This is a null coalescing operator. The method above states x is assigned y&#39;s value, unless y is null, in which case it is assigned z&#39;s value.

From &lt;a href=&quot;http://en.wikipedia.org/wiki/C_Sharp_syntax&quot;&gt;Wikipedia&lt;/a&gt;:

It&#39;s the null-coalesce operator and shorthand for this:

    x = (y != null ? y : z);

Use `y` if not `null`, otherwise use `z`.

If a the value y is null then the value z is assigned.

For example:

    x = Person.Name ?? &quot;No Name&quot;;

If name is null x will have the value &quot;No Name&quot;

As others have stated, it is the null coalescing operator.

MSDN information on this:

https://docs.microsoft.com/dotnet/csharp/language-reference/operators/null-coalescing-operator

.Net framework 2.0 onwards allow null values to Nullable value types.

here in this case, it says
x equals y if it has some value (ie not null) or else equals z



What option do I need to set to make a drop down box readonly when using MVCs Html.DropDownList?

I&#39;ve tried things like....

    Html.DropDownList(&quot;Types&quot;, Model.Types, new { _Enabled = &quot;false&quot; })

...and many different things along this line; alas no joy!

I thought this would be an easy.....and it probably is!

Html.DropDownList - Disabled/Readonly

For instance, to make something blue I would go:

    this.BackColor = Color.LightBlue;

How can I summon the Control color, the khaki one.

Thanks SO.

How can I call the Control color, I mean the default forms color?

&gt; **Possible Duplicates:**  
&gt; [?? Null Coalescing Operator --&amp;gt; What does coalescing mean?](https://stackoverflow.com/questions/770186/-null-coalescing-operator-what-does-coalescing-mean)  
&gt; [What do two question marks together mean in C#?](https://stackoverflow.com/questions/446835/what-do-two-question-marks-together-mean-in-c)  


I couldn&#39;t find this question being asked here so I figured I would ask it. What does a double question mark do in C#?

Example:

    x = y ?? z;

What does a double question mark do in C#?

> Possible Duplicates: 
> <a href="https://stackoverflow.com/questions/770186/-null-coalescing-operator-what-does-coalescing-mean" target="_blank" rel="noopener noreferrer">?? Null Coalescing Operator --> What does coalescing mean?</a> 
> <a href="https://stackoverflow.com/questions/446835/what-do-two-question-marks-together-mean-in-c" target="_blank" rel="noopener noreferrer">What do two question marks together mean in C#?</a>
I couldn't find this question being asked here so I figured I would ask it. What does a double question mark do in C#?
Example:
<pre><code class="hljs language-ini">x = y ?? z;
</code></pre>

The following code throws an compile-time error like

Cannot convert type &#39;string&#39; to &#39;int&#39;

    string name = Session[&quot;name1&quot;].ToString();
    int i = (int)name;
            
whereas the code below compiles and executes successfully:

    string name = Session[&quot;name1&quot;].ToString();
    int i = Convert.ToInt32(name);

I would like to know:

 1. Why does the the first code generate a compile-time error?

 2. What&#39;s the difference between the 2 code snippets?

What is the difference between Convert.ToInt32 and (int)?

How do I use `Assert.Throws` to assert the type of the exception and the actual message wording?

Something like this:

    Assert.Throws&lt;Exception&gt;(
        ()=&gt;user.MakeUserActive()).WithMessage(&quot;Actual exception message&quot;)

The method I am testing throws multiple messages of the same type, with different messages, and I need a way to test that the correct message is thrown depending on the context.


How do I use Assert.Throws to assert the type of the exception?

I have an array and I want to divide them into page according to preset page size.

This is how I do:
     
    private int CalcPagesCount()
    {
        int  totalPage = imagesFound.Length / PageSize;
    
        // add the last page, ugly
        if (imagesFound.Length % PageSize != 0) totalPage++;
        return totalPage;
    }

I feel the calculation is not the simplest (I am poor in math), can you give one simpler calculation formula?



The simplest formula to calculate page count?

I am trying to truncate some long text in C#, but I don&#39;t want my string to be cut off part way through a word. Does anyone have a function that I can use to truncate my string at the end of a word?

