I have tried this:
public static class ListHelper
{
public static string ToString<T>(this IList<String> list)
{
return string.Join(", ", list.ToArray());
}
public static string ToString<T>(this String[] array)
{
return string.Join(", ", array);
}
}
But it does not work, both for string[] and List<string>. Maybe I need some special annotations?
Extension methods are only checked if there are no applicable candidate methods that match. In the case of a call to ToString() there will always be an applicable candidate method, namely, the ToString() on object. The purpose of extension methods is to extend the set of methods available on a type, not to override existing methods; that's why they're called "extension methods". If you want to override an existing method then you'll have to make an overriding method.
It sounds like you want to replace what files.ToString() returns. You will not be able to do that without writing a custom class to assign files as (i.e. inherit from List and override ToString().)
First, get rid of the generic type (<T>), you're not using it. Next, you will need to rename the extension method because calling files.ToString()will just call the List's ToString method.
This does what you're looking for.
static class Program
{
static void Main()
{
var list = new List<string> { {"a"}, {"b"}, {"c"} };
string str = list.ToStringExtended();
}
}
public static class ListHelper
{
public static string ToStringExtended(this IList<String> list)
{
return string.Join(", ", list.ToArray());
}
}
Simply you Shouldn't use the name ToString for the Extension method as it will never be called because that method already exist and you shouldn't use T as its useless there.
For example i tried this and again it returned same thing:
Console.WriteLine(lst.ToString<int>());
output:
shekhar, shekhar, shekhar, shekhar
so this time i used int and it still ran because that T has no use other then changing the Method Prototype.
So simply why are you using ToString Literal as Method name, as it already exist and you can't override it in a Extension method, this is the reason you had to use that T to make it generic. Use some different name like
public static string ToMyString(this IList<String> list)
That way you wouldn't have to use generic as it useless there and you could simply call it as always.
That said your code is working for me. here is what i tried (in LINQPAD):
void Main()
{
List<string> lst = new List<string>();
lst.Add("shekhar");
lst.Add("shekhar");
lst.Add("shekhar");
lst.Add("shekhar");
lst.ToString<string>().Dump();
}
public static class ListHelper
{
public static string ToString<T>(this IList<String> list)
{
return string.Join(", ", list.ToArray());
}
public static string ToString<T>(this String[] array)
{
return string.Join(", ", array);
}
}
And the output was shekhar, shekhar, shekhar, shekhar
Since you have specified that T in ToString<T> you will need to mention a Type like string or int while calling the ToString method.
Related
How can I get both these two methods to compile?
public static IEnumerable<string> DoSomething(params string[] args)
{ // do something }
public static IEnumerable<string> DoSomething(this string[] args)
{ // do something }
I get this compile error:
Type 'Extensions' already defines a member called 'DoSomething' with the same parameter types Extensions.cs
So that I can do this:
new string[] { "", "" }.DoSomething();
Extensions.DoSomething("", "");
Without the params method, I have to do this:
Extensions.DoSomething(new string[] { "", "" });
Update: Based on the answer by O. R. Mapper
public static IEnumerable<string> DoSomething(string arg, params string[] args)
{
// args null check is not required
string[] argscopy = new string[args.Length + 1];
argscopy[0] = arg;
Array.Copy(args, 0, argscopy, 1, args.Length);
return argscopy.DoSomething();
}
Update: I like HugoRune's answer now.
You can add an additional parameter to the params version:
public static IEnumerable<string> DoSomething(string firstArg, params string[] moreArgs)
That should be sufficient for the compiler to distinguish it from the string[] extension method.
As suggested by user SLaks, an additional overload without any arguments should be provided in this case, if the situation with an empty params array needs to be supported:
public static IEnumerable<string> DoSomething()
Late answer:
Another option is to just put both methods in different classes. Since you never us the class name when calling the extension method (the one with the this parameter), the extension method can be in any public static class in the same namespace, without any noticeable difference.
// contains static methods to help with strings
public static class StringTools
{
public static IEnumerable<string> DoSomething(params string[] args)
{
// do something
}
}
// contains only extension methods
public static class StringToolsExtensions
{
public static IEnumerable<string> DoSomething(this string[] args)
{
return StringTools.DoSomething(args);
}
}
This way you avoid copying the string array, you do not need an additional overload with no arguments, and I would say it looks cleaner. I would always separate extension methods and other static methods to avoid confusion.
