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POMEF+MVVM: How to instantiate a class with non-importing or mixed parameters
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  1. POMEF+MVVM: How to instantiate a class with non-importing or mixed parameters
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    copied!<p>I have been experimenting MEF and MVVM. I wanted to let MEF initialize a NonShared ViewModel instance with a string constructor parameter, i.e something like this:</p> <pre><code>// BarViewModel's constructor has one single string parameter IBarViewModel bar = container.GetExportedValue&lt;IBarViewModel&gt;("bar title"); </code></pre> <p>Obviously MEF wouldn't let me do this.</p> <p>I googled and some say ExportFactory is the right tool for this but there are no syntax samples. I have not been able to figure out how to use ExportFactory to initialize an instance with constructor parameters (or should I say non-imported parameter).</p> <p>So then I tried to use a ViewModelFactory to achieve this. I referenced this <a href="http://pglazkov.blogspot.com.au/2011/04/mvvm-with-mef-viewmodelfactory.html" rel="nofollow">article</a></p> <p>and came up something like this:</p> <pre><code>[Export(typeof(IBarViewModelFactory))] public class BarViewModelFactory : IBarViewModelFactory { [Import] public Lazy&lt;CompositionContainer&gt; Container { get; set; } public IBarViewModel CreateBarViewModel(string text) { IBarViewModel result = null; var tempContainer = CreateTemporaryDisposableContainer(Container.Value); try { result = new BarViewModel(text); tempContainer.ComposeParts(result); } catch (Exception ex) { } finally { } return result; } </code></pre> <p>Basically what I'm doing here is (1) import a Container from somewhere else (2) new the VM up with parameter (3) use a temporary container to settle the dependencies of the new'ed instance.</p> <p>This code seems working OK but I then discovered that (a) BarViewModel can no longer have [ImportingConstructor] (b) I cannot use [Import] properties of BarViewModel in its constructor because they are null in the ctor scope.</p> <p>This mean the ViewModel is very limited in usage and also it means it is not possible to initialize a class like this with MEF:</p> <pre><code>[Import] public ILogger Logger {get;set;} [ImportingConstructor] public SomeClass(IDataService service, string text) { Logger.Trace(text); } </code></pre> <p>Now I have no idea how to instantiate this class with MEF. I guess this is a quite common scenario so I wonder if MEF is not capable of handling this?</p>
 

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