.Net Dynamically Load DLL

Question

I am trying to write some code that will allow me to dynamically load DLLs into my application, depending on an application setting. The idea is that the database to be accessed is set in the application settings and then this loads the appropriate DLL and assigns it to an instance of an interface for my application to access.

This is my code at the moment:

        Dim SQLDataSource As ICRDataLayer
    Dim ass As Assembly = Assembly. _
    LoadFrom("M:\MyProgs\WebService\DynamicAssemblyLoading\SQLServer\bin\Debug\SQLServer.dll")

    Dim obj As Object = ass.CreateInstance(GetType(ICRDataLayer).ToString, True)
    SQLDataSource = DirectCast(obj, ICRDataLayer)

    MsgBox(SQLDataSource.ModuleName & vbNewLine & SQLDataSource.ModuleDescription)

I have my interface (ICRDataLayer) and the SQLServer.dll contains an implementation of this interface. I just want to load the assembly and assign it to the SQLDataSource object.

The above code just doesn't work. There are no exceptions thrown, even the Msgbox doesn't appear. I would've expected at least the messagebox appearing with nothing in it, but even this doesn't happen!

Is there a way to determine if the loaded assembly implements a specific interface. I tried the below but this also doesn't seem to do anything!

        For Each loadedType As Type In ass.GetTypes
        If GetType(ICRDataLayer).IsAssignableFrom(loadedType) Then
            Dim obj1 As Object = ass.CreateInstance(GetType(ICRDataLayer).ToString, True)
            SQLDataSource = DirectCast(obj1, ICRDataLayer)
        End If
    Next

---

EDIT: New code from Vlad's examples:

    Module CRDataLayerFactory
    Sub New()
    End Sub
    ' class name is a contract,
    ' should be the same for all plugins
    Private Function Create() As ICRDataLayer
        Return New SQLServer()
    End Function
End Module

Above is Module in each DLL, converted from Vlad's C# example.

Below is my code to bring in the DLL:

Dim SQLDataSource As ICRDataLayer
    Dim ass As Assembly = Assembly. _
    LoadFrom("M:\MyProgs\WebService\DynamicAssemblyLoading\SQLServer\bin\Debug\SQLServer.dll")

    Dim factory As Object = ass.CreateInstance("CRDataLayerFactory", True)
    Dim t As Type = factory.GetType
    Dim method As MethodInfo = t.GetMethod("Create")
    Dim obj As Object = method.Invoke(factory, Nothing)
    SQLDataSource = DirectCast(obj, ICRDataLayer)

---

EDIT: Implementation based on Paul Kohler's code

Dim file As String
        For Each file In Directory.GetFiles(baseDir, searchPattern, SearchOption.TopDirectoryOnly)
            Dim assemblyType As System.Type
            For Each assemblyType In Assembly.LoadFrom(file).GetTypes

                Dim s As System.Type() = assemblyType.GetInterfaces
                For Each ty As System.Type In s

                    If ty.Name.Contains("ICRDataLayer") Then
                        MsgBox(ty.Name)
                        plugin = DirectCast(Activator.CreateInstance(assemblyType), ICRDataLayer)
                        MessageBox.Show(plugin.ModuleName)
                    End If
                Next

I get the following error with this code:

Unable to cast object of type 'SQLServer.CRDataSource.SQLServer' to type 'DynamicAssemblyLoading.ICRDataLayer'.

The actual DLL is in a different project called SQLServer in the same solution as my implementation code. CRDataSource is a namespace and SQLServer is the actual class name of the DLL. The SQLServer class implements ICRDataLayer, so I don't understand why it wouldn't be able to cast it. Is the naming significant here, I wouldn't have thought it would be.

---

Final Working code

Contents of PluginUtility:

enter code here    Public Shared Function GetInstances1(Of Type)(ByVal baseDir As String, ByVal searchPattern As String) As System.Type()
    Dim tmpInstances As New List(Of Type)
    Try
        Dim file As String
        For Each file In Directory.GetFiles(baseDir, searchPattern, SearchOption.TopDirectoryOnly)
            Dim assemblyType As System.Type
            For Each assemblyType In Assembly.LoadFrom(file).GetTypes

                Dim s As System.Type() = assemblyType.GetInterfaces
                Return s.ToArray()

            Next
        Next
    Catch exp As TargetInvocationException
        If (Not exp.InnerException Is Nothing) Then
            Throw exp.InnerException
        End If
    End Try
End Function

Code to load the DLL:

enter code here
    Dim basedir As String = "M:\MyProgs\WebService\DynamicAssemblyLoading\SQLServer\bin\Debug\"
    Dim searchPattern As String = "*SQL*.dll"
    Dim plugin As CRDataLayer.ICRDataLayer

