操作說明：使用 LINQ 查詢字串
會以字元序列的形式儲存字串。 可以使用 LINQ 來查詢字元序列形式的字串。 在本文中有數個範例查詢，其會查詢字串中是否有不同字元或單字、篩選字串，或將查詢與規則運算式混合。
如何查詢字串中的字元
下列範例會查詢字串，以判斷它所包含的數字位數。
string aString = "ABCDE99F-J74-12-89A";
// Select only those characters that are numbers
var stringQuery = from ch in aString
where Char.IsDigit(ch)
select ch;
// Execute the query
foreach (char c in stringQuery)
Console.Write(c + " ");
// Call the Count method on the existing query.
int count = stringQuery.Count();
Console.WriteLine($"Count = {count}");
// Select all characters before the first '-'
var stringQuery2 = aString.TakeWhile(c => c != '-');
// Execute the second query
foreach (char c in stringQuery2)
Console.Write(c);
/* Output:
Output: 9 9 7 4 1 2 8 9
Count = 8
ABCDE99F
*/
上述查詢示範如何將字串視為字元序列。
如何計算單字在字串中出現的次數
以下範例示範如何使用 LINQ 查詢，以計算字串中指定單字的出現次數。 執行計數要先呼叫 Split 方法，才能建立單字陣列。 Split 方法有效能成本。 如果字串上唯一的作業是計算字數，應考慮改用 Matches 或 IndexOf 方法。
string text = """
Historically, the world of data and the world of objects
have not been well integrated. Programmers work in C# or Visual Basic
and also in SQL or XQuery. On the one side are concepts such as classes,
objects, fields, inheritance, and .NET APIs. On the other side
are tables, columns, rows, nodes, and separate languages for dealing with
them. Data types often require translation between the two worlds; there are
different standard functions. Because the object world has no notion of query, a
query can only be represented as a string without compile-time type checking or
IntelliSense support in the IDE. Transferring data from SQL tables or XML trees to
objects in memory is often tedious and error-prone.
""";
string searchTerm = "data";
//Convert the string into an array of words
char[] separators = ['.', '?', '!', ' ', ';', ':', ','];
string[] source = text.Split(separators, StringSplitOptions.RemoveEmptyEntries);
// Create the query. Use the InvariantCultureIgnoreCase comparison to match "data" and "Data"
var matchQuery = from word in source
where word.Equals(searchTerm, StringComparison.InvariantCultureIgnoreCase)
select word;
// Count the matches, which executes the query.
int wordCount = matchQuery.Count();
Console.WriteLine($"""{wordCount} occurrences(s) of the search term "{searchTerm}" were found.""");
/* Output:
3 occurrences(s) of the search term "data" were found.
*/
上述查詢示範如何在將字串分割成單字序列後，將字串視為單字序列。
如何依任何字詞或欄位排序或篩選文字資料
下列範例示範如何依行中的任一欄位，來排序多行結構化文字 (例如逗號分隔值)。 可能會在執行階段動態指定此欄位。 假設 scores.csv 中的欄位各代表學生的學號和四個測驗分數：
111, 97, 92, 81, 60
112, 75, 84, 91, 39
113, 88, 94, 65, 91
114, 97, 89, 85, 82
115, 35, 72, 91, 70
116, 99, 86, 90, 94
117, 93, 92, 80, 87
118, 92, 90, 83, 78
119, 68, 79, 88, 92
120, 99, 82, 81, 79
121, 96, 85, 91, 60
122, 94, 92, 91, 91
下列查詢會根據儲存在第二個資料行的第一個測驗分數，來排序這幾行：
// Create an IEnumerable data source
string[] scores = File.ReadAllLines("scores.csv");
// Change this to any value from 0 to 4.
int sortField = 1;
Console.WriteLine($"Sorted highest to lowest by field [{sortField}]:");
// Split the string and sort on field[num]
var scoreQuery = from line in scores
let fields = line.Split(',')
orderby fields[sortField] descending
select line;
foreach (string str in scoreQuery)
{
Console.WriteLine(str);
}
/* Output (if sortField == 1):
Sorted highest to lowest by field [1]:
116, 99, 86, 90, 94
120, 99, 82, 81, 79
111, 97, 92, 81, 60
114, 97, 89, 85, 82
121, 96, 85, 91, 60
122, 94, 92, 91, 91
117, 93, 92, 80, 87
118, 92, 90, 83, 78
113, 88, 94, 65, 91
112, 75, 84, 91, 39
119, 68, 79, 88, 92
115, 35, 72, 91, 70
*/
上述查詢示範如何將字串分割成欄位，以及查詢個別欄位，以操控字串。
如何查詢包含特定單字的句子
以下範例示範如何在文字檔中尋找句子，內含與每個指定單字組相符的項目。 雖然硬式編碼了搜尋字詞的陣列，但也可以在執行階段以動態方式將其填入。 此查詢會傳回包含 "Historically"、"data" 和 "integrated" 等單字的句子。
string text = """
Historically, the world of data and the world of objects
have not been well integrated. Programmers work in C# or Visual Basic
and also in SQL or XQuery. On the one side are concepts such as classes,
objects, fields, inheritance, and .NET APIs. On the other side
are tables, columns, rows, nodes, and separate languages for dealing with
them. Data types often require translation between the two worlds; there are
different standard functions. Because the object world has no notion of query, a
query can only be represented as a string without compile-time type checking or
IntelliSense support in the IDE. Transferring data from SQL tables or XML trees to
objects in memory is often tedious and error-prone.