You can give one of the two methods a different name. i.e. DoSomething2
You can just use one method. It's the same method with the same parameter list; clearly they're doing the same thing (since you didn't give them different names as per #1). Just combine them.
You can change the parameter list of one of the methods. i.e. (this string[] args, object unusedParameter)
Consider the following method example:
public static string[] ParseOptions()
{
return Environment.GetCommandLineArgs();
}
What would I have to do to create an extension that would make ParseOptions() return all command line arguments in lower case?
I would like to be able to use the extension as follows:
var argArray = ParseOptions().MyExtToLower();
Note: I'm asking this to better understand how to create an extension for a method. I'm not actually interested in getting lower case command line arguments this way.
public static string[] MyExtToLower(this string[] source)
{
for (int i = 0; i < source.Length; i++)
{
source[i] = source[i].ToLower();
}
return source;
}
Notice the this keyword in the parameter list. That is what makes it possible to call the method like this:
var argArray = ParseOptions().MyExtToLower();
To be clear, you're not actually adding an extension to a method here. What you are doing is adding an extension to the type that the method returns.
You seem to be talking about Fluent Interfaces. Look at this example - http://blog.raffaeu.com/archive/2010/06/26/how-to-write-fluent-interface-with-c-and-lambda.aspx
Or, you can create extension methods on the type you are returning ( in your case, string[] ) to get the method chaining - http://msdn.microsoft.com/en-us/library/bb383977.aspx
For the syntax you describe you would have to extend String[] or possibly IEnumerable<String> the following way:
public static class MyExtensions {
public static String[] MyExtToLower(this String[] strings) {
return strings.Select(s => s.toLower()).ToArray();
}
public static IEnumerable<String> MyExtToLower(this IEnumerable<String> strings) {
return strings.Select(s => s.toLower());
}
}
You don't create extension of methods, you create methods which extend objects' capabilities. Those methods must be static and part of a static class. They must have one parameter marked with the this keyword to indicate which object you want to extend. In your case, you must write something like:
// the class must be static, I usually declare a class reserved for extension method.
// I mark it as partial so that I can put every method in the same file where I use it.
public static partial class Extension {
// This is the extension method; it must be static. Note the 'this' keyword before
// the first parameter: it tells the compiler extends the string[] type.
public static MyExtToLower( this string[ ] args ) {
// your code
}
}
Note that you cannot override instance method. Altough you can have a method with the same signature as an instance method that method will be never called due to the way the compiler binds to.
I need to Trim a String. But I want to remove all the repeated blank spaces within the String itself, not only at the end or at the start of it. I could do it with a method like:
public static string ConvertWhitespacesToSingleSpaces(string value)
{
value = Regex.Replace(value, #"\s+", " ");
}
Which I got from here. But I want this piece of code to be called within the String.Trim() itself, so I think I need to extend or overload or override the Trim method... Is there a way to do that?
Since you cannot extend string.Trim(). You could make an Extension method as described here that trims and reduces whitespace.
namespace CustomExtensions
{
//Extension methods must be defined in a static class
public static class StringExtension
{
// This is the extension method.
// The first parameter takes the "this" modifier
// and specifies the type for which the method is defined.
public static string TrimAndReduce(this string str)
{
return ConvertWhitespacesToSingleSpaces(str).Trim();
}
public static string ConvertWhitespacesToSingleSpaces(this string value)
{
return Regex.Replace(value, #"\s+", " ");
}
}
}
You can use it like so
using CustomExtensions;
string text = " I'm wearing the cheese. It isn't wearing me! ";
text = text.TrimAndReduce();
Gives you
text = "I'm wearing the cheese. It isn't wearing me!";
Is it possible? Yes, but only with an extension method
The class System.String is sealed so you can't use overriding or inheritance.
public static class MyStringExtensions
{
public static string ConvertWhitespacesToSingleSpaces(this string value)
{
return Regex.Replace(value, #"\s+", " ");
}
}
// usage:
string s = "test !";
s = s.ConvertWhitespacesToSingleSpaces();
There's a yes and a no to your question.
