Create New Expression from Existing Expression

Question

I have an Expression<Func<T,DateTime>> I want to take the DateTime part of the expression and pull the Month off of it. So I would be turning it into a Expression<Func<T,int>> I'm not really sure how to do this. I looked at the ExpressionTree Visitor but I can't get it to work like I need. Here is an example of the DateTime Expression

DateTimeExpression http://img442.imageshack.us/img442/6545/datetimeexpression.png

Here is an example of what I want to create MonthExpression http://img203.imageshack.us/img203/8013/datetimemonthexpression.png

It looks like I need to create a new MemberExpression that is made up of the Month property from the DateTime expression but I'm not sure.

Answer 1

Yes, that's exactly what you want - and using Expression.Property is the easiest way to do that:

Expression func = Expression.Property(existingFunc.Body, "Month");
Expression<Func<T, int>> lambda = 
    Expression.Lambda<Func<T, int>>(func, existingFunc.Parameters);

I believe that should be okay. It works in this simple test:

using System;
using System.Linq.Expressions;

class Person
{
    public DateTime Birthday { get; set; }
}

class Test
{
    static void Main()
    {
        Person jon = new Person 
        { 
            Birthday = new DateTime(1976, 6, 19)
        };

        Expression<Func<Person,DateTime>> dateTimeExtract = p => p.Birthday;
        var monthExtract = ExtractMonth(dateTimeExtract);
        var compiled = monthExtract.Compile();
        Console.WriteLine(compiled(jon));
    }

    static Expression<Func<T,int>> ExtractMonth<T>
        (Expression<Func<T,DateTime>> existingFunc)
    {
        Expression func = Expression.Property(existingFunc.Body, "Month");
        Expression<Func<T, int>> lambda = 
            Expression.Lambda<Func<T, int>>(func, existingFunc.Parameters);
        return lambda;
    }                                        
}