How to use expression to combine Linq and intersect?
I have code
public static class PredicateExtensions
{
public static Expression<Func<T, bool>> True<T>() { return f => true; }
public static Expression<Func<T, bool>> False<T>() { return f => false; }
public static Expression<Func<T, bool>> Or<T>(this Expression<Func<T, bool>> expression1, Expression<Func<T, bool>> expression2)
{
var invokedExpression = Expression.Invoke(expression2, expression1.Parameters.Cast<Expression>());
return Expression.Lambda<Func<T, bool>>(Expression.Or(expression1.Body, invokedExpression), expression1.Parameters);
}
public static Expression<Func<T, bool>> And<T>(this Expression<Func<T, bool>> expression1, Expression<Func<T, bool>> expression2)
{
var invokedExpression = Expression.Invoke(expression2, expression1.Parameters.Cast<Expression>());
return Expression.Lambda<Func<T, bool>>(Expression.And(expression1.Body, invokedExpression), expression1.Parameters);
}
}
How can I use this code instead of LINQ Union and Intersect methods?
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If you have a union and intersection of a shape:
var union = source.Where(predicate0).Union(source.Where(predicate1));
var inter = source.Where(predicate0).Intersect(source.Where(predicate1));
Then union
will matter for which either predicate0
, or predicate1
were true, and inter
will have the values for which both predicate0
and predicate1
are true.
For this reason, the following result will have the same result:
var union = source.Where(predicate0.Or(predicate1));
var inter = source.Where(predicate0.And(predicate1));
It depends on the union and intersection components that the two queries create Where
. In such cases:
var union = source1.Where(predicate0).Select(item => item.ID)
.Union(source2.Where(predicate1).Select(item => item.ID));
Then the predicates are probably not of the same type, and there are still other cases where combining predicates will not work as a replacement for union
and Intersect
.
While it is helpful to be able to look at the first examples in both directions, from a single understanding of how predicates, unions, and intersections work.
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