如何扩展 C # 内置类型，比如 String？

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();

Extension methods!

public static class MyExtensions
{
public static string ConvertWhitespacesToSingleSpaces(this string value)
{
return Regex.Replace(value, @"\s+", " ");
}
}

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.

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!";

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.