Yes, you can extend existing types by using extension methods. Extension methods, naturally, can only access the public interface of the type.
public static string ConvertWhitespacesToSingleSpaces(this string value) {...}
// some time later...
"hello world".ConvertWhitespacesToSingleSpaces()
No, you cannot call this method Trim(). Extension methods do not participate in overloading. I think a compiler should even give you a error message detailing this.
Extension methods are only visible if the namespace containing the type that defines the method is using'ed.
Extension methods!
public static class MyExtensions
{
public static string ConvertWhitespacesToSingleSpaces(this string value)
{
return Regex.Replace(value, #"\s+", " ");
}
}
Besides using extension methods -- likely a good candidate here -- it is also possible to "wrap" an object (e.g. "object composition"). As long as the wrapped form contains no more information than the thing being wrapped then the wrapped item may be cleanly passed through implicit or explicit conversions with no loss of information: just a change of type/interface.
Happy coding.
I have code that I want to make the following changes:
How do I override ToString()? It says: A static member ...ToString(System.Collections.Generic.List)' cannot be marked as override, virtual, or abstract.
How do I make it generic?
public static override string ToString(this List<int> list) {
string output = "";
list.ForEach(item => output += item.ToString() + "," );
return output;
}
Thanks!
You cannot use extension methods to override an existing method.
From the spec http://msdn.microsoft.com/en-us/library/bb383977.aspx
"You can use extension methods to extend a class or interface, but not to override them. An extension method with the same name and signature as an interface or class method will never be called. At compile time, extension methods always have lower priority than instance methods defined in the type itself."
If you want to override ToString(), you would need to inherit from List<T> rather than try to extend it. You have already seen that you cannot mark the static extension method as override, and overload resolution will always go for the member method over an extension method if it is available. Your options are
Inherit and override
Change your extension method's name to something else ToSpecialString()
Call the method directly using the class name MyExtensions.ToString(myList);
What are you trying to achieve? Often I want to output the contents of a list, so I created the following extension method:
public static string Join(this IEnumerable<string> strings, string seperator)
{
return string.Join(seperator, strings.ToArray());
}
It is then consumed like this
var output = list.Select(a.ToString()).Join(",");
EDIT: To make it easier to use for non string lists, here is another variation of above
public static String Join<T>(this IEnumerable<T> enumerable, string seperator)
{
var nullRepresentation = "";
var enumerableAsStrings = enumerable.Select(a => a == null ? nullRepresentation : a.ToString()).ToArray();
return string.Join(seperator, enumerableAsStrings);
}
public static String Join<T>(this IEnumerable<T> enumerable)
{
return enumerable.Join(",");
}
Now you can consume it like this
int[] list = {1,2,3,4};
Console.WriteLine(list.Join()); // 1,2,3,4
Console.WriteLine(list.Join(", ")); // 1, 2, 3, 4
Console.WriteLine(list.Select(a=>a+".0").Join()); // 1.0, 2.0, 3.0, 4.0
You can only override a method if you inherit the base class.
What I would advocate is calling your extension method .ToCsv().
I have a class MyClass, and I would like to override the method ToString() of instances of List:
class MyClass
{
public string Property1 { get; set; }
public int Property2 { get; set; }
/* ... */
public override string ToString()
{
return Property1.ToString() + "-" + Property2.ToString();
}
}
I would like to have the following:
var list = new List<MyClass>
{
new MyClass { Property1 = "A", Property2 = 1 },
new MyClass { Property1 = "Z", Property2 = 2 },
};
Console.WriteLine(list.ToString()); /* prints: A-1,Z-2 */
Is it possible to do so? Or I would have to subclass List<MyClass> to override the method ToString() in my subclass? Can I solve this problem using extension methods (ie, is it possible to override a method with an extension method)?
Thanks!
Perhaps a bit off-topic, but I use a ToDelimitedString extension method which works for any IEnumerable<T>. You can (optionally) specify the delimiter to use and a delegate to perform a custom string conversion for each element:
// if you've already overridden ToString in your MyClass object...
Console.WriteLine(list.ToDelimitedString());
// if you don't have a custom ToString method in your MyClass object...
