VB.Net Dynamically Load DLL
- by hermiod
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.
