string 种的扩展方法

string 类的扩展方法列表(基本相同于 IEnumerable<T> 接口的成员列表):

Aggregate<>?   ?     //累加
All<>?   ?   ?   ?   //是否都满足条件
Any<>?   ?   ?   ?   //是否有一个满足条件
AsEnumerable<>?      //
AsParallel<>?   ?    //
AsQueryable<>?   ?   //
Average<>?   ?   ?   //平均值
Cast<>?   ?   ?      //
Concat<>?   ?   ?    //连接
Contains<>?   ?      //是否包含
Count<>?   ?   ?     //总数
DefaultIfEmpty<>?    //指定默认空值
Distinct<>?   ?      //去除重复
ElementAt<>?   ?     //获取指定位置的元素
ElementAtOrDefault<> //同 ElementAt, 但获取失败则返回默认的空值
Except<>?   ?   ?    //差集
First<>?   ?   ?     //获取第一个元素
FirstOrDefault<>?    //同 First, 但获取失败返回默认的空值
GroupBy<>?   ?   ?   //分组
GroupJoin<>?   ?     //关联分组
Intersect<>?   ?     //交集
Join<>?   ?   ?      //串联
Last<>?   ?   ?      //获取最后一个元素
LastOrDefault<>?     //同 Last, 但获取失败返回默认的空值
LongCount<>?   ?     //同 Count, 但返回 Int64
Max<>?   ?   ?   ?   //最大值
Min<>?   ?   ?   ?   //最小值
OfType<>?   ?   ?    //
OrderBy<>?   ?   ?   //排序
OrderByDescending<>  //倒排序
Reverse<>?   ?   ?   //翻转
Select<>?   ?   ?    //映射
SelectMany<>?   ?    //深度映射
SequenceEqual<>?     //
Single<>?   ?   ?    //获取只是唯一的元素, 不唯一或获取不到则异常
SingleOrDefault<>?   //获取只是唯一的元素, 不唯一则异常, 获取不到则取默认空值
Skip<>?   ?   ?      //获取指定序号之后的
SkipWhile<>?   ?     //获取指定条件之后的
Sum<>?   ?   ?   ?   //求和
Take<>?   ?   ?      //获取指定序号之前的
TakeWhile<>?   ?     //获取指定条件之前的
ToArray<>?   ?   ?   //
ToCharArray<>?   ?   //
ToDictionary<>?      //
ToList<>?   ?   ?    //
ToLookup<>?   ?      //
Union<>?   ?   ?     //并集
Where<>?   ?   ?     //筛选
Zip<>?   ?   ?   ?   //合并

获取首尾的字符(First、Last、FirstOrDefault、LastOrDefault):

/* 函数及重载 */
First <Char>()?   ?   ?   ?   ?   ?   ?      // 函数名: First; <Char> 中的 Char 是返回值类型; () 表示这是一个无参数的重载
First <Char> (Func <Char, Boolean>)?   ?     // 这里的参数是个函数, 其(函参)返回值是 Boolean 类型, 参数是 Char 类型.
?   ?   ?   ?   ?   ?   ?   ?   ?   ?   ?    // <> 中是函数参数与返回值的列表, 最后一个是返回值.
FirstOrDefault <Char>()?   ?   ?   ?   ?     // 同 First, 但如果找不到则返回默认值 null
FirstOrDefault <Char> (Func <Char, Boolean>) //
Last <Char>()?   ?   ?   ?   ?   ?   ?   ?   // 顾名思义, 同上
Last <Char> (Func <Char, Boolean>)?   ?      //
LastOrDefault <Char>()?   ?   ?   ?   ?      //
LastOrDefault <Char> (Func <Char, Boolean>)  //


//下面例子很容易理解:
protected void Button1_Click(object sender, EventArgs e)
{
?   string str = "ABCDEFG";
?   char c1 = str.First(); // A
?   char c2 = str.Last();  // G

?   TextBox1.Text = string.Concat(c1, "/n", c2);
}

//下面例子使用了其第二种重载, 其参数是个函数, 使用了 Lambda 匿名函数表达式:
protected void Button2_Click(object sender, EventArgs e)
{
?   string str = "ABCDEFG";
?   char c1 = str.First(p => p != 'A'); // B : 这是获取不是 A 的第一个字符
?   char c2 = str.Last(p => p < 70);?   // E : 这是获取字符编码值大于 70(F) 的倒数第一个字符

?   TextBox1.Text = string.Concat(c1, "/n", c2);
}

// Lambda 表达式中, 
// => 前面的是参数列表, 其类型可被自动识别所以可省略, 只有一个参数时可省略括号; 其中的 p 是自定义的变量名.
// => 后面的是函数内容, 通常只有一句话; 但其必须返回指定的类型(本例中是 Boolean).

//测试用于其它数组:
protected void Button3_Click(object sender, EventArgs e)
{
?   int[] nArr = { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 };
?   string[] sArr = { "one", "two", "three", "four", "five", "six", "seven", "eight", "nine", "ten" };

?   int n1 = nArr.First();?   ?   ?   ?   ?    //1
?   int n2 = nArr.First(x => x % 2 == 0);?     //2
?   string s1 = sArr.Last();?   ?   ?   ?      //ten
?   string s2 = sArr.Last(s => s.Length == 5); //eight
?   TextBox1.Text = string.Concat(n1, "/n", n2, "/n", s1, "/n", s2);
}

获取指定位置的元素(ElementAt、ElementAtOrDefault):

protected void Button1_Click(object sender, EventArgs e)
{
?   string str = "Asp.Net 4.0";
?   char c1 = str.ElementAt(1);?   ?   ?   //s
?   char c2 = str.ElementAtOrDefault(1);   //s
?   char c3 = str.ElementAtOrDefault(999); //
?   bool b1 = c3 == '/x00';?   ?   ?   ?   //True

?   TextBox1.Text = string.Concat(c1, "/n", c2, "/n", c3, "/n", b1);
}

统计操作(Aggregate、Count、LongCount、Max、Min、Sum、Average):

//用数字数组测试比较合适
protected void Button1_Click(object sender, EventArgs e)
{
?   int[] nArr = { 2, 1, 4, 3, 6, 5 };

?   int n1 = nArr.Count();?   ?   ?   ?   ?   //6 
?   int n2 = nArr.Count(n => n % 2 == 0);?    //3 : 偶数元素总数
?   int n3 = nArr.Sum();?   ?   ?   ?   ?     //21
?   int n4 = nArr.Max();?   ?   ?   ?   ?     //6
?   int n5 = nArr.Min();?   ?   ?   ?   ?     //1
?   double n6 = nArr.Average();?   ?   ?      //3.5
?   int n7 = nArr.Aggregate((x, y) => x * y); //720 : 累乘
?   Int64 n8 = nArr.LongCount();?   ?   ?     //6 : LongCount 和 Count 只是返回值的类型不同

?   string r = "/n";
?   TextBox1.Text = string.Concat(n1 + r + n2 + r + n3 + r + n4 + r + n5 + r + n6 + r + n7 + r + n8);
}

protected void Button2_Click(object sender, EventArgs e)
{
?   string str = "one two three four five six seven eight nine ten";

?   int n1 = str.Count();?   ?   ?     //48 : 这相当于 str.Length
?   int n2 = str.Count(s => s != ' '); //39 : 这是非空格字符的总数
?   char c1 = str.Max();?   ?   ?      //x  : 这是序号最大的字符
?   char c2 = str.Min();?   ?   ?      //序号最小的字符在这里是空格

?   TextBox1.Text = string.Concat(n1, "/n", n2, "/n", c1, "/n", c2);
}

//通过 Aggregate 翻转字符串中的单词
protected void Button3_Click(object sender, EventArgs e)
{
?   string str = "one two three four five six seven eight nine ten";
?   string[] sArr = str.Split(' ');

?   str = sArr.Aggregate((s1, s2) => s2 + " " + s1); //ten nine eight seven six five four three two one
?   TextBox1.Text = str;
}

集合操作(Intersect、Except、Union):

//测试整数数组的交集、差集、并集
protected void Button1_Click(object sender, EventArgs e)
{
?   int[] nArr1 = { 1, 2, 3, 4, 5, 6 };
?   int[] nArr2 = { 4, 5, 6, 7, 8, 9 };

?   IEnumerable<int> R1 = nArr1.Intersect(nArr2); // 返回类型可以写作隐式类型 var
?   var R2 = nArr1.Except(nArr2);
?   var R3 = nArr1.Union(nArr2);

?   string s1 = string.Join("", R1); //456
?   string s2 = string.Join("", R2); //123
?   string s3 = string.Join("", R3); //123456789

?   TextBox1.Text = string.Concat(s1, "/n", s2, "/n", s3);
}

//测试字符串(字符数组)的交集、差集、并集
protected void Button2_Click(object sender, EventArgs e)
{
?   string str1 = "Asp.Net 3.5";
?   string str2 = "ASP.NET 4.0";

