如何創建動態的Lambda表達式

Question

我有一個問題，如何添加另一個過濾器，並且我必須驗證它是否被選中？

private Expression < Func < Entity.Modelos.Flux, bool >> Filter() { 
    var dateStart = dtpDateStart.Value.Date; 
    var dateEnd = dtpDateEnd.Value.Date; 

    Expression < Func < Entity.Modelos.Flux, bool >> expr = null; 

    expr = f = > f.DatFlux >= dateStart.Date && f.DatFlux <= dateEnd.Date; 

    if (txtDescription.Text != String.Empty) { 
     //add filter 
    } 

    return expr; 
} 

更新：我將在這個函數中使用表達式：

public virtual IQueryable <T> Filter(Expression < Func < T, bool >> expressao) { 
    return DbSet.Where(expressao).AsQueryable <T>(); 
} 

什麼，我試圖做的就是這一點，但與表達

public List <Users> GetUsers(int ? id, string name) { 
    using(DBContext ctx = new DBContext()) { 
     IQueryable query = ctx.Usuarios; 
     if (id.HasValue) 
      query = query.Where(x = > x.ID == id); 

     if (!string.IsNullOrEmpty(name)) 
      query = query.Where(x = > x.Name.StartsWith(name)); 

     return query.ToList(); 

    } 
} 

Answer 1

繼承ExpressionVisitor

public class MyVisitor: ExpressionVisitor 
{ 
    private LambdaExpression visitor; 
    public Expression Modify(Expression expression, LambdaExpression visitor) 
    { 
     this.visitor = visitor; 
     return Visit(expression); 
    } 

    protected override Expression VisitBinary(BinaryExpression b) 
    { 
     var binary = visitor.Body as BinaryExpression; 

     return Expression.MakeBinary(ExpressionType.AndAlso, b, binary, b.IsLiftedToNull, b.Method); 
    } 
} 

您Filter()方法可能是這樣

private Expression<Func<Entity.Modelos.Flux, bool>> Filter() 
    { 
     var dateStart = dtpDateStart.Value.Date; 
     var dateEnd = dtpDateEnd.Value.Date; 
     var description = txtDescription.Text; 

     Expression<Func<Entity.Modelos.Flux, bool>> expr = null; 

     expr = f => f.DatFlux >= dateStart.Date && f.DatFlux <= dateEnd.Date; 

     if (description != String.Empty) 
     { 
      //add filter 
      Expression<Func<Entity.Modelos.Flux, bool>> other = f => f.Description == description; 

      var modifier = new MyVisitor(); 
      expr = (Expression<Func<Entity.Modelos.Flux, bool>>)modifier.Modify((Expression)expr, (LambdaExpression)other); 
     } 

     return expr; 
    } 

example

看看下面的詳細信息

How to: Modify Expression Trees (C# and Visual Basic)

Answer 2

如果這是隻是您的需求演示，那麼您可以使用System.Linq.Expressions命名空間創建並修改Expression Trees。

然而，在你的問題的情況下，它可能會更容易使用EF：

bool filterDescription = !String.IsNullOrEmpty(txtDescription.Text); 

expr = f => 
    (
    (f.DatFlux >= dateStart.Date && f.DatFlux <= dateEnd.Date) 
    && 
    (!filterDescription || ... add filter ...) 
) 
; 

或普通的C＃：

if (String.IsNullOrEmpty(txtDescription.Text)) 
{ 
    expr = f => f.DatFlux >= dateStart.Date && f.DatFlux <= dateEnd.Date; 
} 
else 
{ 
    expr = f => f.DatFlux >= dateStart.Date && f.DatFlux <= dateEnd.Date 
    && 
    ... add filter ... 
    ; 
} 

Answer 3

好，嗯，這裏是如何的樣本嵌入lambda語句。這不是你的例子，但在這裏亞去：

Func<int, int, EventHandler> makeHandler = 
    (dx, dy) => (sender, e) => { 
     var btn = (Button) sender; 
     btn.Top += dy; 
     btn.Left += dx; 
    }; 

btnUp.Click += makeHandler(0, -1); 
btnDown.Click += makeHandler(0, 1); 
btnLeft.Click += makeHandler(-1, 0); 
btnRight.Click += makeHandler(1, 0); 

Answer 4

It is simple.Visit this link

棘手的事情是調用OrderByAlias - 使用MakeGenericMethod可能的方式，如上面的鏈接。

Answer 5

試試這個方法：

private Expression<Func<Entity.Modelos.Flux, bool>> Filter() 
{ 
    var dateStart = dtpDateStart.Value.Date; 
    var dateEnd = dtpDateEnd.Value.Date; 

    Func<Entity.Modelos.Flux, bool> expr = null; 

    expr = f => f.DatFlux >= dateStart.Date && f.DatFlux <= dateEnd.Date; 

    if(txtDescription.Text != String.Empty) 
    { 
     expr = f => expr(f) && f.Title.Equals(txtDescription.Text); // ← Your additional filter 
    } 

    return f => expr(f); 
}