Dynamic Query (Build a QueryExpression)

Hi Guys,

I would like to know if the following is possible:

I want to create the query expression dynamically e.g.:

QueryExpression exp1 = Entity.Attribute(someVar) == someValue;

QueryExpression exp2 = Entity.Attribute(someOtherVar) > someOtherValue;

QueryExpression exp3 = Entity.Attribute(Var1) > Value2;

Once I've got these expessions I would like to add them toghter with either a && or || is this possible?

Regards,

Johan

Hi Johan,

Yes indeed you can use the && and || operators to join togethor parts of a query, these return a new QueryExpression instance.

e.g. QueryExpression exp4 = exp1 && exp2 && exp3;

You can also use this syntax when building up a query:

e.g. QueryExpression exp5 = Entity.Attribute(Var1) > Value 2 && Entity.Attribute("Foo") == "Bar";

Jeremy

Hi Jeremy,

My problem is that I've got a list of expressions that I build up. Once I have the list I would like to join them.

E.g.

List<QueryExpression> experssions = new List<QueryExpression>();

expressions.Add(  Entity.Attribute(Var1) > Value2 );

expressions.Add(  Entity.Attribute("Foo") == "Bar" );

expressions.Add(  Entity.Attribute("Bar") == "Foo" );

Foreach ( var exp in expressions )

x &= exp;

Or something like this.

If you just want to AND together a list of expressions then you can do this using the Aggregate operator:

var combinedQuery = expressions.Aggregate((q1, q2) => q1 && q2);

Similarly if you want to OR together a list of expressions.

If you want to mix ANDs and ORs (e.g. (q1 && q2) || (q3 && q4)) then it's a bit trickier because you need to worry about which pairs to AND and which to OR, grouping/precedence, etc.  If this is the case then have a look at http://www.mindscape.co.nz/forums/Post.aspx?ThreadID=1615&PostID=3609.

Hi Ivan,

Thanks for the response the Aggregate did the trick for me.