Add variable property/field name in the contains when creating an expression tree

Question

Consider expression tree below,

var parameter = Expression.Parameter(typeof(Customer), "b");
var property = Expression.Property(parameter, "FirstName");
var abc = new string[] { "a", "b" };
MethodInfo contains = typeof(Enumerable)
    .GetMethods(BindingFlags.Static | BindingFlags.Public)
    .Where(x => x.Name.Contains(nameof(Enumerable.Contains)))
    .Single(x => x.GetParameters().Length == 2)
    .MakeGenericMethod(property.Type);
var body = Expression.Call(contains, Expression.Constant(abc), property);

when we view this code in into debug we see the query generated by the body (variable) is:

.Call System.Linq.Enumerable.Contains(
    .Constant<System.String[]>(System.String[]),
    $b.FirstName)

and when we inspect the where in (var results = customers.Where(x => abc.Contains(x.FirstName)).ToList();) we get this:

.Call System.Linq.Enumerable.Contains(
    .Constant<Program+<>c__DisplayClass0_0>(Program+<>c__DisplayClass0_0).abc,
    $x.FirstName)

How do you add the abc like in the where in the expression tree ?

Notes abc is: var abc = new string[] { "a", "b" } (see code above)

Probaly `Expression.MakeMemberAccess` will help, but you did not explain what `abc` is. — Olivier Jacot-Descombes, Aug 16 '23 at 19:09
by curiosity how would you add the Expression.MakeMemberAccess in there ? — Zulander, Aug 17 '23 at 01:20
I don't know by heart, but a look at the parameters of the `MakeMemberAccess` method should give you clues (of course only if `.abc` was a real member access). — Olivier Jacot-Descombes, Aug 17 '23 at 09:52

Olivier Jacot-Descombes · Accepted Answer · 2023-08-16T20:09:02.653

You are almost there. Add this:

var lambda = Expression.Lambda(body, parameter);
// Displays as:
// b => value(System.String[]).Contains(b.FirstName)

var compiled = (Func<Customer, bool>)lambda.Compile();

You can test it with:

var cust = customers.Where(compiled).ToList();
// I get the expected result with some test data.

Note: the expression above may look awkward, but this is due to the fact that the expression captures the local variable in a compiler generated class named <>c__DisplayClass0_0.

See this answer form user295190 to the question What are 'closures' in .NET?

You cannot see this compiler generated code in your expression, because it is probably created when being compiled (call to Compile()).

Add variable property/field name in the contains when creating an expression tree

1 Answers1