How to write List All method using expression trees?

Question

I have below code snippet.

var list= new List<CustomClass>(){ new CustomClass(){Id=10}};
var result= list.All(a=>a.Id==10);

How can we write it in an Expression Tree?

Purpose: It's part of a bigger expression tree logic which we are implementing, I am just stuck at generating the expression tree for "All" method of the list.

Thanks

Answer 1

It isn't exactly clear what you are trying to do, but let's start from here...

This is the Enumerable.All method:

static readonly MethodInfo allTSource = (from x in typeof(Enumerable).GetMethods(BindingFlags.Static | BindingFlags.Public)
                                         where x.Name == nameof(Enumerable.All)
                                         let args = x.GetGenericArguments()
                                         where args.Length == 1
                                         let pars = x.GetParameters()
                                         where pars.Length == 2 &&
                                             pars[0].ParameterType == typeof(IEnumerable<>).MakeGenericType(args[0]) &&
                                             pars[1].ParameterType == typeof(Func<,>).MakeGenericType(args[0], typeof(bool))
                                         select x).Single();

Then the lambda expression used in .All(...):

// a => 
var par = Expression.Parameter(typeof(CustomClass), "a");

// a.Id
var id = Expression.Property(par, nameof(CustomClass.Id));

// 10
var val = Expression.Constant(10);

// a.Id == 10
var eq = Expression.Equal(id, val);

// a => a.Id == 10
var lambda = Expression.Lambda<Func<CustomClass, bool>>(eq, par);

Then the problem: it isn't clear if the list is a constant or an external expression... I don't know how to connect my code to yours... Let's say for simplicity that it is a constant:

// Unclear what lst should be, a parameter of an external lambda or a constant
var lst = Expression.Constant(list);

// list.All(...)
var all = Expression.Call(allTSource.MakeGenericMethod(typeof(CustomClass)), lst, lambda);

// () => list.All(...)
Expression<Func<bool>> exp = Expression.Lambda<Func<bool>>(all);
Func<bool> fn = exp.Compile();
bool res = fn();