?   var R1 = str1.Intersect(str2);
?   var R2 = str1.Except(str2);
?   IEnumerable<char> R3 = str1.Union(str2);

?   string s1 = string.Join("", R1); //A.N 
?   string s2 = string.Join("", R2); //spet35
?   string s3 = string.Join("", R3); //Asp.Net 35SPET40

?   TextBox1.Text = string.Concat(s1, "/n", s2, "/n", s3);
}


//定义一个 Person 类, 其有 Name 和 Age 两个属性
public class Person
{
?   public string Name { get; set; }
?   public int Age { get; set; }
?   public Person(string name, int age) { Name = name; Age = age; } // 构造函数
}

//自定义的对比类, 以比较是否是同一个人
public class PersonComparer : IEqualityComparer<Person>
{
?   public bool Equals(Person p1, Person p2)
?   {
?   ?   return p1.Name == p2.Name && p1.Age == p2.Age;
?   }

?   public int GetHashCode(Person obj)
?   {
?   ?   return 0;
?   }
}

//获取两组人的交集、差集、并集
protected void Button3_Click(object sender, EventArgs e)
{
?   string s1, s2, s3;
?   s1 = s2 = s3 = "";

?   Person[] PersonArr1 = { new Person("AA", 11), new Person("BB", 22), new Person("CC", 33) };
?   Person[] PersonArr2 = { new Person("CC", 33), new Person("DD", 44), new Person("AA", 55) };

?   var ps1 = PersonArr1.Intersect(PersonArr2, new PersonComparer());
?   var ps2 = PersonArr1.Except(PersonArr2, new PersonComparer());
?   IEnumerable<Person> ps3 = PersonArr1.Union(PersonArr2, new PersonComparer());

?   foreach (Person p in ps1) { s1 += string.Format("{0}:{1} ", p.Name, p.Age); } //CC:33 
?   foreach (Person p in ps2) { s2 += string.Format("{0}:{1} ", p.Name, p.Age); } //AA:11 BB:22 
?   foreach (Person p in ps3) { s3 += string.Format("{0}:{1} ", p.Name, p.Age); } //AA:11 BB:22 CC:33 DD:44 AA:55

?   TextBox1.Text = string.Concat(s1, "/n", s2, "/n", s3);
}

连接、合并与串联(Concat、Zip、Join):

//Concat
protected void Button1_Click(object sender, EventArgs e)
{
?   int[] nArr1 = { 1, 2, 3 };
?   int[] nArr2 = { 4, 5, 6 };

?   var R1 = nArr1.Concat(nArr2);
?   var R2 = nArr2.Concat(nArr1);
?   var R3 = nArr1.Concat(nArr1);

?   string s1 = string.Join("", R1); //123456
?   string s2 = string.Join("", R2); //456123
?   string s3 = string.Join("", R3); //123123

?   TextBox1.Text = string.Format("{0}/n{1}/n{2}", s1, s2, s3);
}

//Zip
protected void Button2_Click(object sender, EventArgs e)
{
?   string[] sArr = { "one", "two", "there" };
?   int[] nArr = { 1, 2, 3 };

?   var R1 = sArr.Zip(nArr, (x1, x2) => x1 + ":" + x2 + " ");
?   var R2 = sArr.Zip(nArr, (x1, x2) => x2 + ":" + x1 + " ");

?   string s1 = string.Join("", R1); //one:1 two:2 there:3 
?   string s2 = string.Join("", R2); //1:one 2:two 3:there 

?   TextBox1.Text = string.Concat(s1, "/n", s2);
}

//Join
protected void Button3_Click(object sender, EventArgs e)
{
?   int[] nArr = { 1, 2, 3, 4, 5, 6, 7 };
?   string[] sArr = { "one", "two", "three", "four", "five", "six", "seven", "eight", "nine", "ten" };

?   var R = nArr.Join(sArr, n => n, s => s.Length, (n, s) => new { n, s });

?   string str = "";
?   foreach (var obj in R)
?   {
?   ?   str += obj.n + " : " + obj.s + "/n";
?   }
?   TextBox1.Text = str;

/* 输出结果:
?   3 : one
?   3 : two
?   3 : six
?   3 : ten
?   4 : four
?   4 : five
?   4 : nine
?   5 : three
?   5 : seven
?   5 : eight
*/
}

