F# thinks my type is not defined
<p>I am starting to play around with F# so I thought I would start by using it for unit testing. The problem is that Visual Studio keeps telling me that the type is not defined for some of the types I am using and I can't understand why. </p>

<p>I have imported the namespace and the type that I am using is public. Here is the F# code:</p>

<pre><code>namespace OpcPersistenceTest

open System
open NUnit.Framework
open OpcPersistence
open OpcPersistence.Data

[&lt;TestFixture&gt;]
type PushServicesTesting() =

    [&lt;Test&gt;]
    let ``create a subscription, then shutdown service, start service, should retrieve original subscription`` =
        let filters = new ResizeArray&lt;Filter&gt;()
        [new Filter(Operand = "EventType", Operator = "=", Value = "SimpleAlarm");
         new Filter(Operand = "Area", Operator = "=", Value = "Area_A")] 
         |&gt; Seq.iter filters.Add

        let keys = new ResizeArray&lt;string&gt;()
        [Guid.NewGuid().ToString(); Guid.NewGuid().ToString();] |&gt; Seq.iter keys.Add

        let configId = PushServices.CreateSubscription(filters, keys)
        PushServices.Shutdown()
        PushServices.Startup()
        let subscription = PushServices.GetSubscriptions(configId)
</code></pre>

<p>And some of the C# code:</p>

<pre><code>using System;
using System.Text.RegularExpressions;

namespace OpcPersistence.Data
{
    public struct Filter
    {
        public string Operand { get; set; }
        public string Operator { get; set; }
        public object Value { get; set; }
        ...
</code></pre>

<p>The type <em>Filter</em> is apparently undefined (so is the type <em>PushServices</em>) but if I swap it out for another class in the same namespace, I do not have this problem. I have even tried changing the name of the class but to no avail. Any ideas?</p>

<p><strong>Edit:</strong></p>

<p>It also doesn't show up in intellisense as the rest of the namespace does as depicted in the picture below.</p>

<p><a href="https://i.stack.imgur.com/KHCYQ.jpg" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/KHCYQ.jpg" alt="enter image description here"></a></p>