I am working on a program that will communicate with various pieces of hardware. Because of the varied nature of the items it communicates and controls, I need to have a different "driver" for each different piece of hardware. This made me think that MEF would be a great way to make those drivers as plugins that can be added even after the product has been released.
I've looked at a lot of examples of how to use MEF, but the question that I haven't been able to find an answer to is how to populate a MEF plugin with external data (eg. from a database). All the examples I can find have the "data" hard-coded into the assembly, like the following example:
[Export( typeof( IRule ) )]
public class RuleInstance : IRule
{
public void DoIt() {}
public string Name
{
get { return "Rule Instance 3"; }
}
public string Version
{
get { return "1.1.0.0"; }
}
public string Description
{
get { return "Some Rule Instance"; }
}
}
What if I want Name, Version and Description to come from a database? How would I tell MEF where to get that information?
I may be missing something very obvious, but I don't know what it is.
You would have to either pass the information to the plugin after it loaded via properties:
[Export( typeof( IRule ) )]
public class RuleInstance : IRule
{
puliic void DoIt()
{ }
public string Name { get; set; }
}
public class Program
{
[Import(typeof( IRule ))]
public IRule instance { get; set; }
public void Run()
{
// Load the assemblies here
instance.Name = "Rule Instance 3";
}
}
Or the plugin could request the variables though a host interface. You can either pass the IHost instance through a property or though a constructor parameter, but constructor parameters are not simple with MEF. Here is through a property:
[Export( typeof( IRule ) )]
public class RuleInstance : IRule
{
puliic void DoIt()
{ }
public void Initialise()
{
// Load our name from the host, this cannot be done in the constructor
string name = Host.GetName(/* some parameters? */)
}
public IHost Host { get; set; }
public string Name { get; set; }
}
public interface IHost
{
string GetName(/* some parameters? */);
}
public class Program : IHost
{
[Import(typeof( IRule ))]
public IRule instance { get; set; }
public void Run()
{
// Load the assemblies here
// Make sure the plugins know where the host is...
instance.Host = this;
}
}
You could also "Export" the database interface and "Import" it into any plugins that need database access...
You can always export individual values (through contract names), here's an example:
public class Configuration
{
[Export("SomeValue")]
public string SomeValue
{
get { /* return value from database perhaps? */ }
}
}
[Export(typeof(IRule))]
public class RuleInstance : IRule
{
[Import("SomeValue")]
public string SomeValue { get; private set; }
}
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