""";
// Split the text block into an array of sentences.
string[] sentences = text.Split(['.', '?', '!']);
// Define the search terms. This list could also be dynamically populated at run time.
string[] wordsToMatch = [ "Historically", "data", "integrated" ];
// Find sentences that contain all the terms in the wordsToMatch array.
// Note that the number of terms to match is not specified at compile time.
char[] separators = ['.', '?', '!', ' ', ';', ':', ','];
var sentenceQuery = from sentence in sentences
let w = sentence.Split(separators,StringSplitOptions.RemoveEmptyEntries)
where w.Distinct().Intersect(wordsToMatch).Count() == wordsToMatch.Count()
select sentence;
foreach (string str in sentenceQuery)
{
Console.WriteLine(str);
}
/* Output:
Historically, the world of data and the world of objects have not been well integrated
*/
此查詢會先將文字分割成句子，然後將每個句子分割成內含每個單字的字串陣列。 對於每個陣列，Distinct 方法會移除所有重複的字組，接著查詢會對字組陣列和 wordsToMatch 陣列執行 Intersect 作業。 如果交集的計數與 wordsToMatch 陣列的計數相同，則所有字都可以在字組中找到，因而傳回原始句子。
對 Split 的呼叫會使用標點符號當成分隔符號，以便從字串中移除。 例如，如果您未移除標點符號，則字串 “Historically” 與 wordsToMatch 陣列中的 “Historically” 不相符。 根據來源文字中找到的標點符號類型，您可能必須使用額外的分隔符號。
如何合併 LINQ 查詢與規則運算式
以下範例會示範如何使用 Regex 類別來建立規則運算式，以在文字字串中進行更複雜的比對。 LINQ 查詢讓您輕鬆地準確篩選出您想要用規則運算式搜尋的檔案，並調整結果。
string startFolder = """C:\Program Files\dotnet\sdk""";
// Or
// string startFolder = "/usr/local/share/dotnet/sdk";
// Take a snapshot of the file system.
var fileList = from file in Directory.GetFiles(startFolder, "*.*", SearchOption.AllDirectories)
let fileInfo = new FileInfo(file)
select fileInfo;
// Create the regular expression to find all things "Visual".
System.Text.RegularExpressions.Regex searchTerm =
new System.Text.RegularExpressions.Regex(@"microsoft.net.(sdk|workload)");
// Search the contents of each .htm file.
// Remove the where clause to find even more matchedValues!
// This query produces a list of files where a match
// was found, and a list of the matchedValues in that file.
// Note: Explicit typing of "Match" in select clause.
// This is required because MatchCollection is not a
// generic IEnumerable collection.
var queryMatchingFiles =
from file in fileList
where file.Extension == ".txt"
let fileText = File.ReadAllText(file.FullName)
let matches = searchTerm.Matches(fileText)
where matches.Count > 0
select new
{
name = file.FullName,
matchedValues = from System.Text.RegularExpressions.Match match in matches
select match.Value
};
// Execute the query.
Console.WriteLine($"""The term "{searchTerm}" was found in:""");
foreach (var v in queryMatchingFiles)
{
// Trim the path a bit, then write
// the file name in which a match was found.
string s = v.name.Substring(startFolder.Length - 1);
Console.WriteLine(s);
// For this file, write out all the matching strings
foreach (var v2 in v.matchedValues)
{
Console.WriteLine($" {v2}");
}
}
您也可以查詢 RegEx 搜尋所傳回的 MatchCollection 物件。 只會在結果中產生每個相符項目的值。 不過，其也可以使用 LINQ 對該集合執行各種篩選、排序及分組。 因為 MatchCollection 為非泛型 IEnumerable 集合，所以您必須在查詢中明確地陳述範圍變數的類型。
