How to merge two C# Lambda Expressions without an invoke?

Question

I\'d like to merge the following Expressions:

// example class
class Order
{
    List Lines       
}
class OrderLine { }

Expression

        

Answer 1

This extension works:

public static class Utility
    {
        public static Expression Compose(this Expression first, Expression second, Func merge)
        {
            // build parameter map (from parameters of second to parameters of first)
            var map = first.Parameters.Select((f, i) => new { f, s = second.Parameters[i] }).ToDictionary(p => p.s, p => p.f);

            // replace parameters in the second lambda expression with parameters from the first
            var secondBody = ParameterRebinder.ReplaceParameters(map, second.Body);

            // apply composition of lambda expression bodies to parameters from the first expression 
            return Expression.Lambda(merge(first.Body, secondBody), first.Parameters);
        }

        public static Expression> And(this Expression> first, Expression> second)
        {
            return first.Compose(second, Expression.And);
        }

        public static Expression> Or(this Expression> first, Expression> second)
        {
            return first.Compose(second, Expression.Or);
        }
    }

Sample using:

Expression> filter1 = p => a.ProductId == 1;
Expression> filter2 = p => a.Text.StartWith("test");
Expression> filterCombined = filter1.And(filter2);