C # extension method imaginative advanced Article Five: ToString (string format) extension

In. NET, System.Object.ToString () is one of the most used methods, and the ToString () method is defined in the Object class as the Virtual,object class gives it a default implementation:

1　public　virtual　string　ToString()
2　{
3　　return　this.GetType().ToString();
4　}

. NET native class or struct, such as int,datetime, to which it is rewritten (override), so that it returns a more valuable value than the name of the type. The reasonable rewriting of the ToString () method in programming, debugging in the great convenience. But after all, a class has only one ToString () method that does not meet our diverse needs, and many classes are overloaded with ToString (). As follows:

1　string　dateString　=　DateTime.Now.ToString("yyyy");　　//2009
2　string　intString　=　10.ToString("d4");　　//0010

int, DateTime implements the ToString (string format) method, which greatly facilitates our use.

For our own defined types, we should also provide a reasonable ToString () rewrite, if we can provide another ToString (string format), it will make our later work simpler. Try the following types:

1　public　class　People
2　{
3　　private　List<People>　friends　=　new　List<People>();
4　
5　　public　int　Id　{　get;　set;　}
6　　public　string　Name　{　get;　set;　}
7　　public　DateTime　Brithday　{　get;　set;　}
8　　public　People　Son　{　get;　set;　}
9　　public　People[]　Friends　{　get　{　return　friends.ToArray();　}　}
10
11　　public　void　AddFriend(People　newFriend)
12　　{
13　　　if　(friends.Contains(newFriend))　throw　new　ArgumentNullException("newFriend",　"该朋友已添加");
14　　　else　friends.Add(newFriend);
15　　}
16　　public　override　string　ToString()
17　　{
18　　　return　string.Format("Id:　{0},　Name:　{1}",　Id,　Name);
19　　}
20　
21　}

A simple class, we give a ToString () rewrite that returns a string containing the ID and name two key attributes. Now we need a ToString (string format) rewrite to meet the following applications:

1　People　p　=　new　People　{　Id　=　1,　Name　=　"鹤冲天",　Brithday　=　new　DateTime(1990,　9,　9)　};
2　string　s0　=　p.ToString("Name　生日是　Brithday");　//理想输出：鹤冲天　生日是　1990-9-9
3　string　s1　=　p.ToString("编号为：Id，姓名：Name");　//理想输出：编号为：1，姓名：鹤冲天

Think about how to achieve it, and remember that format is variable, what attributes are not used, and what kind of combination ...

Perhaps a class is good to do, if we define a lot of classes, dozens of, hundreds of how to do? One by one implementing ToString (string format) would have worn people out. The good news is that we have an extension method, and we have an extended ToString (string format) for object. NET object is all the base class, and all classes are automatically owned when it is extended. Of course, the existing ToString (string format) is not implemented because the native method has a high priority and is not overwritten by the extension method.

To see how to achieve it (we will improve the step-by-step, in order to distinguish between the various versions, respectively, extended to ToString1, ToString2 ...) Corresponding version one, version two ... ）：

1　public　static　string　ToString1(this　object　obj,　string　format)
2　{
3　　　Type　type　=　obj.GetType();
4　　　PropertyInfo[]　properties　=　　type.GetProperties(
5　　　BindingFlags.GetProperty　|　BindingFlags.Public　|　BindingFlags.Instance);
6
7　　　string[]　names　=　properties.Select(p　=>　p.Name).ToArray();
8　　　string　pattern　=　string.Join("|",　names);
9
10　　　MatchEvaluator　evaluator　=　match　=>
11　　　{
12　　　　PropertyInfo　property　=　properties.First(p　=>　p.Name　==　match.Value);
13　　　　object　propertyValue　=　property.GetValue(obj,　null);
14　　　　if　(propertyValue　!=　null)　return　propertyValue.ToString();
15　　　　else　return　"";
16　　　};
17　　　return　Regex.Replace(format,　pattern,　evaluator);
18　}

The 3~5 row obtains the public, instance's Get property (if it needs to be static or private, modifies line 5th) by reflection, and the 7~8 row dynamically generates a regular expression to match the Format,10~16 row as a successful match. Here is a reflection and regular expression, if not familiar, first debug run it, test the previous mentioned application: