无法将X汇总到Count（）

Question

I'm looking at a solution that began life in .net Framework 2.0 and has now been moved up to Framework 4.0, so I'm probably missing a required reference and/or Imports line somewhere, I think...

I've got a LINQ query in the form:

Dim x = Aggregate myDataRow As System.Data.DataRow In myDataTable _
        Where (booleanCondition1) _
        AndAlso (booleanCondition2) _
        AndAlso (booleanCondition3) _
        Into Count()

I'm getting the compile error " Definition of method 'Count' is not accessible in this context ".

As far as I can tell from the literature the Count() function should be available (and apparently should register as a keyword).

What am I doing wrong?

EDIT:

I've just changed my code to a straight Select method, like so:

Dim x = (From myDataRow As System.Data.DataRow In myDataTable _
        Where (booleanCondition1) _
        AndAlso (booleanCondition2) _
        AndAlso (booleanCondition3) _
        Select myDataRow)

and when I try to get the .Count of x, I get the message " 'Count' is not a member of 'System.Data.EnumerableRowCollection(Of System.Data.DataRow) " and according to the MSDN, it is .

Totally confused, now.

Answer 1

First, let's look at EnumerableRowCollection(Of TRow) , and note that Count is an extension method:

Count<TRow>() Overloaded. Returns the number of elements in a sequence. (Defined by Enumerable .)

Enumerable itself has this information:

Namespace: System.Linq
Assembly: System.Core (in System.Core.dll)

Note that you'll always find information on both assemblies and namespaces in the MSDN help. Both are important, and there's not a one-one correspondence between a type's namespace and the assembly that it's found in.

So, next, let's look at extension methods . Unfortunately, it says this:

Typically, the module in which an extension method is defined is not the same module as the one in which it is called. Instead, the module that contains the extension method is imported, if it needs to be, to bring it into scope

and then doesn't really elaborate fuller on what it means for the module to be in scope. It does hint at it towards the end though:

When two extension methods that have identical signatures are in scope and accessible, the one with higher precedence will be invoked. An extension method's precedence is based on the mechanism used to bring the method into scope. The following list shows the precedence hierarchy, from highest to lowest.

• Extension methods defined inside the current module.

• Extension methods defined inside data types in the current namespace or any one of its parents, with child namespaces having higher precedence than parent namespaces.

• Extension methods defined inside any type imports in the current file.

• Extension methods defined inside any namespace imports in the current file.

• Extension methods defined inside any project-level type imports.

• Extension methods defined inside any project-level namespace imports.

Since the first two bullets aren't relevant to your situation, only the remainder apply. And they all deal with imports

Imports :

The Imports statement enables types that are contained in a given namespace to be referenced directly.

Hopefully, from all of the above, you can see why having a reference to the correct assembly, by itself, was insufficient, and you also needed to add an import.