E.g:

    &quot;This was a long string...&quot;

Not:

    &quot;This was a long st...&quot;

Truncate string on whole words in .NET C#

I have this : `Datetime.Now();` or `23/10/2009`  
I want this : `Friday`

For local date-time (GMT-5) and using Gregorian calendar.

How to get the day name from a selected date?

Something I find myself doing more and more is checking a string for empty (as in `&quot;&quot;` or null) and a conditional operator.

A current example:

    s.SiteNumber.IsNullOrEmpty() ? &quot;No Number&quot; : s.SiteNumber;

This is just an extension method, it&#39;s equivalent to:

    string.IsNullOrEmpty(s.SiteNumber) ? &quot;No Number&quot; : s.SiteNumber;

Since it&#39;s empty and not null, `??` won&#39;t do the trick.  A `string.IsNullOrEmpty()` version of `??` would be the perfect solution.  I&#39;m thinking there has to be a cleaner way of doing this (I hope!), but I&#39;ve been at a loss to find it.  

Does anyone know of a better way to do this, even if it&#39;s only in .Net 4.0?

?? Coalesce for empty string?

null coalescing translates roughly to `return x, unless it is null, in which case return y`

I often need `return null if x is null, otherwise return x.y`

I can use `return x == null ? null : x.y;`

Not bad, but that `null` in the middle always bothers me -- it seems superfluous. I&#39;d prefer something like `return x :: x.y;`, where what follows the `::` is evaluated only if what precedes it is not `null`.

I see this as *almost* an opposite to null coalescence, kind of mixed in with a terse, inline null-check, but I&#39;m [*almost*] certain that there is no such operator in C#.

### Are there other languages that have such an operator? If so, what is it called? ###


*(I know that I can write a method for it in C#; I use `return NullOrValue.of(x, () =&gt; x.y);`, but if you have anything better, I&#39;d like to see that too.)*

Is there an &quot;opposite&quot; to the null coalescing operator? (…in any language?)

In C# there&#39;s a [null-coalescing operator][1] (written as `??`) that allows for easy (short) null checking during assignment:

    string s = null;
    var other = s ?? &quot;some default value&quot;;

Is there a python equivalent?

I know that I can do:

    s = None
    other = s if s else &quot;some default value&quot;

But is there an even shorter way (where I don&#39;t need to repeat `s`)?


  [1]: http://msdn.microsoft.com/en-us/library/ms173224.aspx

Is there a Python equivalent of the C# null-coalescing operator?

Is it possible to do something similar to the following code in Java

    int y = x ?? -1;


[More about `??`][1]


  [1]: https://stackoverflow.com/a/446839

is there a Java equivalent to null coalescing operator (??) in C#?

**Note: this appears to have been fixed in [Roslyn][1]**

This question arose when writing my answer to [this one][2], which talks about the associativity of the [null-coalescing operator][3].

Just as a reminder, the idea of the null-coalescing operator is that an expression of the form

    x ?? y

first evaluates `x`, then:

- If the value of `x` is null, `y` is evaluated and that is the end result of the expression
- If the value of `x` is non-null, `y` is *not* evaluated, and the value of `x` is the end result of the expression, after a conversion to the compile-time type of `y` if necessary

Now *usually* there&#39;s no need for a conversion, or it&#39;s just from a nullable type to a non-nullable one - usually the types are the same, or just from (say) `int?` to `int`. However, you *can* create your own implicit conversion operators, and those are used where necessary.

For the simple case of `x ?? y`, I haven&#39;t seen any odd behaviour. However, with `(x ?? y) ?? z` I see some confusing behaviour.

