Create empty lambda expression

Question

Consider the function:

public async Task<IEnumerable<Purchases>> GetPurchases(User user, Expression<Func<Purchases, bool>> whereClause)
    {           
            using (var context = new UserDbContext())
            {
                context.Users.Attach(user);
                context.Entry(user).Collection(p => p.Purchases)
                .Query()
                .Where(whereClause)
                .Load();

                if (Equals(user.Purchases, null))
                    return new List<Purchases>();
            }

        return user.Purchases;
}

In this function the parameter whereClause can at times be null , I'm wanting to check if its null and then assign an empty Expression if so. This is as close as I've come:

if (Equals(whereClause, null))
       whereClause = () => { };

This was based on the question here , but for the line that makes whereClause empty I'm getting the error.

Error 7 Not all code paths return a value in lambda expression of type 'System.Func'<'Purchases,bool>'

Anyone know how this could be corrected?

Answer 1

"Not all code paths return a value" means your lambda isn't returning anything. I mean, it's got only one code path, and it's not a long one.

Where requires a lambda which takes one parameter, and returns bool . Where won't let you give it an empty lambda expression.

You need to give it something it can use to filter items with.

That's actually easy:

if (Equals(whereClause, null))
    whereClause = o => true;

That's a where clause that always returns true , regardless of what you give it. That's probably what you want: If there's no filter, include everything. For each item in the enumeration, one at a time, Where gives the lambda the item as o . If the lambda returns true , it includes that value of o in the results.

If you want a null filter to return no items, just return false instead:

if (Equals(whereClause, null))
    //  Whatever it is, I'm against it.
    whereClause = o => false;

Answer 2

whereClause must return a boolean value to use it in LinQs Where method.

Assuming you don't want to filter the list (include all items) you must return true , otherwise false :

if (Equals(whereClause, null))
     whereClause = () => true; 

Notice: The { and } are not necessary if you only return a value with a single statement.

But you should know that the most framework methods don't work in this way. They would throw an ArgumentNullException instead of setting a value.

Answer 3

Assuming that when whereClause is null you don't want to apply any filter.

        using (var context = new UserDbContext())
        {
            context.Users.Attach(user);
            context.Entry(user).Collection(p => p.Purchases)
            .Query()
            .Where(whereClause ?? p => true)
            .Load();

            if (Equals(user.Purchases, null))
                return new List<Purchases>();
        }