EntityFramework动态组合Lambda表达式作为数据筛选条件，代替拼接SQL语句

传统的操作数据库方式，筛选数据需要用StringBuilder拼接一大堆的WHERE子句。

在Entity Framework中，代码稍有不慎就会造成巨大性能消耗，如：

using(var db=new MyDbContext())

{

var s=db.Students.ToList().First(s=>s.ID=1200);

}

嘣！进行了全表数据读取！当然一般人也不会犯这种低级的错误，言归正传。

可以简单的这样筛选数据：

using(var db=new MyDbContext())

{

var list =db.Students.AsQueryable();

if(********){list=list.Where(s=>s.ID=1200);}

if(******){list=list.Where(...)}

}

但是有时这种方法不能完成特定需求，如：

using(var db=new MyDbContext())

{

var list =db.Students.AsQueryable();

if(条件1){list=list.Where(s=>s.ID>1200);}

if(条件2){list=list.Where(s=>s.ID<1000);}

}

现在条件1和条件2同时成立，得到的是空结果集而不是ID>1200和ID<1000的结果集。

这只是两个并列简单条件的组合，如果是条件嵌套呢？

下面是假想：

using (var db = new MyDbContext())
{


Expression<Func<Student, bool>> checkStudent1 = s1 => s1.ID > 1200;
Expression<Func<Student, bool>> checkStudent2 = s2 => s2.ID < 1000;
var e =
Expression.Lambda<Func<Student, bool>>(
Expression.Or(checkStudent1.Body, checkStudent2.Body), checkStudent1.Parameters);
var result = db.Students.Where(e).ToList();
}

叫它假想的原因是执行会产生异常”The parameter 's2' was not bound in the specified LINQ to Entities query expression“。

e的内容是{s1 => ((s1.ID > 1200) Or (s2.ID < 1000))}，很明显s2这个参数是没有被定义的。

实际上我们一直操作一个Student表，最终我们想要的也是多Lambda表达式合在一起对该Student表的操作。换句话说，s2应该用s1代替。

有人说了，这样：

Expression<Func<Student, bool>> checkStudent1 = s => s.ID > 1200;
Expression<Func<Student, bool>> checkStudent2 = s => s.ID < 1000;
var e =
Expression.Lambda<Func<Student, bool>>(
Expression.Or(checkStudent1.Body, checkStudent2.Body), checkStudent1.Parameters);
var result = db.Students.Where(e).ToList();

异常：”The parameter 's' was not bound in the specified LINQ to Entities query expression“。

e的内容是{s => ((s.ID > 1200) Or (s.ID < 1000))}，现在参数都一样是s了，但其实它们的GUID不同，也就是说它们还是两个不同的参数。

我们需要做的是手工把checkStudent2.Body里面的参数s换成checkStudent1.Body里面的参数s。

ExpressionVisitor可以很好的完成这步操作。拿个别人现成的例子来用：

public class ParameterRebinder : ExpressionVisitor
    {
        private readonly Dictionary<ParameterExpression, ParameterExpression> map;

        public ParameterRebinder(Dictionary<ParameterExpression, ParameterExpression> map)
        {
            this.map = map ?? new Dictionary<ParameterExpression, ParameterExpression>();
        }

        public static Expression ReplaceParameters(Dictionary<ParameterExpression, ParameterExpression> map, Expression exp)
        {
            return new ParameterRebinder(map).Visit(exp);
        }

        protected override Expression VisitParameter(ParameterExpression p)
        {
            ParameterExpression replacement;
            if (map.TryGetValue(p, out replacement))
            {
                p = replacement;
            }
            return base.VisitParameter(p);
        }
    }

更改后的测试代码：

Expression<Func<Student, bool>> checkStudent1 = s => s.ID > 1200;
Expression<Func<Student, bool>> checkStudent2 = s => s.ID < 1000;

var body2 =
ParameterRebinder.ReplaceParameters(
checkStudent2.Parameters.Select((s,i)=>new{s,f=checkStudent1.Parameters[i]}).ToDictionary(p=>p.s,p=>p.f), checkStudent2.Body);
var e =
Expression.Lambda<Func<Student, bool>>(
Expression.Or(checkStudent1.Body, body2), checkStudent1.Parameters);
var result = db.Students.Where(e).ToList();

