programming error and I would like the compiler to pick up on this.  [Also if I could get this correct then (hopefully) I would get intellisense too - which would be a boon.]  

In order to achieve this I would want to tell the compiler that the type `T` can be one of A, B or C

        public bool Is&lt;T&gt;() where T : A 
                               or T : B // Yes I know this is not legal!
                               or T : C 
        {
            return typeof(T) == type;
        } 

Does anyone have any idea if what I want to achieve is possible?  Or am I just plain stupid for writing this class in the first place?

Thanks in advance.


  [1]: https://stackoverflow.com/questions/126781/c-union-in-c

Discriminated union in C#

Given a folder path (like `C:\Random Folder`), how can I find a file in it that holds a certain extension, like `txt`? I assume I&#39;ll have to do a search for `*.txt` in the directory, but I&#39;m not sure how I&#39;m supposed to start this search in the first place.

Find a file with a certain extension in folder

&gt; **Possible Duplicate:**  
&gt; [How do I calculate someone&amp;rsquo;s age in C#?](https://stackoverflow.com/questions/9/how-do-i-calculate-someones-age-in-c)  

&lt;!-- End of automatically inserted text --&gt;

I want to calculate basically the age of employees -  So we have DOB for each employee, So on 
the C# Side I want to do something like this -

    int age=Convert.Int32(DateTime.Now-DOB);

I can use days and manipulate then get the age...but I wanted to know if there something I can use directly to get the number of years.


Calculate the difference between two dates and get the value in years?

&gt; **Possible Duplicate:**  
&gt; [What is the diffference between the | and || or operators?](https://stackoverflow.com/questions/35301/what-is-the-diffference-between-the-and-or-operators)  

&lt;!-- End of automatically inserted text --&gt;

Logical AND and OR:

    (x &amp; y)
    (x | y)

Conditional AND and OR:

    (x &amp;&amp; y)
    (x || y)

I&#39;ve only known about conditional operands up to this point. I know what it does and how to apply it in if-statements. But what is the purpose of logical operands?

What is the difference between logical and conditional AND, OR in C#?

I&#39;m using an object from a 3rd party API that has a property of type `System.Windows.Media.ImageSource`, yet I can&#39;t seem to find the System.Windows.Media namespace anywhere. If I try to add a reference to my project I don&#39;t see `System.Windows.Media` in the list of options. My project is also targeting .Net 3.5.


Is there something else I need to do to be able to access this namespace?

Can&#39;t find System.Windows.Media namespace?

I&#39;m using NHibernate 2.1.2.400 that is referencing log4net 1.2.10.0.
In the same project, I also use the simply accounting SDK, sadly it is still using log4net 1.2.9.0.

So I can get NHibernate to work if I reference log4net 1.2.10.0 but the simplySDK don&#39;t work.  And vice versa...

I&#39;m guessing most of the problems come from the fact that log4net has changed its assembly key. I tried using a binding redirection without success: the 2 DLLs do not have the same key.

I&#39;m considering recompiling NHibernate to use log4net 1.2.9.0 but it seems like the wrong thing to do and my feeling is that Simply Accounting won&#39;t be updating their SDK to use log4net 1.2.10.0 anytime soon.

What is the best way to handle this? Is it possible to resolve at all?

Referencing 2 different versions of log4net in the same solution

We all know [mutable structs are evil][1] in general. I&#39;m also pretty sure that because `IEnumerable&lt;T&gt;.GetEnumerator()` returns type `IEnumerator&lt;T&gt;`, the structs are immediately boxed into a reference type, costing more than if they were simply reference types to begin with.

So why, in the BCL generic collections, are all the enumerators mutable structs? Surely there had to have been a good reason. The only thing that occurs to me is that structs can be copied easily, thus preserving the enumerator state at an arbitrary point. But adding a `Copy()` method to the `IEnumerator` interface would have been less troublesome, so I don&#39;t see this as being a logical justification on its own.

Even if I don&#39;t agree with a design decision, I would like to be able to understand the reasoning behind it.

  [1]: https://ericlippert.com/2008/05/14/mutating-readonly-structs/

Why do BCL Collections use struct enumerators, not classes?

I&#39;ve been working for a while on my Windows Forms project, and I decided to experiment with keyboard shortcuts. After a bit of reading, I figured I had to just write an event handler and bind it to the form&#39;s KeyDown event:

    private void Form1_KeyDown(object sender, KeyEventArgs e)
    {
        if (e.Control &amp;&amp; e.Alt &amp;&amp; e.KeyCode == Keys.O)
        {
            MessageBox.Show(&quot;Ctrl+Alt+O: magic!&quot;);
        }
    }

I did that the good ol&#39; way of opening the Properties panel of the Visual Studio designer, then double-clicking on the KeyDown event of my form to generate the `Form1_KeyDown` event handler. But on testing my application, the form doesn&#39;t respond at all to the &lt;kbd&gt;Ctrl&lt;/kbd&gt;+&lt;kbd&gt;Alt&lt;/kbd&gt;+&lt;kbd&gt;O&lt;/kbd&gt; keyboard shortcut. The Visual Studio designer did generate the code to bind the event handler to the form though:

    private void InitializeComponent()
    {
        // ...

        this.KeyDown += new System.Windows.Forms.KeyEventHandler(this.Form1_KeyDown);

        // ...
    }

So I tried adding a `Console.WriteLine()` call to the handler to check that it was being called at all, but no luck on that either.

