构建动态LINQ查询的最佳方法

Question

嗨,我正在寻找编写动态LINQ查询的最佳方法.

我有一个像这样的功能

public IQueryable<Student> FindByAllStudents(int? id, string Name, int? CourseID, bool? IsActive) // like this way, all field values are passed
    {    
        // code for compairision
        return db.Student;
    }

我们也可以写db.Students.where(谓词)

要么

像这样的查询

var students = from s in db.students where s.Name.Contains(Name)
                s.ID.Equals(id)
                //and so on....

如果我没有传递ID(即Null),这种方法会起作用吗？是适合所有数据类型的方法吗？

关键是函数可以将所有空值作为select*from语句等价的参数.

任何人都可以帮我用示例代码构建最佳查询？

Answer 1

好吧,它并不完全清楚你想要什么,但是如果你只想添加非null的参数where子句,你可以这样做:

public IQueryable<Student> FindByAllStudents
    (int? id, string name, int? courseID, bool? isActive)
{    
    IQueryable<Student> query = db.Student;
    if (id != null)
    {
        query = query.Where(student => student.ID == id.Value);
    }
    if (name != null)
    {
        query = query.Where(student => student.Name.Contains(name));
    }
    if (courseID != null)
    {
        query = query.Where(student => student.CourseID == courseID.Value);
    }
    if (isActive != null)
    {
        query = query.Where(student => student.IsActive == isActive.Value);
    }
    return query;
}

我并没有说,这是可能是的LINQ to SQL会感到困惑的代码,找到空值类型的值.您可能需要编写如下代码:

    if (courseID != null)
    {
        int queryCourseID = courseID.Value;
        query = query.Where(student => student.CourseID == queryCourseID);
    }

首先尝试更简单的形式值得:)

当然,这一切都有点刺激.一种有用的扩展方法可以使生活更简洁:

public static IQueryable<TSource> OptionalWhere<TSource, TParameter>
    (IQueryable<TSource> source,
     TParameter? parameter, 
     Func<TParameter, Expression<Func<TSource,bool>>> whereClause)
    where TParameter : struct
{
    IQueryable<TSource> ret = source;
    if (parameter != null)
    {
        ret = ret.Where(whereClause(parameter.Value));
    }
    return ret;
}

然后你会像这样使用它:

public IQueryable<Student> FindByAllStudents
    (int? id, string name, int? courseID, bool? isActive)
{    
    IQueryable<Student> query = db.Student
        .OptionalWhere(id, x => (student => student.ID == x))
        .OptionalWhere(courseID, x => (student => student.CourseID == x))
        .OptionalWhere(isActive, x => (student => student.IsActive == x));
    if (name != null)
    {
        query = query.Where(student => student.Name.Contains(name));
    }
    return query;
}

如果您对此不太满意,使用这样的高阶函数可能会让人感到困惑,所以如果你没有做很多这样的查询,你可能想要坚持使用更长但更简单的代码.