Question

I’m new to lambda. So excuse me if my question is simple.

I have a method that uses reflection to set a property on some types:

public void WriteId(object obj, int id) {
    var type = obj.GetType();
    var prop = type.GetProperties(BindingFlags.Public | BindingFlags.Instance)
                   .Where(p => p.CanRead && p.CanWrite)
                   .Where(p => p.Name == "Id")
                   .Where(p.PropertyType == typeof(int))
                   .FirstOrDefault();
    if(prop != null)
        prop.SetValue(obj, id, null);
}

Can you show me please how can I create a lambda that do same job? Actually I want to create a lambda for each type, compile it, and cache it. Thanks in advance.

Answer 1

I would install FastMember from NuGet, and then use:

var wrapped = ObjectAccessor.Create(obj);
obj["Id"] = id;

which does pretty much what you say, except it happens to use ILGenerator via TypeBuilder (rather than Expression) – but all the caching etc is there.

A second cheaky approach is to get dynamic to do it all for you:

((dynamic)obj).Id = id;

But if you want to use Expression for other reasons:

using System;
using System.Linq.Expressions;   

static class Program
{
    static void Main()
    {
        var obj = new Foo { Id = 2 };
        WriteId(obj, 6);
        Console.WriteLine(obj.Id); // 6
    }

    private static class SneakyCache<T>
    {
        public static readonly Action<T, int> SetId;
        static SneakyCache()
        {
            var obj = Expression.Parameter(typeof(T), "obj");
            var id = Expression.Parameter(typeof(int), "id");
            SetId = Expression.Lambda<Action<T, int>>(
                  Expression.Assign(Expression.Property(obj, "Id"), id),
                  obj, id).Compile();
        }
    }
    public static void WriteId<T>(T obj, int id) where T : class
    {
        SneakyCache<T>.SetId(obj, id);
    }
}
class Foo
{
    public int Id { get; set; }
}