Question

I am trying to create an extension “WhereNot”

So I can use:

Dim x = "Hello world "
Dim y = x.Split.WhereNot(AddressOf String.IsNullOrEmpty)

Note that my aim here is to learn linq expressions; not solve my issue.

I craated this function:

 <Extension()> _
 Public Function WhereNot(Of TElement)(ByVal source As IQueryable(Of TElement), ByVal selector As Expression(Of Func(Of TElement, Boolean))) As IQueryable(Of TElement)
  Return source.Where(GetWhereNotExpression(selector))
 End Function

I don’t know how to switch the boolean flag, will the function Negate do it?

answers in both vb.net and C# are welcommed

Answer 1

I realize this is a very old answered question, but I think the selected answer is misleading, because the asker was looking for an expression, and the selected answer provides a lambda.

This means that its invocation from an IQueryable will return an IEnumerable<T>, not an IQueryable<T>. This forces the expression to compile, and can lead to poorly optimized access to Linq providers.

Here is an answer that directly addresses the original question.

public static class NegationExpressionHelper
{
    public static IQueryable<T> WhereNot<T>(this IQueryable<T> queryable, Expression<Func<T,bool>> predicate)
    {
        return queryable.Where(predicate.Invert());
    }
    public static Expression<Func<T, bool>> Invert<T>(this Expression<Func<T, bool>> original)
    {
        return Expression.Lambda<Func<T, bool>>(Expression.Not(original.Body), original.Parameters.Single());
    }
}