我可以使用扩展方法继承吗？

Question

I have the following:

public static class CityStatusExt
{
    public static string D2(this CityStatus key)
    {
        return ((int) key).ToString("D2");
    }


public static class CityTypeExt
{
    public static string D2(this CityType key)
    {
        return ((int) key).ToString("D2");
    }

Plus other classes with similar extensions that return the key formatted as a "D2"

Is there a way I could inherit from a base class and have the base class provide the functionality so don't I don't have to repeat the same extension method code?

Update. I am sorry I did not mention this but my classes like CityType are Enums.

Answer 1

You can make the method generic. C# will infer the type:

public static class Extension 
{ 
    public static string D2<T> (this T key) 
    { 
        return ((int)(object) key).ToString("D2"); 
    } 
}

Answer 2

From the comment below, CityType and CityStatus are enums. Therefore you can do this:

public static class Extensions
{
    public static string D2(this Enum key)
    {
        return Convert.ToInt32(key).ToString("D2");
    }
}

---

Original answer:

You can use a generic method and an interface ID2Able :

public static class Extensions
{ 
    public static string D2<T>(this T key) where T : ID2Able
    { 
        return ((int) key).ToString("D2"); 
    } 
}

This way the extension method won't show up for absolutely every type; it'll only be available for things you inherit ID2Able from.

Answer 3

Your enums already all inherit from a common base class, namely System.Enum . So you can do this (Enums don't accept "D2" as a format string, but they accept "D", so I added a call to PadLeft):

public static class EnumExtensions
{
    public static string D2(this Enum e)
    {
        return e.ToString("D").PadLeft(2, '0');
    }
}