动态Linq查询帮助？

Question

如何为以下简单搜索条件编写动态Linq查询？ 1）StudentNumber 2）姓氏 3）姓氏和名字

//if (!String.IsNullOrEmpty(StudentNumber)) 
var results = (from s in Students    
       where s.StudentNumber == 1001 
       select s 
       ); 

//else if (!String.IsNullOrEmpty(LastName) & (String.IsNullOrEmpty(FirstName)) 

var results = (from s in Students    
       where s.LastName == "Tucker" 
       select s 
       ); 

//else if (!String.IsNullOrEmpty(LastName) & (!String.IsNullOrEmpty(FirstName))    
var results = (from s in Students    
       where s.LastName == "Tucker" && s.FirstName == "Ron" 
       select s 
       ); 

Answer 1

需要声明任何个人查询您的results变量之外。这将允许您根据不同的标准附加不同的过滤器，并根据需要添加尽可能多的过滤器。一个例子：

var results = Students.AsEnumerable(); // use .AsQueryable() for EF or Linq-to-SQL 

if (!string.IsNullorEmpty(StudentNumber)) 
{ 
    results = results.Where(s => s.StudentNumber.Equals(StudentNumber)); 
} 
else if (!string.IsNullOrEmpty(LastName)) 
{ 
    results = results.Where(s => s.LastName.Equals(LastName)); 

    if (!string.IsNullOrEmpty(FirstName)) 
    { 
     results = results.Where(s => s.FirstName.Equals(FirstName)); 
     // filter is in addition to predicate against LastName 
    } 
} 

// results can be used here 

如果处理的LINQ到实体或-SQL，与Students.AsQueryable();键入初始查询，从而过滤发生在数据库，而不是在应用程序内。

---

有没有一种方法，我可以先构建WHERE子句，并用它在 Linq查询，而不IF ... ELSE

如果你想建立之前，整个where查询的第一步，这是相同的逻辑。你有条件地构建谓词，所以你会有某种if/else参与。但是，要首先构建整个谓词，可以针对Linq to Objects构建一个Func<Student, bool>。

Func<Student, bool> predicate; 
if (!string.IsNullOrEmpty(StudentNumber)) 
{ 
    predicate = s => s.StudentNumber.Equals(StudentNumber); 
} 
else if (!string.IsNullOrEmpty(LastName)) 
{ 
    predicate = s => s.LastName.Equals(LastName); 

    if (!string.IsNullOrEmpty(FirstName)) 
    { 
     Func<Student, bool> p = predicate; 
     predicate = s => p(s) && s.FirstName.Equals(FirstName); 
    } 
} 
else 
{ 
    predicate = s => true; 
} 

var query = Students.Where(predicate); 

您会注意到它与if/else结构完全相同。你可以把它折叠成一个复杂的条件表达式

Func<Student, bool> predicate; 
predicate = s => 
    !string.IsNullOrEmpty(StudentNumber) 
    ? s.StudentNumber.Equals(StudentNumber) 
    : !string.IsNullOrEmpty(LastName) 
     ? !string.IsNullOrEmpty(FirstName) 
      ? s.LastName.Equals(LastName) && s.FirstName.Equals(FirstName) 
      : s.LastName.Equals(LastName) 
     : true; 

var query = Students.Where(predicate); 

但是我觉得很难跟得上，当然和if/else的时间长了一样。这个谓词也比我们通过if/else构建的更大，因为这个逻辑包含了全部，它不仅仅是我们有条件添加的逻辑。