Syntax to refer a method returning an Expression to another method?

Question

I found a piece of code of the following form:

public static Expression<Func<Invoice, CustomerContact>> GetCustomerContact()
{
   return i => new CustomerContact {
                 FirstName = i.Customer.FirstName,
                 LastName = i.Customer.LastName,
                 Email = i.Customer.Email,
                 TelMobile = i.Customer.TelMobile,
               };
}

In other parts of the code, I want to get the same lightweight CustomerContact object, only not from the Invoice, but from the Customer itself. So the obvious thing to do would be to have:

public static Expression<Func<Customer, CustomerContact>> GetCustomerContact()
{
   return c => new CustomerContact {
                 FirstName = c.FirstName,
                 LastName = c.LastName,
                 Email = c.Email,
                 TelMobile = c.TelMobile,
               };
}

and then change the Expression taking Invoice as input to refer to this method, i.e. something like this:

public static Expression<Func<Invoice, CustomerContact>> GetCustomerContact()
{
   return i => GetCustomerContact(i.Customer); // doesn't compile
}

What's the correct syntax for this?

Answer 1

You can use Expression.Invoke:

var paramExpr = Expression.Parameter(typeof(Invoice), "i");
var propertyEx = Expression.Property(paramExpr, "Customer");

var body = Expression.Invoke(GetCustomerContactFromCustomer(), propertyEx);

return Expression.Lambda<Func<Invoice, CustomerContact>>(body, paramExpr);

Do note that some LINQ providers have problems with such invocation-expressions.

The easiest way to work around this (and to give you more convenient syntax) is to use LINQKit:

var expr = GetCustomerContactFromCustomer();   
Expression<Func<Invoice, CustomerContact>> result = i => expr.Invoke(i.Customer);    
return result.Expand();