LINQ查询中的where子句不根据数据求值

Question

I'm populating an ObservableCollection with the following statement:

foreach (var customer in
    collListItem.Select(
        item =>
        new
            {
                ResourceName = item["Resource_x0020_Name"].ToString(),
                Quantity = (item["Quantity_x0020_Ordered"] ?? 0).ToString(),
                CustomerName = item["Title"].ToString()
        })
        .Select(r => new Customer { ResourceName = r.ResourceName, Quantity = int.Parse(r.Quantity) })
        .Where(r => r.CustomerName == "test"))
{
    resources.Add(customer);
}

When I remove the .Where() portion of my LINQ query the statement will fetch all customers. I would like to restrict this to a customer I specify. Whenever I specify a customer in the where clause it doesn't appear to evaluate properly. I'm sure my problem is syntactical but after referring to the LINQ documentation I'm a bit unsure where I'm going wrong.

Answer 1

You have two calls to Select . The first one projects the original collection to a collection of dynamic objects with three properties ( ResourceName , Quantity , and CustomerName ). Then your second call to Select projects that collection into another collection of Customer instances, but only two properties of that class are set ( ResourceName and Quantity ). You are not setting CustomerName in the second select therefore it is uninitialized when Where is called.

I think you really just want one call to Select , which combines the two calls and skips creating of the dynamic objects.

For example:

foreach (var customer in
    collListItem.Select(
        item =>
        new Customer {
                ResourceName = item["Resource_x0020_Name"].ToString(),
                Quantity = (item["Quantity_x0020_Ordered"] ?? 0),
                CustomerName = item["Title"].ToString()
        }
    )
    .Where(r => r.CustomerName == "test"))
{
    resources.Add(customer);
}

I'm assuming that item["Quantity_x0020_Ordered"] is already an integer since you have it combined with an integer via the ?? operator. Therefore there's no need for the ToString call from the first select or the int.Parse call from the second.

Answer 2

Just switch the last Select with the Where

foreach (var customer in
    collListItem.Select(
        item =>
        new
            {
                ResourceName = item["Resource_x0020_Name"].ToString(),
                Quantity = (item["Quantity_x0020_Ordered"] ?? 0).ToString(),
                CustomerName = item["Title"].ToString()
            })
        .Where(r => r.CustomerName == "test")
        .Select(r => new Customer { ResourceName = r.ResourceName, Quantity = int.Parse(r.Quantity) }))
{
    resources.Add(customer);
}

or collapse it like this

resources.AddRange(collListItem
    .Where(item => item["Title"].ToString() == "test")
    .Select(item =>
        new Customer {
            ResourceName = item["Resource_x0020_Name"].ToString(),
            Quantity = item["Quantity_x0020_Ordered"] ?? 0
        }
     )
 );

You always want to query first ( Where before Select ) because Select actually creates an entirely new array each time, and it can be expensive to do so, so you want to make sure you do that with the least amount of items as possible, by using all the queries first, so that you have all the items you need first then you can create the array.