You need `Type.GetElementType()` to get the non-array type:

    object x = new Int32[7];
    Type t = x.GetType();
    object y = Array.CreateInstance(t.GetElementType(), 7);

Alternatively, if you can get the type of the element directly, use that:

    Type t = typeof(int);
    object y = Array.CreateInstance(t, 7);

Basically, `Array.CreateInstance` needs the element type of the array to create, not the final array type.

Just to add to Jon&#39;s answer. The reason attempt 1 fails is because there&#39;s no default constructor for `Int32[]`. You need to supply a length. If you use the overload, which takes an array of arguments it will work:

    // attempt1 
    object y1 = Activator.CreateInstance(t, new object[] { 1 }); // Length 1

Is there any convenient way to automatically parse command line arguments passed to R scripts?

Something like perl&#39;s `Getopt::Long`?


Parsing command line arguments in R scripts

I want to find out if there could ever be _conflicts_ between two known regular expressions, in order to allow the user to construct a list of mutually exclusive regular expressions.

For example, we know that the regular expressions below are quite different but they both match `xy50`:

    &#39;^xy1\d&#39;
    &#39;[^\d]\d2$&#39;

Is it possible to determine, using a computer algorithm, if two regular expressions can have such a _conflict_? How?

Regex: Determine if two regular expressions could match for the same input?

I can&#39;t figure out how to make this work:

    object x = new Int32[7];
    Type t = x.GetType();

    // now forget about x, and just use t from here.
    
    // attempt1 
    object y1 = Activator.CreateInstance(t); // fails with exception
    
    // attempt2
    object y2 = Array.CreateInstance(t, 7);  // creates an array of type Int32[][] ! wrong


What&#39;s the secret sauce?  I can make the second one work if I can get the type of the elements of the array, but I haven&#39;t figured that one out either.

How do I create a C# array using Reflection and only type info?

I can't figure out how to make this work:
<pre><code class="hljs language-ini">object x = new Int32[7];
Type t = x.GetType();

// now forget about x, and just use t from here.

// attempt1 
object y1 = Activator.CreateInstance(t); // fails with exception

// attempt2
object y2 = Array.CreateInstance(t, 7); // creates an array of type Int32[][] ! wrong
</code></pre>
What's the secret sauce? I can make the second one work if I can get the type of the elements of the array, but I haven't figured that one out either.

I have a form containing a `TextBox` in C# which I set to a string as follows:

    textBox.Text = str;

When the form is displayed, why does the text in the texbox appear highlighted/selected?

Why is text in TextBox highlighted (selected) when form is displayed?

Anyone out there using the new CSHTML pages feature and is finding that they prefer this new view engine syntax over the existing ASP.NET MVC default view engine or over web forms, and if so, why?  What about CSHTML gives you an advantage over MVC or web forms, or vice versa?

Just curious to hear people&#39;s take on it.

Razor/CSHTML - Any Benefit over what we have?

I&#39;m trying to include in my XAML some classes which convert values.  However, I&#39;m getting the following error when I compile:

&gt; Undefined CLR namespace. The &#39;clr-namespace&#39; URI refers to a namespace &#39;View.Summary.Converters&#39; that is not included in the assembly.(View\View)

And the XAML it&#39;s erroring on:

    xmlns:c=&quot;clr-namespace:View.Summary.Converters&quot;

Also, here is the outline of my conversion classes/namespace:

    namespace View.Summary.Converters
    {
        class CollapsedIfNegative : IValueConverter { }
    
        class VisibleIfNegative : IValueConverter { }
    
        class ErrorCodeToString : IValueConverter { }
    }

I had to remove the guts of the code because the project I am working on is highly confidential.

The &#39;clr-namespace&#39; URI refers to a namespace that is not included in the assembly

I want the user to be able to put the cell into editing mode and highlight the row the cell is contained in with a single click. By default, this is double click. 

How do I override or implement this?

Single click edit in WPF DataGrid

I read some articles about the `volatile` keyword but I could not figure out its correct usage.  Could you please tell me what it should be used for in C# and in Java?

What is the &quot;volatile&quot; keyword used for?

This is a simple problem, and I&#39;ve done it before. I just can&#39;t remember how, or what exactly it was called.

In python I can do this:

    arr = [&#39;one&#39;, &#39;two&#39;]
    one, two = arr

how do I do that in JavaScript?