至此表达式顺利拼接完成。当然这样使用还是麻烦，借用别人的扩展类稍微修改一下：

    public static class PredicateBuilder
    {

        public static Expression<Func<T, bool>> True<T>() { return f => true; }
        public static Expression<Func<T, bool>> False<T>() { return f => false; }
        public static Expression<T> Compose<T>(this Expression<T> first, Expression<T> second, Func<Expression, Expression, Expression> merge)
        {
            // build parameter map (from parameters of second to parameters of first)
            var map = first.Parameters.Select((f, i) => new { f, s = second.Parameters[i] }).ToDictionary(p => p.s, p => p.f);

            // replace parameters in the second lambda expression with parameters from the first
            var secondBody = ParameterRebinder.ReplaceParameters(map, second.Body);

            // apply composition of lambda expression bodies to parameters from the first expression 
            return Expression.Lambda<T>(merge(first.Body, secondBody), first.Parameters);
        }

        public static Expression<Func<T, bool>> And<T>(this Expression<Func<T, bool>> first, Expression<Func<T, bool>> second)
        {
            return first.Compose(second, Expression.And);
        }

        public static Expression<Func<T, bool>> Or<T>(this Expression<Func<T, bool>> first, Expression<Func<T, bool>> second)
        {
            return first.Compose(second, Expression.Or);
        }
    }

参考：http://blogs.msdn.com/b/meek/archive/2008/05/02/linq-to-entities-combining-predicates.aspx
完美的动态拼接Lambda表达式如下：

using (var db = new MyDbContext())
{
var predicate = PredicateBuilder.True<Student>();
predicate=predicate.And(s => s.ID > 1200);
predicate=predicate.Or(s => s.ID < 1000);
var result = db.Students.Where(predicate).ToList();
}

下面是一个我自己使用的例子，仅供参考：

using (var db = new SHTrackerDbContext())
            {

                var predicate = PredicateBuilder.True<Course>();
                settings = DecorateSettings(settings);

                Expression<Func<Course, bool>> checkCourse = c => db.Students.Any(s => s.CourseID == c.ID);
                if (!string.IsNullOrEmpty(settings.Quater_Year))
                {

                    checkCourse =
                        c => db.Students.Any(s => s.CourseID == c.ID && db.Student2CBOs.Any(
                            s2c => s2c.StudentID == s.ID && s2c.Quater_Year.Equals(settings.Quater_Year)));
                }
                if (settings.QuaterYearArray != null)
                {
                    checkCourse =
                        c => db.Students.Any(s => s.CourseID == c.ID && db.Student2CBOs.Any(
                            s2c =>
                            s2c.StudentID == s.ID && settings.QuaterYearArray.Any(qy => qy.Equals(s2c.Quater_Year))));
                }

                if (!string.IsNullOrEmpty(settings.DPU_ID))
                {
                    checkCourse =
                        checkCourse.And(
                            c => db.Students.Any(s => s.CourseID == c.ID && s.DPU_ID.Equals(settings.DPU_ID)));
                }
                predicate = predicate.And(checkCourse);

                if (settings.IsCheckInstructorName)
                {
                    predicate = predicate.And(c => c.InstructorName.Equals(settings.InstructorName));
                }
                if (!string.IsNullOrEmpty(settings.Term))
                {
                    predicate = predicate.And(c => c.TermDescription.Equals(settings.Term));
                }
                if (settings.TermArray != null)
                {
                    predicate = predicate.And(c => settings.TermArray.Any(t => t.Equals(c.TermDescription)));
                }
                if (settings.CourseType != CourseType.All)
                {
                    predicate = predicate.And(c => c.Type == (int) settings.CourseType);
                }
                var cc =
                    new CourseCollection(
                        db.Courses.AsNoTracking()
                          .Where(predicate)
                          .OrderByDescending(m => m.ID)
                          .Skip((pageIndex - 1)*pageSize)
                          .Take(pageSize)
                          .ToList(),
                        db.Courses.AsNoTracking().Where(predicate).Count())
                        {
                            PageIndex = pageIndex,
                            PageSize = pageSize,
                            Settings = DecorateSettings(settings)
                        };

                return cc;
            }