Console.WriteLine(list.ToDelimitedString(x => x.Property1 + "-" + x.Property2));
// ...
public static class MyExtensionMethods
{
public static string ToDelimitedString<T>(this IEnumerable<T> source)
{
return source.ToDelimitedString(x => x.ToString(),
CultureInfo.CurrentCulture.TextInfo.ListSeparator);
}
public static string ToDelimitedString<T>(
this IEnumerable<T> source, Func<T, string> converter)
{
return source.ToDelimitedString(converter,
CultureInfo.CurrentCulture.TextInfo.ListSeparator);
}
public static string ToDelimitedString<T>(
this IEnumerable<T> source, string separator)
{
return source.ToDelimitedString(x => x.ToString(), separator);
}
public static string ToDelimitedString<T>(this IEnumerable<T> source,
Func<T, string> converter, string separator)
{
return string.Join(separator, source.Select(converter).ToArray());
}
}
You'll need to subclass to override any method. The point of generics is to say that you want the same behaviour regardless of the type of T. If you want different behaviour for a specific type of T then you are breaking that contract and will need to write your own class:
public class MyTypeList : List<MyClass>
{
public override string ToString()
{
return ...
}
}
Edited to add:
No, you can't override a method by creating an extension, but you could create a new method with a different signature that is specific to this list type:
public static string ExtendedToString(this List<MyClass> list)
{
return ....
}
Used with
List<MyClass> myClassList = new List<MyClass>
string output = myClassList.ExtendedToString();
I still think you're better off subclassing though...
You can actually use a unicode trick to allow you to define an alternate ToString method directly against your generic list.
If you enable hex character input into visual studio then you can create invisible characters by holding down the Alt key, then pressing the following on your numeric keypad + F F F 9 (now release Alt)
So we can create the following function with an invisible character placed next to its name... (yes i know its VB code, but the concept will still work work for C#)
<Extension()> _
Public Function ToString(ByVal source As Generic.List(Of Char)) As String
Return String.Join(separator:="", values:=source.ToArray)
End Function
Now in visual studio, when you access intellisense against your list, you will be able to choose between either the standard ToString or your custom function.
To enable hex character input into visual studio you may need to edit your registry
open HKEY_CURRENT_USER\Control Panel\Input Method
and create a REG_SZ called EnableHexNumpad set this to 1
You will also need to disable the & shortcuts for the File, Edit, Debug, Data menus,
In visual studio, open the tools menu, select customize, then open the commands tab, and using the modify selection button for any menu item that uses either of the ABCDEF charactes for its short cut, by removing the &
Otherwise you will end up opening popup menus, instead of typing hex characters.
If you method must be named ToString you will have to derive a class from List. You can make it a generic:
static class MyList<T> : List<T>
{
public override string ToString()
{
// ...
}
}
In this case, you would have to use MyList instead of List throughout your application if you wish to have your custom conversion.
However, if you can choose a different name for your method, you can use extension methods and achieve the same effect, with almost no modifications to your code:
You can use extension methods to make this more generic:
static class ListExtension
{
public static void ConvertToString<T>(this IEnumerable<T> items)
{
// ...
}
}
You can use it on any instance of IEnumerable<T> just as if it were an ordinary method:
List<MyClass> list = new List<MyClass> { ... };
Console.WriteLine(list.ConvertToString());
int[] array_of_ints = {1,2,3,4,5};
Console.WriteLine(array_of_ints.ConvertToString());
You would have to create your own custom class that inherits from Collection and then overwride the ToString() method of that class specifically.
No its not possible. ToString of TList will give you the string representation of the list object.
Your options are:
Derive from TList and override the .ToString() method as you mentioned. (in this example I wouldn't say its worth doing so)
Create a helper method that converts a TList list to a comma delimited string e.g. extension method (probably best suggestion)
Use a foreach statement at the Console.WriteLine stage.
Hope that helps!
Depending on the exact reason you have for wanting to override List<T>.ToString() to return something specific it might be handy to have a look at custom TypeConverter implementations.
If you simply want a List<T> of specific T to show itself a certain way as a string in locations where TypeConverters are used, like in the debugger or in string.Format("List: {0}", listVariable) type situations, this might be enough.
You might just have seen the result of ToString() being shown somewhere and wanted to change that, without knowing about the existence of TypeConverter and locations where they are used. I believe many/most/all (not sure which?) of the default TypeConverters in the .NET Framework simply use ToString() when converting any type for which they are defined for to a string.