📅 最后修改于: 2020-11-21 07:06:06 🧑 作者: Mango
形成查询模式的一组扩展方法称为LINQ标准查询运算符。作为LINQ查询表达式的构建块，这些运算符提供了一系列查询功能，例如过滤，排序，投影，聚合等。
LINQ标准查询运算符可以根据其功能分为以下几种。
过滤是一种限制结果集的操作，以使其仅具有满足特定条件的选定元素。
| Operator | Description | C# Query Expression Syntax | VB Query Expression Syntax |
|---|---|---|---|
| where | Filter values based on a predicate function | where | Where |
| OfType | Filter values based on their ability to be as a specified type | Not Applicable | Not Applicable |
联接是指以直接方式难以相互遵循的数据源为目标的操作。
| Operator | Description | C# Query Expression Syntax | VB Query Expression Syntax |
|---|---|---|---|
| Join | The operator join two sequences on basis of matching keys | join … in … on … equals … | From x In …, y In … Where x.a = y.a |
| GroupJoin | Join two sequences and group the matching elements | join … in … on … equals … into … | Group Join … In … On … |
投影是一种将对象转换为仅具有特定属性的全新形式的操作。
| Operator | Description | C# Query Expression Syntax | VB Query Expression Syntax |
|---|---|---|---|
| Select | The operator projects values on basis of a transform function | select | Select |
| SelectMany | The operator project the sequences of values which are based on a transform function as well as flattens them into a single sequence | Use multiple from clauses | Use multiple From clauses |
排序操作允许根据单个或多个属性对序列的元素进行排序。
| Operator | Description | C# Query Expression Syntax | VB Query Expression Syntax |
|---|---|---|---|
| OrderBy | The operator sort values in an ascending order | orderby | Order By |
| OrderByDescending | The operator sort values in a descending order | orderby … descending | Order By … Descending |
| ThenBy | Executes a secondary sorting in an ascending order | orderby …, … | Order By …, … |
| ThenByDescending | Executes a secondary sorting in a descending order | orderby …, … descending | Order By …, … Descending |
| Reverse | Performs a reversal of the order of the elements in a collection | Not Applicable | Not Applicable |
运算符基于共同的共享属性将数据分为几组。
| Operator | Description | C# Query Expression Syntax | VB Query Expression Syntax |
|---|---|---|---|
| GroupBy | Organize a sequence of items in groups and return them as an IEnumerable collection of type IGrouping |
group … by -or- group … by … into … | Group … By … Into … |
| ToLookup | Execute a grouping operation in which a sequence of key pairs are returned | Not Applicable | Not Applicable |
运算符更改输入对象的类型，并在各种应用中使用。
| Operator | Description | C# Query Expression Syntax | VB Query Expression Syntax |
|---|---|---|---|
| AsEnumerable | Returns the input typed as IEnumerable |
Not Applicable | Not Applicable |
| AsQueryable | A (generic) IEnumerable is converted to a (generic) IQueryable | Not Applicable | Not Applicable |
| Cast | Performs casting of elements of a collection to a specified type | Use an explicitly typed range variable. Eg:from string str in words | From … As … |
| OfType | Filters values on basis of their , depending on their capability to be cast to a particular type | Not Applicable | Not Applicable |
| ToArray | Forces query execution and does conversion of a collection to an array | Not Applicable | Not Applicable |
| ToDictionary | On basis of a key selector function set elements into a Dictionary |
Not Applicable | Not Applicable |
| ToList | Forces execution of a query by converting a collection to a List |
Not Applicable | Not Applicable |
| ToLookup | Forces execution of a query and put elements into a Lookup |
Not Applicable | Not Applicable |
执行两个序列的串联操作，就其操作而言，它与Union运算符非常相似，不同之处在于它不会删除重复项。
| Operator | Description | C# Query Expression Syntax | VB Query Expression Syntax |
|---|---|---|---|
| Concat | Two sequences are concatenated for the formation of a single one sequence. | Not Applicable | Not Applicable |
执行任何类型的所需聚合，并允许在LINQ中创建自定义聚合。
| Operator | Description | C# Query Expression Syntax | VB Query Expression Syntax |
|---|---|---|---|
| Aggregate | Operates on the values of a collection to perform custom aggregation operation | Not Applicable | Not Applicable |
| Average | Average value of a collection of values is calculated | Not Applicable | Aggregate … In … Into Average() |
| Count | Counts the elements satisfying a predicate function within collection | Not Applicable | Aggregate … In … Into Count() |
| LonCount | Counts the elements satisfying a predicate function within a huge collection | Not Applicable | Aggregate … In … Into LongCount() |
| Max | Find out the maximum value within a collection | Not Applicable | Aggregate … In … Into Max() |
| Min | Find out the minimum value existing within a collection | Not Applicable | Aggregate … In … Into Min() |
