c# build generic Expression tree for list contains

Question

Hello I would like to create a generic Expression Tree that returns a list contains result.

public static class Extension{
    public static List<T> WhereIn<T, T1>(IQueryable<T> query, IEnumerable<T1> keys, Expression<Func<T, T1>> param)
    {
    }
}

The problem is that I would also like to create something like this:

var result = Extension.WhereIn(customers.AsQueryable(), stringList, c => c.Number.ToString());

so far this would work for a static property name:

public static Expression<Func<T, bool>> FilterByCode<T, T1>(List<T1> codes, string propName)
{
    var methodInfo = typeof(List<T1>).GetMethod("Contains", 
        new Type[] { typeof(T1) });

    var list = Expression.Constant(codes);

    var param = Expression.Parameter(typeof(T), "j");
    var value = Expression.Property(param, propName);
    var body = Expression.Call(list, methodInfo, value);

    // j => codes.Contains(j.Code)
    return Expression.Lambda<Func<T, bool>>(body, param);
}

I don't get your question. What should WhereInA do? Can you hardcode expected expression or add an example of how it should work? — Aleks Andreev
– Aleks Andreev, Commented Jul 11, 2019 at 13:36
Basically the WhereIn (WhereInA had an A too much) method should create a contains Expression. But like you see in the second code Extension.WhereIn... It should get a queryable, a list and an expression as Parameter. The tricky part for me is to pass an expression that is not a member Expression but something like : c => c.Number.ToString() — Franki1986
– Franki1986, Commented Jul 11, 2019 at 19:26

Franki1986 · Accepted Answer · 2019-07-12 07:48:59Z

Thanks to Marc Gravell I got the solution:

  public List<T> WhereIn<T, TValue>(IQueryable<T> source, IEnumerable<TValue> keys, Expression<Func<T, TValue>> selector)
  {
     MethodInfo method = null;
     foreach (MethodInfo tmp in typeof(Enumerable).GetMethods(
        BindingFlags.Public | BindingFlags.Static))
     {
        if (tmp.Name == "Contains" && tmp.IsGenericMethodDefinition
                                   && tmp.GetParameters().Length == 2)
        {
           method = tmp.MakeGenericMethod(typeof(TValue));
           break;
        }
     }
     if (method == null) throw new InvalidOperationException(
        "Unable to locate Contains");
     var row = Expression.Parameter(typeof(T), "row");
     var member = Expression.Invoke(selector, row);
     var values = Expression.Constant(keys, typeof(IEnumerable<TValue>));
     var predicate = Expression.Call(method, values, member);
     var lambda = Expression.Lambda<Func<T, bool>>(
        predicate, row);
     return source.Where(lambda).ToList();
  }

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