去除重复(Distinct):

protected void Button1_Click(object sender, EventArgs e)
{
?   string str = "aabbccc";
?   var cs = str.Distinct();
?   foreach (char c in cs) { TextBox1.Text += c; } //abc
}

protected void Button2_Click(object sender, EventArgs e)
{
?   int[] nArr = { 1, 3, 2, 4, 3, 5, 4, 6, 5, 7, 6, 8, 7, 9, 8, 0 };
?   var ns = nArr.Distinct().OrderBy(x => -x);
?   foreach (int n in ns) { TextBox1.Text += n.ToString(); } //9876543210
}

排序(OrderBy、OrderByDescending):

protected void Button1_Click(object sender, EventArgs e)
{
?   int[] nArr = { 1, 3, 2, 4, 3, 5 };
?   string[] sArr = { "one", "two", "three", "four", "five", "six", "seven", "eight", "nine", "ten" };
?   string str = "Asp.Net";

?   var R1 = nArr.OrderBy(x => x);
?   var R2 = nArr.OrderByDescending(x => x);
?   var R3 = sArr.OrderBy(x => x);
?   var R4 = sArr.OrderByDescending(x => x);
?   var R5 = str.OrderBy(x => x.ToString().ToLower());
?   var R6 = str.OrderByDescending(x => x.ToString().ToLower());

?   string s1 = string.Join("", R1);  //123345
?   string s2 = string.Join("", R2);  //543321
?   string s3 = string.Join(" ", R3); //eight five four nine one seven six ten three two 
?   string s4 = string.Join(" ", R4); //two three ten six seven one nine four five eight 
?   string s5 = string.Join("", R5);  //.AeNpst
?   string s6 = string.Join("", R6);  //tspNeA. 

?   TextBox1.Text = string.Concat(s1, "/n", s2, "/n", s3, "/n", s4, "/n", s5, "/n", s6);
}

翻转(Reverse):

protected void Button1_Click(object sender, EventArgs e)
{
?   string str = "Asp.Net";
?   var cs = str.Reverse();
?   foreach (char c in cs) { TextBox1.Text += c; } //teN.psA
}

筛选(Where):

protected void Button1_Click(object sender, EventArgs e)
{
?   int[] nArr = { 1, 2, 3, 4, 5, 6, 7, 8, 9 };
?   string[] sArr = { "one", "two", "three", "four", "five", "six", "seven", "eight", "nine", "ten" };

?   var R1 = nArr.Where(n => n % 2 == 0);
?   var R2 = sArr.Where(s => s.Length == 3);

?   string s1 = string.Join(", ", R1); //2, 4, 6, 8
?   string s2 = string.Join(", ", R2); //one, two, six, ten

?   TextBox1.Text = string.Concat(s1, "/n", s2);
}

映射(Select、SelectMany):

//Select
protected void Button1_Click(object sender, EventArgs e)
{
?   int[] nArr = { 1, 2, 3, 4, 5, 6, 7, 8, 9 };
?   string[] sArr = { "one", "two", "three", "four", "five", "six", "seven", "eight", "nine", "ten" };

?   var R1 = nArr.Select(n => n * n);
?   var R2 = sArr.Select((str, index) => (index+1) + ":" + str.ToUpper());

?   string s1 = string.Join(",", R1); //1,4,9,16,25,36,49,64,81
?   string s2 = string.Join(" ", R2); //1:ONE 2:TWO 3:THREE 4:FOUR 5:FIVE 6:SIX 7:SEVEN 8:EIGHT 9:NINE 10:TEN

?   TextBox1.Text = string.Concat(s1, "/n", s2);
}

//Select 与 SelectMany
protected void Button2_Click(object sender, EventArgs e)
{
?   string[] sArr = { "AA:BB:CC", "DD:EE:FF", "GG:HH:II" };

?   var R1 = sArr.Select(arr => arr.Split(':'));
?   var R2 = sArr.SelectMany(arr => arr.Split(':'));

?   var s1 = string.Join(" ", R1); //System.String[] System.String[] System.String[]
?   var s2 = string.Join(" ", R2); //AA BB CC DD EE FF GG HH II

?   var s3 = "";
?   foreach (string[] ss in R1) { s3 += string.Join(" ", ss) + "; "; } //AA BB CC; DD EE FF; GG HH II; 

