Question

I want to take a class, loop through it’s properties, get the property value, and call a method passing that property value in. I think I can get the property values, but what does the lambda expression’s body look like? What body is used to call a method on each property?

This is what I have so far…

Action<T> CreateExpression<T>( T obj )
{
 foreach( var property in typeof( T ).GetProperties() )
 {
  Expression value = Expression.Property( Expression.Constant( obj ), property );
  var method = Expression.Call( typeof( SomeType ), "SomeMethod", null, value );
 }

 // What expression body can be used that will call
 // all the method expressions for each property?
 var body = Expression...
 return Expression.Lambda<Action<T>>( body, ... ).Compile();
}

Answer 1

It depends on a few things.

• does the method return anything? Expression in 3.5 can’t do multiple separate “action” operations (a statement body), but you can cheat if you can do something with a fluent API:

  SomeMethod(obj.Prop1).SomeMethod(obj.Prop2).SomeMethod(obj.Prop3);
  

  (perhaps using generics to make it simpler)

• do you have access to 4.0? In 4.0 there are additional Expression types allowing statement bodies and exactly what you ask for. I discuss some similar examples in an article here (look for Expression.Block, although this is based on a beta a while ago – it may have been renamed by now).

Alternative; since you are compiling to a delegate, consider that an Action<T> is multicast; you could build a set of simple operations, and combine them in the delegate; this would work in 3.5; for example:

using System;
using System.Linq.Expressions;
static class SomeType
{
    static void SomeMethod<T>(T value)
    {
        Console.WriteLine(value);
    }
}
class Customer
{
    public int Id { get; set; }
    public string Name { get; set; }
}
static class Program
{
    static readonly Action<Customer> action = CreateAction<Customer>();
    static void Main()
    {
        Customer cust = new Customer { Id = 123, Name = "Abc" };
        action(cust);
    }
    static Action<T> CreateAction<T>()
    {
        Action<T> result = null;
        var param = Expression.Parameter(typeof(T), "obj");
        foreach (var property in typeof(T).GetProperties(
            BindingFlags.Instance | BindingFlags.Public))
        {
            if (property.GetIndexParameters().Length > 0) continue;
            var propVal = Expression.Property(param, property);
            var call = Expression.Call(typeof(SomeType), "SomeMethod", new Type[] {propVal.Type}, propVal);
            result += Expression.Lambda<Action<T>>(call, param).Compile();
        }
        return result;
    }
}