Code First Entity Framework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
Expression
var e =
Expression.Lambda
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
Expression
var e =
Expression.Lambda
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 map;
public ParameterRebinder(Dictionary map)
{
this.map = map ?? new Dictionary();
}
public static Expression ReplaceParameters(Dictionary 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
Expression
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
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> True() { return f => true; }
public static Expression> False() { return f => false; }
public static Expression Compose(this Expression first, Expression second, Func 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(merge(first.Body, secondBody), first.Parameters);
}
public static Expression> And(this Expression> first, Expression> second)
{
return first.Compose(second, Expression.And);
}
public static Expression> Or(this Expression> first, Expression> second)
{
return first.Compose(second, Expression.Or);
}
} 完美的动态拼接Lambda表达式如下：
using (var db = new MyDbContext())
{
var predicate = PredicateBuilder.True
predicate=predicate.And(s => s.ID > 1200);
predicate=predicate.Or(s => s.ID < 1000);
var result = db.Students.Where(predicate).ToList();
}
using (var db = new SHTrackerDbContext())
{
var predicate = PredicateBuilder.True();
settings = DecorateSettings(settings);
Expression> 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;
}