傳統的操作數據庫方式，篩選數據需要用StringBuilder拼接一大堆的WHERE子句。
在Entity Framework中，代碼稍有不慎就會造成巨大性能消耗，如：
using(var db=new MyDbContext())
{
var s= db.Students.ToList().First(s=>s.ID=1200);
}
嘣！進行了全表數據讀取！當然一般人也不會犯這種低級的錯誤，言歸正傳。
可以簡單的這樣篩選數據：
using(var db=new MyDbContext())
{
var list =db.Students.AsQueryable();
if(********){list=list.Where(s=>s.ID=1200);}
if(******){list=list.Where(...)}
}
但是有時這種方法不能完成特定需求，如：
using(var db=new MyDbContext())
{
var list =db.Students.AsQueryable();
if(條件1){list=list.Where(s=>s.ID>1200);}
if(條件2){list=list.Where(s=>s.ID<1000);}
}
現在條件1和條件2同時成立，得到的是空結果集而不是ID>1200和ID<1000的結果集。
這只是兩個並列簡單條件的組合，如果是條件嵌套呢？
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();
}
叫它假想的原因是執行會產生異常”The parameter 's2' was not bound in the specified LINQ to Entities query expression“。
e的內容是{s1 => ((s1.ID > 1200) Or (s2.ID < 1000))}，很明顯s2這個參數是沒有被定義的。
實際上我們一直操作一個Student表，最終我們想要的也是多Lambda表達式合在一起對該Student表的操作。換句話說，s2應該用s1代替。
有人說了，這樣：
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();
異常：”The parameter 's' was not bound in the specified LINQ to Entities query expression“。
e的內容是{s => ((s.ID > 1200) Or (s.ID < 1000))}，現在參數都一樣是s了，但其實它們的GUID不同，也就是說它們還是兩個不同的參數。
我們需要做的是手工把checkStudent2.Body里面的參數s換成checkStudent1.Body里面的參數s。
ExpressionVisitor可以很好的完成這步操作。拿個別人現成的例子來用：
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);
}
}
更改后的測試代碼：
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();
至此表達式順利拼接完成。當然這樣使用還是麻煩，借用別人的擴展類稍微修改一下：
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);
}
}
參考：http://blogs.msdn.com/b/meek/archive/2008/05/02/linq-to-entities-combining-predicates.aspx
完美的動態拼接Lambda表達式如下： 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();
}
下面是一個我自己使用的例子，僅供參考：
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;
}
參考：http://blog.csdn.net/leewhoee/article/details/8968023