Unpacking array into separate variables in JavaScript

So I&#39;m declaring and initializing an int array:

    static final int UN = 0;
    int[] arr = new int[size];
    for (int i = 0; i &lt; size; i++) {
    	arr[i] = UN;
    }

Say I do this instead... 

    int[] arr = new int[5];
    System.out.println(arr[0]);

... `0` will print to standard out. Also, if I do this:

    static final int UN = 0;
    int[] arr = new int[5];
    System.out.println(arr[0]==UN);

... `true` will print to standard out. So how is Java initializing my array by default? Is it safe to assume that the default initialization is setting the array indices to `0` which would mean I don&#39;t have to loop through the array and initialize it? 

Thanks.

What is the default initialization of an array in Java?

First of all, I understand in 90% of applications the performance difference is completely irrelevant, but I just need to know which is the faster construct. That and...

The information currently available on them on the net is confusing. A lot of people say foreach is bad, but technically it should be faster since it&#39;s suppose to simplify writing a array traversal using iterators. Iterators, which are again suppose to be faster, but in PHP are also apparently dead slow (or is this not a PHP thing?). I&#39;m talking about the array functions: next() prev() reset() etc. well, if they are even functions and not one of those PHP language features that look like functions.

_To narrow this down a little_: I&#39;m not interesting in traversing arrays in steps of anything more than 1 (no negative steps either, ie. reverse iteration). I&#39;m also not interested in a traversal to and from arbitrary points, just 0 to length. I also don&#39;t see manipulating arrays with more than 1000 keys happening on a regular basis, but I do see a array being traversed multiple times in the logic of a application! Also as for operations, largely only string manipulation and echo&#39;ing.

Here are few reference sites:  
[http://www.phpbench.com/](http://www.phpbench.com/)  
[http://www.php.lt/benchmark/phpbench.php](http://www.php.lt/benchmark/phpbench.php)

What I hear everywhere:
&lt;ul&gt;
&lt;li&gt;&lt;code&gt;foreach&lt;/code&gt; is slow, and thus &lt;code&gt;for&lt;/code&gt;/&lt;code&gt;while&lt;/code&gt; is faster&lt;/li&gt;
&lt;li&gt;PHPs &lt;code&gt;foreach&lt;/code&gt; copies the array it iterates over; to make it faster you need to use references&lt;/li&gt;
&lt;li&gt;code like this: &lt;code&gt;$key = array_keys($aHash); $size = sizeOf($key); &lt;br /&gt;for ($i=0; $i &amp;lt; $size; $i++)&lt;/code&gt; is faster than a &lt;code&gt;foreach&lt;/code&gt;&lt;/li&gt;
&lt;/ul&gt;

Here&#39;s my problem. I wrote this test script: http://pastebin.com/1ZgK07US and no matter how many times I run the script, I get something like this:

    foreach 1.1438131332397
    foreach (using reference) 1.2919359207153
    for 1.4262869358063
    foreach (hash table) 1.5696921348572
    for (hash table) 2.4778981208801

In short:
&lt;ul&gt;
&lt;li&gt;&lt;code&gt;foreach&lt;/code&gt; is faster than &lt;code&gt;foreach&lt;/code&gt; with reference&lt;/li&gt;
&lt;li&gt;&lt;code&gt;foreach&lt;/code&gt; is faster than &lt;code&gt;for&lt;/code&gt;&lt;/li&gt;
&lt;li&gt;&lt;code&gt;foreach&lt;/code&gt; is faster than &lt;code&gt;for&lt;/code&gt; for a hash table&lt;/li&gt;
&lt;/ul&gt;

Can someone explain?
&lt;ol&gt;
&lt;li&gt;Am I doing something wrong?&lt;/li&gt;
&lt;li&gt;Is PHP foreach reference thing really making a difference? I mean why would it not copy it if you pass by reference?&lt;/li&gt;
&lt;li&gt;What&#39;s the equivalent iterator code for the foreach statement; I&#39;ve seen a few on the net but each time I test them the timing is way off; I&#39;ve also tested a few simple iterator constructs but never seem to get even decent results -- are the array iterators in PHP just awful?&lt;/li&gt;
&lt;li&gt;Are there faster ways/methods/constructs to iterate though a array other than FOR/FOREACH (and WHILE)?&lt;/li&gt;
&lt;/ol&gt;

PHP Version 5.3.0
&lt;hr /&gt;
&lt;strong&gt;Edit: Answer&lt;/strong&gt;
With help from people here I was able to piece together the answers to all question. I&#39;ll summarize them here:  

