By default the latest version of an assembly is loaded from the GAC (Global Assembly Cache) when the assembly is instantiated. While working on a side project the need arose to load older versions of the assembly without hard coding the assembly versions. The answer, load the existing assembly library from the assembly DLL and interrogate it to find the assembly information. Then ask the GAC to provide a handle to the specific object. The result allows multiple versions of the same assembly to be loaded loaded at the same time.
The Visual Basic .NET example code below loads SampleAppV1\DotNetLibrary.DLL and AnotherSampleAppV2\DotNetLibrary.DLL which are different versions of the same assembly.
'Load assembly using reflection from the file specified so we can get information about it Dim DLLAssemblyInfo As System.Reflection.Assembly DLLAssemblyInfo = System.Reflection.Assembly.LoadFrom("\\\\appserver.company.int\\SampleAppV1\\DotNetLibrary.DLL") 'Now load the assembly from the GAC using the information in the file. Dim GACDLL As System.Reflection.Assembly GACDLL = System.Reflection.Assembly.Load(AssemblyInfo.FullName, DLLAssemblyInfo.Evidence)
'Load the second version of the same assembly from the newer application
Dim DLLAssemblyInfoV2 As System.Reflection.Assembly
DLLAssemblyInfoV2 = System.Reflection.Assembly.LoadFrom("\\\\appserver.company.int\\AnotherSampleAppV2\\DotNetLibrary.DLL")
'Again, use the DLL information from the assembly file to load the assembly from the GAC Dim GACDLLv2 As System.Reflection.Assembly GACDLLv2 = System.Reflection.Assembly.Load(AssemblyInfoV2.FullName, DLLAssemblyInfoV2.Evidence)
'At this point in the code we have references to both assemblies loaded from the GAC. The next step is to instantiate an object from each of the references.
'Get an MyAssembly.Library from the old version and run the version method Dim GACDLLType As Type = GACDLL.GetType("MyAssembly.Library") Dim MyLibV1 As Object = Activator.CreateInstance(GACDLLType) Dim v1 As String v1 = MyLibV1.Version
'Finally, do the same and get the new MyAssembly.Library Dim GACDLLTypeV2 As Type = GACDLLV2.GetType("MyAssembly.Library") Dim MyLibV2 As Object = Activator.CreateInstance(GACDLLTypeV2) Dim v2 As String v2 = MyLibV2.Version
The variables v1 and v2 were both retrieved from the MyAssembly.Library.Version() method but the variables will not contain the same value because different versions of the assembly were loaded from the GAC during the System.Reflection.Assembly.Load() call.
Why not just load the assembly from each of the DLL files instead of the GAC? Security restrictions prevented untrusted assemblies from being loaded over the network. A possible work around would be to trust the network location or sign the assemblies and trust the signer but that remains for another posting.