?   TextBox1.Text = string.Concat(s1, "/n", s2 + "/n" + s3);
}

分组(GroupBy、GroupJoin):

public class Person
{
?   public string Name { get; set; }
?   public int Age { get; set; }
}
protected void Button1_Click(object sender, EventArgs e)
{
?   Person[] pArr = 
?   {
?   ?   new Person { Name="AA", Age=11 },
?   ?   new Person { Name="BB", Age=22 },
?   ?   new Person { Name="CC", Age=33 },
?   ?   new Person { Name="DD", Age=11 },
?   ?   new Person { Name="EE", Age=22 },
?   ?   new Person { Name="FF", Age=33 },
?   ?   new Person { Name="GG", Age=11 }
?   };

?   var R1 = pArr.GroupBy(p => p.Age, p => p.Name);
?   var R2 = pArr.GroupBy(p => p.Age, p => p);

?   string str1 = "";
?   foreach (IGrouping<int, string> group in R1)
?   {
?   ?   str1 += group.Key + "/n";
?   ?   foreach (string n in group) { 
?   ?   ?   str1 += "/t" + n + "/n"; 
?   ?   }
?   }

?   string str2 = "";
?   foreach (IGrouping<int, Person> group in R2)
?   {
?   ?   str2 += group.Key + "/n";
?   ?   foreach (Person p in group)
?   ?   {
?   ?   ?   str2 += string.Format("/t{0}:{1}/n", p.Name, p.Age);
?   ?   }
?   }

?   TextBox1.Text = string.Format("{0}----------/n{1}", str1, str2);

/* 输出结果:
11
	AA
	DD
	GG
22
	BB
	EE
33
	CC
	FF
----------
11
	AA:11
	DD:11
	GG:11
22
	BB:22
	EE:22
33
	CC:33
	FF:33
*/
}

//使用 GroupJoin, 测试效果同上
protected void Button1_Click(object sender, EventArgs e)
{
?   int[] nArr = { 11, 22, 33 }; 
?   Person[] pArr = 
?   {
?   ?   new Person { Name="AA", Age=11 },
?   ?   new Person { Name="BB", Age=22 },
?   ?   new Person { Name="CC", Age=33 },
?   ?   new Person { Name="DD", Age=11 },
?   ?   new Person { Name="EE", Age=22 },
?   ?   new Person { Name="FF", Age=33 },
?   ?   new Person { Name="GG", Age=11 }
?   };

?   var R1 = nArr.GroupJoin(pArr, n => n, p => p.Age, (n, ps) => new { MyKey = n, MyNames = ps.Select(p => p.Name) });
?   var R2 = nArr.GroupJoin(pArr, n => n, p => p.Age, (n, ps) => new { MyKey = n, MyNames = ps });

?   string str1 = "";
?   foreach (var obj in R1)
?   {
?   ?   str1 += obj.MyKey + "/n";
?   ?   foreach (string s in obj.MyNames)
?   ?   {
?   ?   ?   str1 += string.Format("/t{0}/n", s);
?   ?   }
?   }

?   string str2 = "";
?   foreach (var obj in R2)
?   {
?   ?   str2 += obj.MyKey + "/n";
?   ?   foreach (var p in obj.MyNames)
?   ?   {
?   ?   ?   str2 += string.Format("/t{0}:{1}/n", p.Name, p.Age);
?   ?   }
?   }

?   TextBox1.Text = string.Format("{0}----------/n{1}", str1, str2);
}

获取指定条件之后的(Skip、SkipWhile)或之前的(Take、TakeWhile):

//Skip、SkipWhile
protected void Button1_Click(object sender, EventArgs e)
{
?   int[] nArr = { 1, 3, 2, 4, 3, 5 };

?   var R1 = nArr.Skip(3);?   ?   ?   ?   ?   ?      //取第 3 个元素之后的
?   var R2 = nArr.OrderBy(n => n).Skip(3);?   ?      //先排序, 再取第 3 个元素之后的
?   var R3 = nArr.OrderByDescending(n => n).Skip(3); //先倒排序, 再取第 3 个元素之后的

?   var R4 = nArr.OrderBy(n => n).SkipWhile(n => n <= 3);   //先排序(123345), 从不满足条件开始, 之后的
?   var R5 = nArr.SkipWhile((num, index) => num != index); // 从第二个元素开始, 数字和序号就不一样了

?   string s1 = string.Join("", R1); //435
?   string s2 = string.Join("", R2); //345
?   string s3 = string.Join("", R3); //321
?   string s4 = string.Join("", R4); //45
?   string s5 = string.Join("", R5); //2435