Also, I tried to set a breakpoint on the event binding call (shown just above) and found that the program reaches that breakpoint just fine. But any breakpoints I set within the method definition itself are never reached.

To make sure I was doing the first few steps correctly, I tried repeating them with:

 - **A new form in the same solution.**  
&lt;del&gt;Same issue: the form doesn&#39;t respond when I press my &lt;kbd&gt;Ctrl&lt;/kbd&gt;+&lt;kbd&gt;Alt&lt;/kbd&gt;+&lt;kbd&gt;O&lt;/kbd&gt; keyboard shortcut and the debugger isn&#39;t even stepping into the event handler.&lt;/del&gt; Tried this again and it works.

 - **A brand new WinForms solution.**  
It works perfectly: the message dialog appears (the `Console.WriteLine()` call also works).

So I&#39;m quite lost here. What&#39;s preventing all the forms in this one project from receiving KeyDown events?

Forms not responding to KeyDown events

&gt; **Possible Duplicate:**  
&gt; [How to get the MonthName in c#?](https://stackoverflow.com/questions/975531/how-to-get-the-monthname-in-c)  

&lt;!-- End of automatically inserted text --&gt;

I used the following c# syntax to get month name from month no but i get `August` i want only `Aug`..


    System.Globalization.DateTimeFormatInfo mfi = new 
    System.Globalization.DateTimeFormatInfo();
    string strMonthName = mfi.GetMonthName(8).ToString();

Any suggestion...

Get Month name from month number

I have a page where the input type always varies, and I need to get the values depending on the input type. So if the type is a radio, I need to get which is checked, and if it is a checkbox I need to now which are checked, and if it is a drop down I need to know which is selected, and I if it is a text/textarea I need to know the values.

Any idea on how to do that?

How to get input type using jquery?

What type for array index in C99 should be used? It have to work on LP32, ILP32, ILP64, LP64, LLP64 and more. It doesn&#39;t have to be a C89 type.

I have found 5 candidates:

* `size_t`
* `ptrdiff_t`
* `intptr_t` / `uintptr_t`
* `int_fast*_t` / `uint_fast*_t`
* `int_least*_t` / `uint_least*_t`

There is simple code to better illustrate problem. What is the best type for `i` and `j` in these two particular loops. If there is a good reason, two different types are fine too.

    for (i=0; i&lt;imax; i++) {
            do_something(a[i]);
    }
    /* jmin can be less than 0 */
    for (j=jmin; j&lt;jmax; j++) {
            do_something(a[j]);
    }

P.S.
In the first version of question I had forgotten about negative indexes.

P.P.S.
I am not going to write a C99 compiler. However any answer from a compiler programmer would be very valuable for me.

Similar question:

* https://stackoverflow.com/questions/1464174/size-t-vs-intptr-t  
The context of this question if different though.

What is the correct type for array indexes in C?

The following snippet is annotated with the output ([as seen on ideone.com](http://ideone.com/1z1r0)):

    print &quot;100&quot; &lt; &quot;2&quot;      # True
    print &quot;5&quot; &gt; &quot;9&quot;        # False
     
    print &quot;100&quot; &lt; 2        # False
    print 100 &lt; &quot;2&quot;        # True
     
    print 5 &gt; &quot;9&quot;          # False
    print &quot;5&quot; &gt; 9          # True

    print [] &gt; float(&#39;inf&#39;) # True
    print () &gt; []          # True

Can someone explain why the output is as such?

---

### Implementation details

* Is this behavior mandated by the language spec, or is it up to implementors?
* Are there differences between any of the major Python implementations?
* Are there differences between versions of the Python language?

How does Python 2 compare string and int? Why do lists compare as greater than numbers, and tuples greater than lists?

Given an arbitrary python object, what&#39;s the best way to determine whether it is a number? Here `is` is defined as `acts like a number in certain circumstances`.

For example, say you are writing a vector class. If given another vector, you want to find the dot product. If given a scalar, you want to scale the whole vector.

Checking if something is `int`, `float`, `long`, `bool` is annoying and doesn&#39;t cover user-defined objects that might act like numbers. But, checking for `__mul__`, for example, isn&#39;t good enough because the vector class I just described would define `__mul__`, but it wouldn&#39;t be the kind of number I want.

What is the most pythonic way to check if an object is a number?

Is `void` a data type in the C programming language? If so, what type of values can it store? If we have `int`, `float`, `char`, etc., to store values, why is `void` needed? And what is the range of void?

Is void a data type in C?

