无法解析可枚举和可查询候选之间的方法

Question

I have this method in a class called Invoice:

public static Expression<Func<Invoice, bool>> IsAllocated()
{
    return i => i.TotalAmountDue == i.GetAllocationsTotal();
}

I have a list like this:

IQueryable<Invoice> invoices

And I need to filter it like that (it's Linq to Entity):

var filteredInvoices = invoices.Where(i => Invoice.IsAllocated());

In this line I'm getting two errors:

Cannot resolve method ... candidates are .... one in Enumerable and the other on in Queryable.

And also:

Cannot convert expression type Expression<Func<Invoice,bool>> to return type 'bool'

I've tried a lot of things I've found in SO with no luck. Can someone say me what is missing here or at least, which one of the two errors is at the root of the problem?

Answer 1

您的方法已经返回了适当的表达式树-您只需要调用它，而不用在lambda表达式中调用它：

var filteredInvoices = invoices.Where(Invoice.IsAllocated());

Answer 2

Expression are representation and not delegate by themselves. You should create a delegate out of it first

static Expression<Func<Invoice, bool>> IsAllocatedExpr()
{
    return i => i.TotalAmountDue == i.GetAllocationsTotal();
}

public static Func<Invoice, bool> IsAllocated = IsAllocatedExpr().Compile();

and then

var filteredInvoices = invoices.Where(i => Invoice.IsAllocated(i));