?   TextBox1.Text = string.Format("{0}/n{1}/n{2}/n{3}/n{4}", s1, s2, s3, s4, s5);
}

//Take、TakeWhile 用法同上, 只是获取之前的
protected void Button2_Click(object sender, EventArgs e)
{
?   int[] nArr = { 1, 3, 2, 4, 3, 5 };

?   var R1 = nArr.Take(3);
?   var R2 = nArr.OrderBy(n => n).Take(3);
?   var R3 = nArr.OrderByDescending(n => n).Take(3);

?   var R4 = nArr.OrderBy(n => n).TakeWhile(n => n <= 3);
?   var R5 = nArr.TakeWhile((num, index) => num != index);

?   string s1 = string.Join("", R1); //132
?   string s2 = string.Join("", R2); //123
?   string s3 = string.Join("", R3); //543
?   string s4 = string.Join("", R4); //1233
?   string s5 = string.Join("", R5); //13

?   TextBox1.Text = string.Format("{0}/n{1}/n{2}/n{3}/n{4}", s1, s2, s3, s4, s5);
}

All 与 Any:

//都满足条件时 All 才返回 True
protected void Button1_Click(object sender, EventArgs e)
{
?   int[] nArr = { 1, 3, 5, 7, 9 };
?   bool b1 = nArr.All(n => n % 2 == 0); //False
?   bool b2 = nArr.All(n => n % 2 == 1); //True

?   TextBox1.Text = string.Concat(b1, "/n", b2);
}

//其中一个满足条件 Any 就返回 True
protected void Button2_Click(object sender, EventArgs e)
{
?   string str1 = "Delphi";
?   string str2 = "Asp.Net";
?   string str3 = "";

?   bool b1 = str1.Any();?   ?   ?     //True
?   bool b2 = str2.Any();?   ?   ?     //True
?   bool b3 = str3.Any();?   ?   ?     //False

?   bool b4 = str1.Any(c => c == 'D'); //True
?   bool b5 = str2.Any(c => c == 'D'); //False
?   bool b6 = str2.Any(c => c == 'D'); //False

?   TextBox1.Text = string.Concat(b1, "/n", b2, "/n", b3, "/n", b4, "/n", b5, "/n", b6);
}

是否包含(Contains):

protected void Button1_Click(object sender, EventArgs e)
{
?   string str = "Asp.Net 4.0";
?   bool b1 = str.Contains('4');?      //True
?   bool b2 = str.Contains("Net 4.0"); //True
?   bool b3 = str.Contains("Net4.0");  //False

?   TextBox1.Text = string.Concat(b1, "/n", b2, "/n", b3, "/n");
}

获取只是唯一的元素(Single、SingleOrDefault):

protected void Button1_Click(object sender, EventArgs e)
{
?   string str = "Microsoft";
?   char c1, c2, c3, c4, c5, c6;
?   c1 = c2 = c3 = c4 = c5 = c6 = '*';

?   //Single
?   c1 = str.Single(c => c == 'M'); //M

?   try { c2 = str.Single(c => c == 'o'); }
?   catch (Exception err) { Response.Write(err.Message + "<br/>"); } // 序列包含一个以上的匹配元素 

?   try { c3 = str.Single(c => c == 'A'); }
?   catch (Exception err) { Response.Write(err.Message + "<br/>"); } // 序列不包含任何匹配元素

?   //SingleOrDefault
?   c4 = str.SingleOrDefault(c => c == 'M'); //M

?   try { c5 = str.SingleOrDefault(c => c == 'o'); }
?   catch (Exception err) { Response.Write(err.Message + "<br/>"); } // 序列包含一个以上的匹配元素 

?   c6 = str.SingleOrDefault(c => c == 'A'); // 这不会激发异常

?   TextBox1.Text = string.Format("{0}{1}{2}{3}{4}{5}", c1, c2, c3, c4, c5, c6); //M**M*
}

指定默认空值(DefaultIfEmpty):

protected void Button1_Click(object sender, EventArgs e)
{
?   string str1 = "Asp.Net 3.5";
?   string str2 = "";

?   string r1 = string.Join("|", str1.DefaultIfEmpty());?   //A|s|p|.|N|e|t| |3|.|5
?   string r2 = string.Join("|", str2.DefaultIfEmpty());?   //
?   string r3 = string.Join("|", str2.DefaultIfEmpty('*')); //*

?   TextBox1.Text = string.Concat(r1, "/n", r2, "/n", r3);
}