I want to override ToString() on IEnumerable<Int32>.
I was thinking to use Extension methods.
But when I do this below, it still calls the ToString() on System.Object. When I rename my method, then it calls my method.
As my extension method is in a static class, I am not able to override.
How can I achieve this so that my ToString() implementation is called when I call .ToString() on List<Int32> for example?
public static class ExtensionMethods
{
public static new string ToString(this IEnumerable<Int32> set)
{
var sb = new StringBuilder();
// Do some modifications on sb
return sb.ToString();
}
}
How can I achieve this so that my ToString() implementation is called when I call .ToString() on List for example?
You can't, basically. Extension methods are only used if no matching instance method can be found.
I suggest you give your method a different name, avoiding the problem - and the potential confusion your method would cause.
Note that even if extension methods were matched in preference to (say) methods declared on object, it would only make a difference for your own code being compiled with an appropriate using directive - not any other code which has already bound the call to the normal one.
If you can give more information about what you're trying to achieve, we may be able to help you more - but for the moment, something like ToDelimitedString (or whatever your method does) sounds like the best bet to me.
You cannot replace a method using extension methods.
Method resolution will check for a method belonging to the type, before trying to find matching extension methods.
In other words, you cannot replace ToString, but yes, you can create your own method.
Either create your own IEnumerable<T> type with an overridden ToString method, or use a different method name. Of course, using your own type will of course only work when you're actually using that type.
It's not possible to override ToString, but you can create a wrapper, that you can call in every place, where you are using IEnumerable<Int32>
To output a collection as string I am using an extension method
public static string ToString<T>( this IEnumerable<T> messages)
{
return ToString<T>(messages, Environment.NewLine, "" );
}
ToString<T>( this IEnumerable<T>messages, string separator, string sComment)
is described in my post
ToString function for Generic List
See also similar Overriding ToString() of List<MyClass>
Similar function implemented as an extension method described in post:
Separator Delimited ToString for Array, List, Dictionary, Generic IEnumerable
You cannot override ToString but you can create a generic method.
This is a simple solution based on Michael Freidgeim's answer (its link is broken):
static public class Extensions {
// extension for arrays, lists, any Enumerable -> AsString
public static string AsString<T>(this IEnumerable<T> enumerable) {
var sb = new StringBuilder();
int inx = 0;
foreach (var item in enumerable) {
sb.Append($"{inx}: {item}\r\n");
inx++;
}
return sb.ToString();
}
}
Usage:
Console.WriteLine(arr.AsString());
Console.WriteLine(list.AsString());
Console.WriteLine(linqResult.AsString());
Related
If this question is too simple I apologize in advance.
Why does the method NthIndexOf found in the link below require a static class and also static class member?
C# - indexOf the nth occurrence of a string?
Because it is an extension method (MSDN) -- notice the keyword this before the first parameter.
This allows you to use the syntax:
var result = "foo bar".NthIndexOf("o", 1);
…as though you had added the NthIndexOf method to the System.String type. This would be available anywhere the namespace for that static class was available (using MyExtensions; for example).
Extension methods must be declared as static methods of public, non-nested static classes, but the same logic can be encapsulated without using an extension method, in which case there would be no requirement to use a static class & method.
Because it's an EXTENSION METHOD (EM).
Extension methods enable you to "add" methods to existing types without creating a new derived type, recompiling, or otherwise modifying the original type. Extension methods are a special kind of static method, but they are called as if they were instance methods on the extended type. For client code written in C# and Visual Basic, there is no apparent difference between calling an extension method and the methods that are actually defined in a type.
For example:
String is a .Net type. Prior to EM, without deriving, you cannot add your own method to string type. Even if you did so by deriving, that method would be counted as method of derived type and not of string type.
But now with EM's you can do so without deriving from it.
Requirements of EM (in C#)
They have to be public and static
The class in which they are defined is also public and static.
Most Important: First parameter should be that type for which you are creating your EM and should have this keyword in front of it. Without it, your method would not be an EM and would be a static method only.
In your earlier scenaro your EM NthIndexOf is defined in static class StringExtender
public static class StringExtender
{
public static int NthIndexOf(this string target, string value, int n)
{
....
}
}
Now since first parameter contains this in front of string so you can call it as
int result = "My string".NthIndexOf("M", 0);
If it was not EM and a plain static method like this
public static class StringExtender
{
public static int NthIndexOf(string target, string value, int n)
{
....
