c# - 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) 学号2) 姓氏3) 姓和名

//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

您需要申报您的 results任何单个查询之外的变量。这将允许您根据不同的标准附加不同的过滤器，并根据需要附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 Linq-to-Entities 或 -Sql，请使用 Students.AsQueryable(); 键入初始查询这样过滤就发生在数据库而不是应用程序内部。

---

Is there a way I can construct the WHERE clause first and use it in a Linq query without if...else

如果你想构建整个where在查询的第一步之前，它是相同的逻辑。您正在有条件地构建谓词，因此您将涉及某种 if/else。但是，要首先构建整个谓词，您可以针对 Func<Student, bool> 构建对于 Linq to Objects。

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);

您会注意到它是完全相同的 if/else 结构。你可以把它分解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);

但我发现这很难理解，尤其是与较长的 if/else 相比。这个谓词也比我们通过 if/else 构建的谓词更大，因为这个谓词包含了所有逻辑，而不仅仅是我们有条件添加的逻辑。