C #, WinForms and Extension Methods

Question

Apart from all the obvious answers, what will call the extension methods to generate compiler errors such as:

'DataType' does not contain a definition for 'YourExtensionMethodName'

I have a real stone here, and this is detailed below. I have exhausted every possible reason I can think of.

Scenario

• I have several extension methods defined in various static classes in a DLL that are used by a WinForms application.
• Extension signatures do not conflict with class I method signatures that are extended ( String
  
  in this case).
• Both DLL and WinForms applications are written in C #.
• Both DLL and WinForms applications are configured to install on .NET 3.5.
• The consumption classes include a reference to the namespace that defines the extension method. Its spelling has been checked.
• If I reference the extension class directly, the extension methods appear. For example, if I type StringExtensions.
  
  , Intellisense looks like normal and all my extension methods are listed.
• EDIT . Errors occur in a WinForms application, but only for some extension methods, not all.

Code (or excerpt)

(Yes, this is the violation code)

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace Roswell.Framework
{
    public static class StringBuilderExtensions
    {
        public static string ToSentenceCase(this string value)
        {
            return value.Substring(0, 1).ToUpper() + value.Substring(1).ToLower();
        }

        public static string ToTitleCase(this string value)
        {
            string[] parts = value.Split(new string[] {" "}, StringSplitOptions.None);
            System.Text.StringBuilder builder = new System.Text.StringBuilder();
            foreach (string part in parts)
            {

                builder.Append(part.ToSentenceCase());
                builder.Append(" ");
            }
            return builder.ToString();
        }

    }
}

      

        
        
        
      

    

And this is the code that consumes it:

using Roswell.Framework;

namespace Roswell.Windows.Command
{
    /// <summary>
    /// Views the SQL for an object in the database window.
    /// </summary>
    internal class ViewObjectDdlCommand
        : MainWindowCommand
    {

        public override void Execute()
        {
           // ...

           OpenCodeWindow(
               string.Format("{0} - {1} - {2}", 
                             dsn.Name, 
                             objectName, 
                             info.ToTitleCase()),
                schemaItemType,
                objectName);
         }
    }
}