&lt;ol&gt;
&lt;li&gt;&lt;em&gt;&quot;Am I doing something wrong?&quot;&lt;/em&gt; The consensus seems to be: yes, I can&#39;t use echo in benchmarks. Personally, I still don&#39;t see how echo is some function with random time of execution or how any other function is somehow any different -- that and the ability of that script to just generate the exact same results of foreach better than everything is hard to explain though just &quot;you&#39;re using echo&quot; (well what should I have been using). However, I concede the test should be done with something better; though a ideal compromise does not come to mind.&lt;/li&gt;
&lt;li&gt;&lt;em&gt;&quot;Is PHP foreach reference thing really making a difference? I mean why would it not copy it if you pass by reference?&quot;&lt;/em&gt; ircmaxell shows that yes it is, further testing seems to prove in most cases reference should be faster -- though given my above snippet of code, most definitely doesn&#39;t mean all. I accept the issue is probably too non-intuitive to bother with at such a level and would require something extreme such as decompiling to actually determine which is better for each situation.&lt;/li&gt;
&lt;li&gt;&lt;em&gt;&quot;What&#39;s the equivalent iterator code for the foreach statement; I&#39;ve seen a few on the net but each time I test them the timing is way off; I&#39;ve also tested a few simple iterator constructs but never seem to get even decent results -- are the array iterators in PHP just awful?&quot;&lt;/em&gt; ircmaxell provided the answer bellow; though the code might only be valid for PHP version &gt;= 5&lt;/li&gt;
&lt;li&gt;&lt;em&gt;&quot;Are there faster ways/methods/constructs to iterate though a array other than FOR/FOREACH (and WHILE)?&quot;&lt;/em&gt; Thanks go to Gordon for the answer. Using new data types in PHP5 should give either a performance boost or memory boost (either of which might be desirable depending on your situation). While speed wise a lot of the new types of array don&#39;t seem to be better than array(), the splpriorityqueue and splobjectstorage do seem to be substantially faster. Link provided by Gordon: &lt;a href=&quot;http://matthewturland.com/2010/05/20/new-spl-features-in-php-5-3/&quot;&gt;http://matthewturland.com/2010/05/20/new-spl-features-in-php-5-3/&lt;/a&gt;&lt;/li&gt;
&lt;/ol&gt;

I&#39;ll likely stick to foreach (the non-reference version) for any simple traversal.

Performance of FOR vs FOREACH in PHP

While browsing some source code I came across a function like this:

    void someFunction(char someArray[static 100])
    {
        // do something cool here
    }

With some experimentation it appears other qualifiers may appear there too:

    void someFunction(char someArray[const])
    {
        // do something cool here
    }

It appears that qualifiers are only allowed inside the `[` `]` when the array is declared as a parameter of a function. What do these do? Why is it different for function parameters?

What is the purpose of static keyword in array parameter of function like &quot;char s[static 10]&quot;?

What exactly is the difference between `array_map`, `array_walk` and `array_filter`. What I could see from documentation is that you could pass a callback function to perform an action on the supplied array. But I don&#39;t seem to find any particular difference between them.

Do they perform the same thing?  
Can they be used interchangeably?

I would appreciate your help with illustrative example if they are different at all.


Difference between array_map, array_walk and array_filter

I have the following class:

    public class Test {
        public static int a = 0;
        public int b = 1;
    }

Is it possible to use reflection to get a list of the static fields only? I&#39;m aware I can get an array of all the fields with `Test.class.getDeclaredFields()`. But it seems there&#39;s no way to determine if a `Field` instance represents a static field or not.

Retrieve only static fields declared in Java class

I have a custom attribute:

    public class MenuItemAttribute : Attribute
    {
    }

and a class with a few methods:

    public class HelloWorld
    {
        [MenuItemAttribute]
        public void Shout()
        {
        }

        [MenuItemAttribute]
        public void Cry()
        {
        }

        public void RunLikeHell()
        {
        }
    }

How can I get only the methods that are decorated with the custom attribute?

So far, I have this:

    string assemblyName = fileInfo.FullName;
    byte[] assemblyBytes = File.ReadAllBytes(assemblyName);
    Assembly assembly = Assembly.Load(assemblyBytes);

    foreach (Type type in assembly.GetTypes())
    {
         System.Attribute[] attributes = System.Attribute.GetCustomAttributes(type);

         foreach (Attribute attribute in attributes)
         {
             if (attribute is MenuItemAttribute)
             {
                 //Get me the method info
                 //MethodInfo[] methods = attribute.GetType().GetMethods();
             }
         }
    }


What I need now is to get the method name, the return type, as well as the parameters it accepts.