}
}
then it had to be called like
int result = NthIndexOf("My string", "M", 0);
It's an extension method which requires the static keyword.
I have a list that contains FrameworkElements and I want to create an extension method called MoveToTop. All this will do is accept an item that is part of that list and move it to the beginning of the list. I know this could be accomplished without the use of an extension method, but I would like it to be implemented as an extension method.
I am having trouble trying to figure out the syntax for creating an extension method that accepts a generic parameter. I know this isn't correct, but if someone could give me an idea how how to accomplish this, I would appreciate it.
public static class Extensions
{
public static void MoveToTop(this ICollection<T> sequence)
{
//logic for moving the item goes here.
}
}
You were close, just need the <T> after the method name before the parenthesis. That's where the generic type parameter list for generic methods is placed. It declares the generic type parameters the method will accept, which then makes them available to be used in the arguments, return values, and method body.
public static class Extensions
{
public static void MoveToTop<T>(this ICollection<T> sequence)
{
//logic for moving the item goes here.
}
}
I'm having trouble getting the C# compiler to call an extension method I created, since its preferring an instance method with a params argument instead.
For example, say I have the following class and its method:
public class C
{
public void Trace(string format, params object[] args)
{
Console.WriteLine("Called instance method.");
}
}
And and extension:
public static class CExtensions
{
public void Trace(this C #this, string category, string message, params Tuple<string, decimal>[] indicators)
{
Console.WriteLine("Called extension method.");
}
}
In my sample program:
public void Main()
{
var c = new C();
c.Trace("Message");
c.Trace("Message: {0}", "foo");
c.Trace("Category", "Message", new KeyValuePair<string, decimal>("key", 123));
}
All calls print Called instance method..
I do not have access to class C, obviously, or I wouldn't bother creating extension methods, and my extension is important because it would allow my users to continue using a class that they already know with some added value.
From what I've understood, the compiler will favor instance methods over extension methods, but is this the only rule? That would mean that any class with a method that looks like Method(string format, params object[] args) cannot have extension methods with a first parameter of type string.
Any explanation on the reasons for this behavior or a way to work around it (that is not "simply call CExtensions.Trace(c, "Category", ...") would be greatly appreciated.
You can't use extensions to "take over" existing class methods.
If the call works without the extension, the behaviour should not change when you add the extension. The reason for this is that existing code should not break by introducing an extension later on.
You have to use a different name for it, or use parameter types different from the class method.
You cannot directly. A method on the target instance is always preferred over an extension method. The only way to do this (while keeping the names the same etc) is to use the CExtensions.Trace approach (in the question).
In some cases, a trick here would be to use some base-class of C or interface that C implements, but which does not have a Trace method, and re-type the variable, and add an overload on CExtensions, i.e.
IFoo foo = c;
foo.Trace(...);
Tuple<string, decimal> is not the same as KeyValuePair<string, decimal>. Hence KeyValuePair<string, decimal> passed in is taken just as an object hence member method with params object[] args is used.
In fact KeyValuePair is a structure.
You can do it with named arguments:
c.Trace(
"Category",
"Message",
indicators: new Tuple<string, decimal>("key", 123));
but you loose the params functionality and you would need to explicitly pass an array for the indicators argument, like the following:
c.Trace(
"Category",
"Message",
indicators: new Tuple<string, decimal>[]
{
new Tuple<string, decimal>("key", 123),
new Tuple<string, decimal>("key2", 123)
});
I guess you could change first parameter type, or function name (TraceFormat?)
This params version looks very greedy, so if you preserve first argument as string it will always catch all calls and ignore extension method I believe.
public static class MyClass
{
public static void Add<T>(this List<T> list, T item)
{
list.Add(item);
Console.WriteLine(item.ToString());
}
}
then
List<string> list = new List<string>(){"1","2"};
list.Add("3");
But the member method would be called.
Is there anyway to call my Extension Method this way?
I don't want to call it like this:
MyClass.Add(list, item)
You can't. Instance methods always take precedence over extension methods, assuming they're applicable. Member resolution will only consider extension methods once it's failed to find a non-extension-method option.
I would suggest you simply rename your method - unless the point was to call this method transparently with existing code.
