How to create an extension method for ToString?

问题:

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?

回答1:

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.

回答2:

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());
    }
}

回答3:

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.