Dynamic LINQ, Select function, works on Enumerable, but not Queryable

问题

I have been fiddling with dynamic LINQ for some time now, but I have yet to learn its secrets.

I have an expression that I want to parse that looks like this:

"document.LineItems.Select(i => i.Credit).Sum();"

During parsing of this I reach a point where I need to call a Select function on LineItems Collection. I am using factory method of Expression.Call:

Expression.Call(
    typeof(Queryable),
    "Select",
    new Type[] { typeof(LineItem), typeof(decimal?) },
    expr,
    Expression.Lambda(expression, new ParameterExpression[] { Expression.Parameter(typeof(LineItem) }));

At this moment

expr:          document.LineItems    [ICollection<LineItem>]  
expression:    LineItem.Credit       [decimal?]

none of which is materialized yet. I am just building Expression Tree at the moment.

Now, the problem:

This Expresssion.Call throws and Exception: "No generic method 'Select' on type 'System.Linq.Queryable' is compatible with the supplied type arguments and arguments";

I resolve it easily by looking for 'Select' in 'System.Linq.Enumerable' instead of 'Queryable' by changing first argument of Expression.Call.

But, that's not quite that I want. I don't want all LineItems hauled in only to calculate Sum(), which would obviously be case with Enumerable. I want Queryable to work.

Also, for the last part of parsing - Sum(), I also need to go with Enumerable Sum(), because Queryable Sum() throws same Exception.

I have checked MSDN, both signatures of 'Select' function are identical, so i really cannot see why would one work and other not.

Any help or pointers would be aprreciated.

Regards,

回答1:

The problem is that LineItems is an ICollection<LineItem>, while the first parameter to Queryable.Select requires an IQueryable<LineItem>. ICollection<T> only implements IEnumerable<T> which is why it works for Enumerable.Select.

You will need to change the type of expr to be IQueryable<LineItem>.

You should be able to do this with the Queryable.AsQueryable method. The following function creates an expression to sum the credit property values for a given Document instance:

public static Expression<Func<decimal?>> CreateSumLineItemsExpr(Document document)
{
    var docExpr = Expression.Constant(document);
    var itemsExpr = Expression.Property(docExpr, "LineItems");

    Expression<Func<LineItem, decimal?>> selector = i => i.Credit;
    var queryableExpr = Expression.Call(typeof(Queryable), "AsQueryable", new[] { typeof(LineItem) }, itemsExpr);
    var selectExpr = Expression.Call(typeof(Queryable), "Select", new[] { typeof(LineItem), typeof(decimal?) }, queryableExpr, selector);
    var sumExpr = Expression.Call(typeof(Queryable), "Sum", null, selectExpr);

    return Expression.Lambda<Func<decimal?>>(sumExpr);
}

来源：https://stackoverflow.com/questions/13777781/dynamic-linq-select-function-works-on-enumerable-but-not-queryable

标签

linq

iqueryable

expression

enumerable