Combining two expressions (Expression>)

Question

I have two expressions of type Expression> and I want to take to OR, AND or NOT of these and get a new expression of the same type

Answer 1

You can use Expression.AndAlso / OrElse to combine logical expressions, but you have to make sure the ParameterExpressions are the same.

I was having trouble with EF and the PredicateBuilder so I made my own without resorting to Invoke, that I could use like this:

var filterC = filterA.And(filterb);

Source code for my PredicateBuilder:

public static class PredicateBuilder {

    public static Expression> And(this Expression> a, Expression> b) {    

        ParameterExpression p = a.Parameters[0];

        SubstExpressionVisitor visitor = new SubstExpressionVisitor();
        visitor.subst[b.Parameters[0]] = p;

        Expression body = Expression.AndAlso(a.Body, visitor.Visit(b.Body));
        return Expression.Lambda>(body, p);
    }

    public static Expression> Or(this Expression> a, Expression> b) {    

        ParameterExpression p = a.Parameters[0];

        SubstExpressionVisitor visitor = new SubstExpressionVisitor();
        visitor.subst[b.Parameters[0]] = p;

        Expression body = Expression.OrElse(a.Body, visitor.Visit(b.Body));
        return Expression.Lambda>(body, p);
    }   
}

And the utility class to substitute the parameters in a lambda:

internal class SubstExpressionVisitor : System.Linq.Expressions.ExpressionVisitor {
        public Dictionary subst = new Dictionary();

        protected override Expression VisitParameter(ParameterExpression node) {
            Expression newValue;
            if (subst.TryGetValue(node, out newValue)) {
                return newValue;
            }
            return node;
        }
    }