Create Linq Expression ( Combine Expressions )

Question

I'm using entity framework 6.1.3 and .net framework 4.5.1 with C# lang.

What I want to do is; I want to combine expressions with if-else statements.
Here is my expression

Expression<Func<Article, bool>> expression = 
    q => (!newsDayStart.HasValue || q.PublishedOn >= newsDayStart) &&
         (!newsDayEnd.HasValue || q.PublishedOn <= newsDayEnd) &&
         (!categoryId.HasValue || q.CategoryId == categoryId.Value) &&
         (string.IsNullOrEmpty(searchText) || q.Title.Contains(searchText) &&
         (!isActive != null || q.IsActive == isActive.Value));

to

Expression<Func<Article, bool>> expression = ......;

if ( newsDayStart.HasValue )
{
   //Obviosly += this statement will not work.
   expression += q => q.PublishedOn > = newsDayStart

}

//TODO write other if else statements...

//Send expression
_context.Articles.Where(expression).Count();

This looks like it might be a duplicate of "Combining two expressions (Expression<Func<T, bool>>)" stackoverflow.com/questions/457316/… — Chris Wyatt
– Chris Wyatt, Commented Nov 17, 2015 at 15:16

Chris Wyatt · Accepted Answer · 2015-11-17 15:19:49Z

2

If this is specifically for use with EF queries then you might find it easier to chain Where() calls to achieve the same effect.

Expression<Func<Article, bool>> expression = ......;

//Send expression
var query = _context.Articles.Where(expression)

if ( newsDayStart.HasValue )
{
    query = query.Where(q => q.PublishedOn > = newsDayStart);
}

query.Count();

*Edit

You could try using this third-party library, PredicateBuilder http://www.albahari.com/nutshell/predicatebuilder.aspx

edited Nov 17, 2015 at 15:19

answered Nov 17, 2015 at 12:26

Chris Wyatt

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2 Comments

Lost_In_Library Over a year ago

Thank you for your answer. But I'm sorry, I can't do that for our abstraction. We are generating filter business code in service layer and pass it to data layer. That's why we need to build an expression and send it as method's parameter.

Servy Over a year ago

@Lost_In_Library Then just restructure your layer so that it accepts a query and returns a query, rather than providing an expression. It's quite a bit easier that way.

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