I was looking at [this article][1] and am struggling to follow the VB.NET example that explains lifted operators. There doesn&#39;t seem to be an equivalent C# example or tutorial. I don&#39;t have much experience with operator overloading in general, so trying to come to terms with the VB.NET equivalent whilst reading up on nullable types probably isn&#39;t the best place to start...

Would anyone be able to provide an explanation of lifted operators and how they are used by nullable types? Does it just mean that the nullable type does not itself overload operators and will use the operators from the underlying type that it represents?

There doesn&#39;t seem to be much information on SO about lifted operators, so hopefully this can help some others out too.

  [1]: http://msdn.microsoft.com/en-us/library/bb981315(VS.80).aspx#_Toc175387342

What are lifted operators?

In C#, say that you want to pull a value off of PropertyC in this example and ObjectA, PropertyA and PropertyB can all be null.

ObjectA.PropertyA.PropertyB.PropertyC

How can I get PropertyC safely with the least amount of code?

Right now I would check:

    if(ObjectA != null &amp;&amp; ObjectA.PropertyA !=null &amp;&amp; ObjectA.PropertyA.PropertyB != null)
    {
        // safely pull off the value
        int value = objectA.PropertyA.PropertyB.PropertyC;
    }

It would be nice to do something more like this (pseudo-code).

    int value = ObjectA.PropertyA.PropertyB ? ObjectA.PropertyA.PropertyB : defaultVal;

Possibly even further collapsed with a null-coalescing operator.

**EDIT** Originally I said my second example was like js, but I changed it to psuedo-code since it was correctly pointed out that it would not work in js.

C# elegant way to check if a property&#39;s property is null

let&#39;s say I have a column on my table defined the following:

    &quot;MyColumn&quot; smallint NULL

Storing a value like 0, 1 or something else should need 2 bytes (1). But how much space is needed if I set &quot;MyColumn&quot; to NULL? Will it need 0 bytes?

Are there some additional needed bytes for administration purpose or such things for every column/row?


(1) http://www.postgresql.org/docs/9.0/interactive/datatype-numeric.html



How much disk-space is needed to store a NULL value using postgresql DB?

I have an enum

    string name;

    public enum Color
    {
      Red,
      Green,
      Yellow
    }

How to set it to NULL on load.

    name = &quot;&quot;;
    Color color = null; //error


Edited: 
My bad, I didn&#39;t explain it properly. But all the answers related to nullable is perfect. My situation is What if, I have get/set for the enum in a class with other elements like name, etc. On page load I initiallize the class and try to default the values to null. Here is the scenario (Code is in C#):

    namespace Testing
    {
        public enum ValidColors
        {
            Red,
            Green,
            Yellow
        }
    
        public class EnumTest
        {
            private string name;
            private ValidColors myColor;
    
            public string Name
            {
                get { return name; }
                set { name = value; }
            }
    
            public ValidColors MyColor
            {
                get { return myColor; }
                set { myColor = value; }
            }
    
        }
    
        public partial class _Default : System.Web.UI.Page
        {       
            protected void Page_Load(object sender, EventArgs e)
            {
                EnumTest oEnumTest = new EnumTest();
                oEnumTest.Name = &quot;&quot;;
                oEnumTest.MyColor = null; //???
            }
        }
    
    }

Then using the suggestions below I changed the above code to make it work with get and set methods. I just need to add &quot;?&quot; in EnumTest class during declaration of private enum variable and in get/set method:


    public class EnumTest
    {
        private string name;
        private ValidColors? myColor; //added &quot;?&quot; here in declaration and in get/set method

        public string Name
        {
            get { return name; }
            set { name = value; }
        }

        public ValidColors? MyColor
        {
            get { return myColor; }
            set { myColor = value; }
        }

    }

Thanks all for the lovely suggestions.

How to set enum to null

I have two variables of type int? (or Nullable&amp;lt;int&amp;gt; if you will). I wanted to do a greater-than-or-equal (&gt;=) comparison on the two variables but as it turns out, this returns false if both variables are null, while obviously the == operator returns true.

Can someone explain to me why that is logical because the semantical definition of the &gt;= operator contains the word &quot;or&quot;?


Content Type	Original Author	Original Content on Stackoverflow
Question	Vaibhav Jain	View Question on Stackoverflow
Solution 1 - C#	kemiller2002	View Answer on Stackoverflow
Solution 2 - C#	ChrisF	View Answer on Stackoverflow
Solution 3 - C#	Stephen Cleary	View Answer on Stackoverflow
Solution 4 - C#	Matthew Abbott	View Answer on Stackoverflow
Solution 5 - C#	Damien Dennehy	View Answer on Stackoverflow
Solution 6 - C#	devlife	View Answer on Stackoverflow

Are nullable types reference types?

C# Problem Overview

C# Solutions

Solution 1 - C#

Solution 2 - C#

Solution 3 - C#

Solution 4 - C#

Solution 5 - C#

Solution 6 - C#

How can I easily fixup a past commit?

Should a Netflix or Twitter-style web service use REST or SOAP?

Attributions