invert.cs

using System;
using System.Linq;
using System.Linq.Expressions;

class Program
{
    static void Main(string[] args)
    {
        new [] {"foo", "", "bar", null, "baz", "  "}
            .Where(Invert<Func<String, Boolean>>(String.IsNullOrWhiteSpace))
            //           ~~~~~~~~~~~~~~~~~~~~~~~ (>_<
            // .IsSequenceEqual(new [] { "foo", "bar", "baz", })
            .ToList()
            .ForEach(Console.WriteLine);
    }

    public static TDelegate Invert<TDelegate>(TDelegate predicate)
    {
        if (predicate.GetType().GetMethod("Invoke").ReturnType != typeof(Boolean))
        {
            throw new ArgumentException("predicate");
        }
        var parameters = predicate
            .GetType()
            .GetMethod("Invoke")
            .GetParameters()
            .Select(p => Expression.Parameter(p.ParameterType, p.Name))
            .ToArray();
        return Expression.Lambda<TDelegate>(
            Expression.Not(
                Expression.Invoke(Expression.Constant(predicate), parameters)
            ),
            parameters
        ).Compile();
    }
}