    Try
        Dim file As String
        For Each file In Directory.GetFiles(baseDir, searchPattern, SearchOption.TopDirectoryOnly)
            Dim assemblyType As System.Type
            For Each assemblyType In Assembly.LoadFrom(file).GetExportedTypes

                If assemblyType.GetInterface("CRDataLayer.ICRDataLayer") IsNot Nothing Then
                    plugin = DirectCast(Activator.CreateInstance(assemblyType), CRDataLayer.ICRDataLayer)
                    MessageBox.Show(plugin.ModuleDescription)
                End If

            Next
        Next
    Catch exp As TargetInvocationException
        If (Not exp.InnerException Is Nothing) Then
            Throw exp.InnerException
        End If
    Catch ex As Exception
        MsgBox(ex.Message)
        Clipboard.SetText(ex.Message)
    End Try

Answer 1

Version 2 - This sample loads up a DLL from it current directory. There are 2 projects, 1 console application project and a "module" project (the module 'coppies' its DLL to the working directory of the console app).

The sample below simply demonstrates dynamically loading a DLL that implements an interface. The IModule interface just reports its name. PlugInUtility.GetInstances(Of IModule)(Environment.CurrentDirectory, "*.Module.dll") will create an instance of any IModule instance found in a DLL within the current directory ending with ".Module.dll". It's a reflected VB.NET version straight out of Mini SQL Query.

With that in mind something like:

Dim modules As IModule() = PlugInUtility.GetInstances(Of ICRDataLayer)(Environment.CurrentDirectory, "*.Server.dll")

Should satisfy your requirement. Then you just need to chose which one to execute!

The code:

In "VB.LoaderDemo Colsole App"

' IModule.vb
Public Interface IModule
    Property ModuleName() As String
End Interface

' PlugInUtility.vb
Imports System.IO
Imports System.Reflection
Public Class PlugInUtility
    Public Shared Function GetInstances(Of T)(ByVal baseDir As String, ByVal searchPattern As String) As T()
        Dim tmpInstances As New List(Of T)
        Try
            Dim file As String
            For Each file In Directory.GetFiles(baseDir, searchPattern, SearchOption.TopDirectoryOnly)
                Dim assemblyType As Type
                For Each assemblyType In Assembly.LoadFrom(file).GetTypes()
                    If (Not assemblyType.GetInterface(GetType(T).FullName) Is Nothing) Then
                        tmpInstances.Add(DirectCast(Activator.CreateInstance(assemblyType), T))
                    End If
                Next
            Next
        Catch exp As TargetInvocationException
            If (Not exp.InnerException Is Nothing) Then
                Throw exp.InnerException
            End If
        End Try
        Return tmpInstances.ToArray()
    End Function
End Class

' MainModule.vb
Module MainModule
    Sub Main()
        Dim plugins As IModule() = PlugInUtility.GetInstances(Of IModule)(Environment.CurrentDirectory, "*.Module.dll")
        Dim m As IModule
        For Each m In plugins
            Console.WriteLine(m.ModuleName)
        Next
    End Sub
End Module

In "Sample1 DLL" (references 'VB.LoaderDemo' for IModule)

Imports VB.LoaderDemo

Public Class MyModule1
    Implements IModule

    Dim _name As String

    Public Sub New()
        _name = "Sample 1, Module 1"
    End Sub

    Public Property ModuleName() As String Implements IModule.ModuleName
        Get
            Return _name
        End Get
        Set(ByVal value As String)
            _name = value
        End Set
    End Property

End Class

The output is:

> Sample 1, Module 1

Answer 2

Is the type ICRDataLayer defined in the DLL you are going to load? If so, you seem to already reference the DLL in your project settings.

You need to work with just reflection:

Dim obj As Object = ass.CreateInstance("ICRDataLayer", True)
Dim t as Type = obj.GetType()
Dim method as MethodInfo = t.GetMethod("DoSomething")
method.Invoke(obj, ...)

---

Edit: If ICRDataLayer is implemented in the application, and the plugin just implements the interface, you need the plugin to provide a factory for you: (sorry for C# code, I am not familiar with VB.NET's syntax)

// in each of plugins:
static class CRDataLayerFactory // class name is a contract,
{                               // should be the same for all plugins
    static ICRDataLayer Create()
    {
        return new CRDataLayerImplementation();
    }
}

The application's code should look like this:

Dim factory As Object = ass.CreateInstance("CRDataLayerFactory", True)
Dim t as Type = factory.GetType()
Dim method as MethodInfo = t.GetMethod("Create")
Dim obj as Object = method.Invoke(factory, null)

SQLDataSource = DirectCast(obj, ICRDataLayer)