How can I create dynamic Lambda Expressions

Question

I have a question, how do I add another filter and this I have to validate that was selected?

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;
}

Update: I'll use the expression in this function:

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

What I was trying to do is this but with an Expression

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();

    }
}

Are you saying you are trying to pass back another Expression with your statement or you are trying to embed lambda expressions? I'm not understanding exactly what you are trying to do — corylulu, Mar 19 '13 at 17:48
Okay, if that's the case, then I provided an example of an embedded lambda statement. But I'm not sure what you are expecting as a returning value, so I don't know how you'd like to modify your example. — corylulu, Mar 19 '13 at 17:58

score 0 · Answer 1 · answered Mar 19 '13 at 17:51

If this is just a demo of your requirements, then you can create and modify Expression Trees using the System.Linq.Expressions namespace.

However, in the case of your question, it would probably be easier to use EF:

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

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

or normal 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 ...
  ;
}

score 0 · Answer 2 · answered Mar 19 '13 at 17:56

Okay, well here is a sample on how to embed lambda statements. It's not your example, but here ya go:

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);

score 0 · Answer 3 · edited May 23 '17 at 11:50

0

It is simple.Visit this link

the tricky thing is invoking the OrderByAlias - using MakeGenericMethod may be the way, as shown in the link above.

edited May 23 '17 at 11:50

Community

1
1

answered Mar 19 '13 at 17:59

Rahul Ranjan

1,028
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3

I would not classify expression trees as "simple"... (also: this answer should probably be a comment, IMO) – Marc Gravell Mar 19 '13 at 18:09

score 0 · Answer 4 · answered Mar 19 '13 at 18:04

Try this way:

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);
}

Get error "The LINQ expression node type 'Invoke' is not supported in LINQ to Entities" — davidterra, Mar 20 '13 at 00:14

score 0 · Accepted Answer · 2013-03-19T21:07:53.020

inherit from 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);
    }
}

your Filter() method might look like this

    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

have a look at the following for more information

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

Return error {"Value cannot be null.\r\nParameter name: right"} — davidterra, Mar 20 '13 at 01:03
return Expression.MakeBinary(ExpressionType.AndAlso, b, binary, b.IsLiftedToNull, b.Method); — davidterra, Mar 20 '13 at 12:18

How can I create dynamic Lambda Expressions

5 Answers5