Question

I have seen this done in C#, such as here although, I cannot seem to figure out how to do this in VB.NET. For some background, I have created a custom ComboBox control as a .dll, and I need to implement it in another .dll(ArcMap Component).

Unfortunately, ArcMap does not allow for “third-party” DLL’s to be loaded along with the component, because there is no option to reference any third-party assemblies for your add-in.

If someone could point me in the right direction, it would be more than appreciated.

Answer 1

We use this technique in VB.NET in Visual Studio 2008…

First, the project needs to know to include the “other” dll as an Embedded Resource. In the Solution Explorer, add the dll as a file to your project (not as a Reference). Then, open the Properties for the file and set the Build Action to “Embedded Resource.” It is recommended that you create a local copy of the dll file in the structure of your project rather than linking to some other location. Once the project includes the dll file, you can then add the reference to that copy of the dll so that you can use its contents at design-time.

That ensures that the “other” dll is included in your compiled dll, but it doesn’t make it automatically load when needed. That’s where the following code comes in:

Public Module Core

Private _initialized As Boolean

Public Sub EnsureInitialized()
  If Not _initialized Then
    AddHandler AppDomain.CurrentDomain.AssemblyResolve, AddressOf AssemblyResolve
    _initialized = True
  End If
End Sub

Private Function AssemblyResolve(ByVal sender As Object, ByVal e As ResolveEventArgs) As Assembly
  Dim resourceFullName As String = String.Format("[CONTAINER ASSEMBLY].{0}.dll", e.Name.Split(","c)(0))
  Dim thisAssembly As Assembly = Assembly.GetExecutingAssembly()
  Using resource As Stream = thisAssembly.GetManifestResourceStream(resourceFullName)
    If resource IsNot Nothing Then Return Assembly.Load(ToBytes(resource))
    Return Nothing
  End Using
End Function

Private Function ToBytes(ByVal instance As Stream) As Byte()
    Dim capacity As Integer = If(instance.CanSeek, Convert.ToInt32(instance.Length), 0)

    Using result As New MemoryStream(capacity)
        Dim readLength As Integer
        Dim buffer(4096) As Byte

        Do
            readLength = instance.Read(buffer, 0, buffer.Length)
            result.Write(buffer, 0, readLength)
        Loop While readLength > 0

        Return result.ToArray()
    End Using
End Function

End Module

Place this Module somewhere in your project and be sure to call the EnsureInitialized method to attach the AssemblyResolve handler before calling any other code in your dll.

NOTE: You’ll need to replace [CONTAINER ASSEMBLY] with the name of your dll.

Also note that the above code is a stripped-down version of what we actually use because ours includes log4net logging messages at strategic places. The logging messages aren’t necessary for the true functionality, so I removed them for brevity and clarity.

The major caveat to this approach is that the AssemblyResolve handler has to be attached manually. Even if you can’t set things up so that EnsureInitialized is called only once during the initialization of the consuming code, you can call EnsureInitialized within any of your own modules that require the “other” dll for execution. This makes the code a little more delicate because you have to remember to make that initialization call, but it does allow you to sleep at night knowing that the dll will be available when you need it.

In my experience, some “other” dlls don’t play well when they are provided as embedded resources, so you might need to play around a bit to get things working.

Final Note: I’ve never used ArcMap Component, so Your Mileage May Vary!