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I have two expression trees of type : Expression<Func<string, bool>> and I would like to obtain a single Expression that will do the OR of the two expressions (passing the same string parameter to both expressions) Any idea?

Daniel Hilgarth
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Dave
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2 Answers2

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You can use PredicateBuilder from LINQKit to do this. For example:

Expression<Func<string, bool>> e1 = …;
Expression<Func<string, bool>> e2 = …;
Expression<Func<string, bool>> combined = e1.Or(e2).Expand();
svick
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  • It works, but as I am going to use it with EF in a LinqToEntities query I can not use Invoke there. – Dave Jul 12 '13 at 22:59
  • @Dave That's what the `Expand()` is there for, it takes care of the `Invoke`. – svick Jul 13 '13 at 09:14
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You can try combining them in a Expression.Lambda expression and then using Expression.Or to check if one of them is true.

Here is an example:

Expression<Func<Car, bool>> theCarIsRed = c1 => c1.Color == "Red";
Expression<Func<Car, bool>> theCarIsCheap = c2 => c2.Price < 10.0;
Expression<Func<Car, bool>> theCarIsRedOrCheap = Expression.Lambda<Func<Car, bool>>(
    Expression.Or(theCarIsRed.Body, theCarIsCheap.Body), theCarIsRed.Parameters.Single());
var query = carQuery.Where(theCarIsRedOrCheap);

Maybe you can get more info here

mathifonseca
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    That doesn't work, because the parameter used in the body of `theCarIsCheap` no longer exists in the resulting expression. You need to use the long way with the ExpressionVisitor. – Daniel Hilgarth Jul 11 '13 at 15:22