| Sum | Find out the sum of a values within a collection | Not Applicable | Aggregate … In … Into Sum() |
当序列中的某些或所有元素满足特定条件时，这些运算符返回布尔值，即True或False。
| Operator | Description | C# Query Expression Syntax | VB Query Expression Syntax |
|---|---|---|---|
| All | Returns a value ‘True’ if all elements of a sequence satisfy a predicate condition | Not Applicable | Aggregate … In … Into All(…) |
| Any | Determines by searching a sequence that whether any element of the same satisfy a specified condition | Not Applicable | Aggregate … In … Into Any() |
| Contains | Returns a ‘True’ value if finds that a specific element is there in a sequence if the sequence doe not contains that specific element , ‘false’ value is returned | Not Applicable | Not Applicable |
将输入序列分为两个单独的部分，而无需重新排列序列的元素，然后返回其中一个。
| Operator | Description | C# Query Expression Syntax | VB Query Expression Syntax |
|---|---|---|---|
| Skip | Skips some specified number of elements within a sequence and returns the remaining ones | Not Applicable | Skip |
| SkipWhile | Same as that of Skip with the only exception that number of elements to skip are specified by a Boolean condition | Not Applicable | Skip While |
| Take | Take a specified number of elements from a sequence and skip the remaining ones | Not Applicable | Take |
| TakeWhile | Same as that of Take except the fact that number of elements to take are specified by a Boolean condition | Not Applicable | Take While |
代数运算符创建了一个新的值序列。
| Operator | Description | C# Query Expression Syntax | VB Query Expression Syntax |
|---|---|---|---|
| DefaultIfEmpty | When applied to an empty sequence, generate a default element within a sequence | Not Applicable | Not Applicable |
| Empty | Returns an empty sequence of values and is the most simplest generational operator | Not Applicable | Not Applicable |
| Range | Generates a collection having a sequence of integers or numbers | Not Applicable | Not Applicable |
| Repeat | Generates a sequence containing repeated values of a specific length | Not Applicable | Not Applicable |
设置操作有四个运算符，每个运算符都基于不同的标准得出结果。
| Operator | Description | C# Query Expression Syntax | VB Query Expression Syntax |
|---|---|---|---|
| Distinct | Results a list of unique values from a collection by filtering duplicate data if any | Not Applicable | Distinct |
| Except | Compares the values of two collections and return the ones from one collection who are not in the other collection | Not Applicable | Not Applicable |
| Intersect | Returns the set of values found t be identical in two separate collections | Not Applicable | Not Applicable |
| Union | Combines content of two different collections into a single list that too without any duplicate content | Not Applicable | Not Applicable |
比较两个句子(可枚举)并确定它们是否完全匹配。
| Operator | Description | C# Query Expression Syntax | VB Query Expression Syntax |
|---|---|---|---|
| SequenceEqual | Results a Boolean value if two sequences are found to be identical to each other | Not Applicable | Not Applicable |
除DefaultIfEmpty外，其余所有八个标准查询元素运算符返回集合中的单个元素。
| Operator | Description | C# Query Expression Syntax | VB Query Expression Syntax |
|---|---|---|---|
| ElementAt | Returns an element present within a specific index in a collection | Not Applicable | Not Applicable |
| ElementAtOrDefault | Same as ElementAt except of the fact that it also returns a default value in case the specific index is out of range | Not Applicable | Not Applicable |
| First | Retrieves the first element within a collection or the first element satisfying a specific condition | Not Applicable | Not Applicable |
| FirstOrDefault | Same as First except the fact that it also returns a default value in case there is no existence of such elements | Not Applicable | Not Applicable |
| Last | Retrieves the last element present in a collection or the last element satisfying a specific condition | Not Applicable | Not Applicable |
| LastOrDefault | Same as Last except the fact that it also returns a default value in case there is no existence of any such element | Not Applicable | Not Applicable |
| Single | Returns the lone element of a collection or the lone element that satisfy a certain condition | Not Applicable | Not Applicable |
| SingleOrDefault | Same as Single except that it also returns a default value if there is no existence of any such lone element | Not Applicable | Not Applicable |
| DefaultIfEmpty | Returns a default value if the collection or list is empty or null | Not Applicable | Not Applicable |