Find methods that have custom attribute using reflection

I have a method where I need to resolve the Type of a class. This class exists in another assembly with the namespace similar to:

    MyProject.Domain.Model

I am attempting to perform the following:

    Type.GetType(&quot;MyProject.Domain.Model.&quot; + myClassName);

This works great if the code that is performing this action is in the same assembly as the class whose type I am trying to resolve, however, if my class is in a different assembly, this code fails.

I am sure there is a far better way to accomplish this task, but I have not had a lot of experience with resolving assemblies and traversing namespaces within to resolve the type of the class I am looking for. Any advice or tips to accomplish this task more gracefully?

Resolve Type from Class Name in a Different Assembly

I want to convert a string to an object property value, whose name I have as a string. I am trying to do this like so:

    string modelProperty = &quot;Some Property Name&quot;;
    string value = &quot;SomeValue&quot;;
    var property = entity.GetType().GetProperty(modelProperty);
    if (property != null) {
        property.SetValue(entity, 
            Convert.ChangeType(value, property.PropertyType), null);
    }

The problem is this is failing and throwing an Invalid Cast Exception when the property type is a nullable type.  This is not the case of the values being unable to be Converted - they will work if I do this manually (e.g. `DateTime? d = Convert.ToDateTime(value);`)  I&#39;ve seen some similiar questions but still can&#39;t get it to work.

Convert.ChangeType() fails on Nullable Types

I have a simple Enum

     public enum TestEnum
     {
         TestOne = 3,
         TestTwo = 4
     }

    var testing = TestEnum.TestOne;

And I want to retrieve its value (3) via reflection. Any ideas on how to do this?

Getting Enum value via reflection

Why is type inference not supported for constructors the way it is for generic methods? 

    public class MyType&lt;T&gt;
    {
       private readonly T field;
       public MyType(T value) { field = value; }
    }

    var obj = new MyType(42); // why can&#39;t type inference work out that I want a MyType&lt;int&gt;?

Though you could get around this with a factory class,

    public class MyTypeFactory
    {
       public static MyType&lt;T&gt; Create&lt;T&gt;(T value)
       {
          return new MyType&lt;T&gt;(value);
       }
    }
    var myObj = MyTypeFactory.Create(42);

Is there a practical or philosophical reason why the constructor can&#39;t support type inference?

Why can&#39;t the C# constructor infer type?

I would like to instantiate an object from its `Class` object, using the constructor that accepts a single `String` argument.

Here is some code that approaches what I want: 

    Object object = null;
    Class classDefinition = Class.forName(&quot;javax.swing.JLabel&quot;);
    object = classDefinition.newInstance();

However, it instantiates the `JLabel` object with no text.  I would like to use the `JLabel` constructor that accepts a string as the initial text.  Is there a way to select a specific constructor from a `Class` object?

Instantiate a class object with constructor that accepts a string parameter?

Is it not supported, is it supported but I have to do some tricks?

Example:

    class Foo
    {
      public Foo&lt;T1,T2&gt;(Func&lt;T1,T2&gt; f1,Func&lt;T2,T1&gt; f2)
      {
         ...
      }
    }

the generics are only used in constructor, there is no field/property depended on them, I use it (generics) to enforce the type correlation for f1 and f2.

**Remark**: I found the workaround -- static method Create, but anyway I am curious why I have problem with straightforward approach.

Is generic constructor in non-generic class supported?

For a constructor with a single parameter, is it OK to throw an ArgumentNullException inside the constructor if the parameter is null/empty? OR, should it be thrown in the method that actually uses the argument? Thanks.

Throwing ArgumentNullException in constructor?

The whole question is in the title. For example:

    enum enumTest {
    
            TYPE1(4.5, &quot;string1&quot;), TYPE2(2.79, &quot;string2&quot;);
            double num;
            String st;
    
            enumTest(double num, String st) {
                this.num = num;
                this.st = st;
            }
        }

The constructor is fine with the default or `private` modifier, but gives me a compiler error if given the `public` or `protected` modifiers.


Content Type	Original Author	Original Content on Stackoverflow
Question	Mark Lakata	View Question on Stackoverflow
Solution 1 - C#	Jon Skeet	View Answer on Stackoverflow
Solution 2 - C#	Brian Rasmussen	View Answer on Stackoverflow

How do I create a C# array using Reflection and only type info?

C# Problem Overview

C# Solutions

Solution 1 - C#

Solution 2 - C#

Regex: Determine if two regular expressions could match for the same input?

Parsing command line arguments in R scripts

Attributions