如何组合两个 LINQ select 表达式？

Question

I am trying to have a select expression that can be incrementally updated depending on what I receive from the input, something like this:

// Init expression
Expression<Func<Order, object>> selectExpression = x => new
{
    x.Id
};

if(selectInput.Title){
    // something like this
    selectExpression = selectExpression.Add(x => new
    {
        x.Title
    });
}
if(selectInput.ClientFullName){
    // something like this
    selectExpression = selectExpression.Add(x => new
    {
        ClientFullname = x.Client.Fullname
    });
}

// Use expression in a LINQ query (for an EF Core query)
var result = await queryable.Select(selectExpression).ToListAsync();

Then I would expect to have a result like this

{
    "Id": 568,
    "Title": "Order 567",
    "ClientFullname": "John Smith"
}

Is this something that is possible? The only examples I found online are about.Where(). Am I going into the wrong direction?

Thank you!

Answer 1

If you would do this, then the problem is, that your compiler would not know whether property ClientFullName is a property in your result or not, thus your compiler can't use it.

Of course you can use the name of the property as an identifier, and put everything in a Dictionary. like @Canton7 suggested.

If you don't expect null values, you can return a class with all properties. The properties that have value null will not have been selected:

class MyItem
{
    public int Id {get; set;}              // Id always fetched
    public string Title {get; set;}
    public DateTime? Date {get; set;}
    ... // etc: all nullable properties
}

As an extension method:

public static IQueryable<MyItem> ToSelectedItems(
    this IQueryable<Order> orders,
    SelectedInput selectedInput)
{
    // TODO: exception if orders or selectedInput null

    return orders.Select(order => new MyItem
    {
        Id = order.Id,

        // properties null if not in selectedInput
        Title = selectedInput.Title ? order.Title : null,
        ClientFullName = selectedInput.ClientFullName ? order.ClientFullName : null,
        Date = selectedInput.Date ? order.Date : null,
     })

Answer 2

You might find LinqKit useful: ( http://www.albahari.com/nutshell/linqkit.aspx ) I have used this many times when trying to write variable where clauses against IQueryable data sources. The PredicateBuilder ( http://www.albahari.com/nutshell/predicatebuilder.aspx ) is really useful for constructing dynamic query expressions.

Sample code would be something like this...

    public void QueryItems(string id, string name, string fullname)
    {
        // this would be the collection od data to be filtered
        var items = new List<Item>();

        // initialise predicate of for querying objects of type Item
        var predicate = PredicateBuilder.New<Item>();

        // dynamically add clauses dependent on available filter values
        if (!string.IsNullOrEmpty(id))
        {
            predicate = predicate.And(x => x.Id == id);
        }
        if (!string.IsNullOrEmpty(name))
        {
            predicate = predicate.And(x => x.Name == name);
        }
        if(!string.IsNullOrEmpty(fullname))
        {
            predicate = predicate.And(x => x.FullName == fullname);
        }

        // evaluate the result of the dynamic query
        var result = items.Where(predicate).ToList();
    }