Here&#39;s a short but complete test program - the results are in the comments:

    using System;
    
    public struct A
    {
        public static implicit operator B(A input)
        {
            Console.WriteLine(&quot;A to B&quot;);
            return new B();
        }
    
        public static implicit operator C(A input)
        {
            Console.WriteLine(&quot;A to C&quot;);
            return new C();
        }
    }
    
    public struct B
    {
        public static implicit operator C(B input)
        {
            Console.WriteLine(&quot;B to C&quot;);
            return new C();
        }
    }
    
    public struct C {}
    
    class Test
    {
        static void Main()
        {
            A? x = new A();
            B? y = new B();
            C? z = new C();
            C zNotNull = new C();
    
            Console.WriteLine(&quot;First case&quot;);
            // This prints
            // A to B
            // A to B
            // B to C
            C? first = (x ?? y) ?? z;
    
            Console.WriteLine(&quot;Second case&quot;);
            // This prints
            // A to B
            // B to C
            var tmp = x ?? y;
            C? second = tmp ?? z;
    
            Console.WriteLine(&quot;Third case&quot;);
            // This prints
            // A to B
            // B to C
            C? third = (x ?? y) ?? zNotNull;
        }
    }

So we have three custom value types, `A`, `B` and `C`, with conversions from A to B, A to C, and B to C.

I can understand both the second case and the third case... but *why* is there an extra A to B conversion in the first case? In particular, I&#39;d *really* have expected the first case and second case to be the same thing - it&#39;s just extracting an expression into a local variable, after all.

Any takers on what&#39;s going on? I&#39;m extremely hesistant to cry &quot;bug&quot; when it comes to the C# compiler, but I&#39;m stumped as to what&#39;s going on...

EDIT: Okay, here&#39;s a nastier example of what&#39;s going on, thanks to configurator&#39;s answer, which gives me further reason to think it&#39;s a bug. EDIT: The sample doesn&#39;t even need two null-coalescing operators now...

    using System;
    
    public struct A
    {
        public static implicit operator int(A input)
        {
            Console.WriteLine(&quot;A to int&quot;);
            return 10;
        }
    }
    
    class Test
    {
        static A? Foo()
        {
            Console.WriteLine(&quot;Foo() called&quot;);
            return new A();
        }
        
        static void Main()
        {
            int? y = 10;
            
            int? result = Foo() ?? y;
        }
    }

The output of this is:

    Foo() called
    Foo() called
    A to int

The fact that `Foo()` gets called twice here is hugely surprising to me - I can&#39;t see any reason for the expression to be *evaluated* twice.


  [1]: https://github.com/dotnet/roslyn
  [2]: https://stackoverflow.com/questions/6238074
  [3]: http://msdn.microsoft.com/en-us/library/ms173224.aspx

Curious null-coalescing operator custom implicit conversion behaviour

How can I write a shorthand of the following scenario?

    get
    {
        if (_rows == null)
        {
            _rows = new List&lt;Row&gt;();
        }

        return _rows;
    }



Is there a shorthand way to return values that might be null?

While I was studying the _delegate_ which _is_ actually an abstract class in [`Delegate.cs`][1], I saw the following method in which I don&#39;t understand

 - Why the return value uses `?` though it&#39;s already a reference(_class_) type
 - `?[]?` meaning on the parameter

Could you explain?

    public static Delegate? Combine(params Delegate?[]? delegates)
    {
        if (delegates == null || delegates.Length == 0)
            return null;

        Delegate? d = delegates[0];
        for (int i = 1; i &lt; delegates.Length; i++)
            d = Combine(d, delegates[i]);

        return d;
    }

  [1]: https://github.com/dotnet/corert/blob/master/src/System.Private.CoreLib/shared/System/Delegate.cs


Content Type	Original Author	Original Content on Stackoverflow
Question	tom d	View Question on Stackoverflow
Solution 1 - C#	Alexis Abril	View Answer on Stackoverflow
Solution 2 - C#	Austin Salonen	View Answer on Stackoverflow
Solution 3 - C#	CodeSpeaker	View Answer on Stackoverflow
Solution 4 - C#	Michael Edwards	View Answer on Stackoverflow
Solution 5 - C#	Spencer Ruport	View Answer on Stackoverflow
Solution 6 - C#	itsmatt	View Answer on Stackoverflow
Solution 7 - C#	123Developer	View Answer on Stackoverflow

What does a double question mark do in C#?

C# Problem Overview

C# Solutions

Solution 1 - C#

Solution 2 - C#

Solution 3 - C#

Solution 4 - C#

Solution 5 - C#

Solution 6 - C#

Solution 7 - C#

How can I call the Control color, I mean the default forms color?

Html.DropDownList - Disabled/Readonly

Attributions