If you made it take an IList<T> instead of List<T>, you could create a wrapper type which implements IList<T> and delegates all calls onto the wrapped list, performing any extra tasks as you go. You could then also write an extension method to IList<T> which created the wrapper - which would allow for more fluent syntax in some cases. Personally I prefer the wrapper approach to deriving a new collection type, as it means you can use it with your existing collections, making the code changes potentially smaller... but it all depends on what you're trying to do.
Instance methods always take precedence over extension methods, so no.
The correct thing to do here would appear to be polymorphism - but note that List<T> doesn't provide virtual methods. Collection<T> does, though:
using System;
using System.Collections.ObjectModel;
class MyClass<T> : Collection<T> {
protected override void InsertItem(int index, T item) {
base.InsertItem(index, item);
Console.WriteLine("Added:" + item.ToString());
}
protected override void SetItem(int index, T item) {
base.SetItem(index, item);
Console.WriteLine("Set (indexer):" + item.ToString());
}
// see also ClearItems and RemoveItem
}
See i have a situation like this...
object myRoledata = List<Roles>() --> (some list or Ienumerable type)
Now i have a generic method which creates an XML object from List<T> -
Something like this..
public string GetXML<T>(object listdata)
{
List<T> objLists = (List<T>)Convert.ChangeType(listData, typeof(List<T>));
foreach(var obj in listdata)
{
//logic to create xml
}
}
Now in order to run this method I have to do like this:
string xml = GetXML<Roles>(myRoledata);
Now i dont know what Type may come to me to be passed to GetXML method. I have a method which will call GetXML for different Types e.g. Roles, Users etc
now i can get the Type within the List<> like this
Type genericType = obj.GetType().GetGenericArguments()[0];
but cannot pass it like this
string xml = GetXML<genericType>(myRoledata);
Is there anyway in which i can pass any genericTypes to GetXML method?
To do that, you need to use reflection;
typeof(SomeClass).GetMethod("GetXML").MakeGenericMethod(genericType)
.Invoke(inst, new object[] {myRoleData});
where inst is null if it is a static method, this for the current instance (in which case you can also use GetType() instead of typeof(SomeClass)), or the target object otherwise.
Since you cast your listdata parameter as a List< T> in the first line of your method, why don't you just change the method signature to
public string GetXML<T>(List<T> listdata)
That way, you don't have to use reflection to get the generic arguments.
EDIT: I see that you need to be able to accept IEnumerable collections, and not just lists. So, consider changing your method signature to
public string GetXML<T>(IEnumerable<T> listdata)
This is a problem you probably want to avoid solving. It is possible, via reflection, to call methods dynamically without statically resolving them - but it kind of defeats the whole point of the type-annotations.
Either do this:
public string GetXML(IEnumerable listdata) {
foreach(object obj in listdata)
//logic to create xml
}
... which you now can call with any IEnumerable, or write it the "modern" way as:
public string GetXML(IEnumerable<object> listdata) {
foreach(object obj in listdata)
//logic to create xml
}
... which you can call with any IEnumerable via GetXML(someEnumerable.Cast<object>()) and in C# 4.0 even directly by covariance.
If you need the type of an element runtime, you can get it using .GetType() on each element, or you can just pass it in as a parameter (and provide an override for backwards-compatibility):
public string GetXML(Type elementType, IEnumerable<object> listdata) {
foreach(object obj in listdata)
//logic to create xml
}
public string GetXML<T>(IEnumerable<T> listdata) {
return GetXML(typeof(T),listdata.Cast<object>());
}
Incidentally, if you're constructing XML, a string is probably a less robust return-type choice: if possible, you could work with something like an XElement instead - and get xml-validity guarantee's to boot.
I don't know your situation, but is it possible to rewrite your function as:
public string GetXML<T>(IEnumerable<T> listdata)
{
foreach(var obj in listdata)
{
//logic to create xml
}
}
Then it can be called as:
List<Role> myList;
GetXML(myList);
You can add type parameters as far back as needed to support it, till you get to somewhere that does know what the solid type is.
You have the right idea, but you are using the wrong method. Have a look at Type.MakeGenericType or MethodInfo.MakeGenericMethod. It will take a few more lines than your example, but it should be simple to solve.
GetGenericArguments() can be used to get the Roles type from a List. It's the differnt way around.
Btw: Looks like your implementing some kind of XML serialization. Make sure you check existing classes, before reinventing the wheel. ;-)