Using Expressions to build Array.Contains for Entity Framework

Question

I want to have a variable field in both Where-Contains and Select. "field_a" is the guy I want to be variable (sometimes I want field_b or _c; they are strings). The code below properly builds the Select as Select(x => x.field_a). How do I get the second part of the Where clause to say, && targetCodes.Contains(x.field_a)? [db is a DbContext, itemsArray is an array of objects that have a string property called Code.]

    using (var db = dbFactory.CreateInstance())
    {
        var parameter = Expression.Parameter(typeof(myTable), "x");
        var field = Expression.Property(parameter, "field_a");
        var selector = Expression.Lambda(field, parameter);
        var targetCodes = itemsArray.Select(i => i.Code).ToArray();
        var query = db.myTables
            .Where(x => x.companyId == 1 && targetCodes.Contains(x.field_a))
            .Select((Expression<Func<myTable, string>>)selector);
        return await query.ToArrayAsync();
    }

Answer 1

Probably the most difficult part is to find MethodInfo of the .Contains() method. You could use typeof(IEnumerable<string>).GetMethod(...).Where(...), but it is usually difficult to do it right for generic method with several overloads. This is a small trick that employs C# compiler to find the correct overload for you by creating temporary expression:

Expression<Func<IEnumerable<string>, bool>> containsExpr = (IEnumerable<string> q) => q.Contains((string)null);
var containsMethod = (containsExpr.Body as MethodCallExpression).Method;
// containsMethod should resolve to this overload:
// System.Linq.Enumerable.Contains<string>(IEnumerable<string>, string)

The rest of the program just builds expression by calling appropriate Expression.XYZ() methods:

var companyIdEquals1 = Expression.Equal(
    Expression.Property(parameter, nameof(myTable.companyId)),
    Expression.Constant(1));

var targetCodesContains = Expression.Call(
    containsMethod,
    Expression.Constant(targetCodes),
    field/*reuses expression you already have*/);

var andExpr = Expression.And(companyIdEquals1, targetCodesContains);
var whereExpr = (Expression<Func<myTable, bool>>)Expression.Lambda(andExpr, parameter);

var query = db//.myTables
    .Where(whereExpr)
    .Select((Expression<Func<myTable, string>>)selector);