MefContainerDebugHelper.cs

/// <summary>
/// Static helper class to debug failed MEF composition.
/// </summary>
public static class MefDebuggingHelper
{
  /// <summary>
  /// Writes missing import definitions to the debug console.
  /// </summary>
  /// <param name="container"></param>
  [Conditional("DEBUG")]
  public static void PrintMissingImports(CompositionContainer container)
  {
    var missingImports = GetMissingImports( container );

    if( missingImports.Count == 0 )
    {
      return;
    }
            
    Debug.WriteLine( string.Empty );
    Debug.WriteLine( "=============== Missing Imports ===============" );
            
    foreach( var missing in missingImports )
    {
      Debug.WriteLine( missing );
    }
            
    Debug.WriteLine( "===============================================" );
    Debug.WriteLine( string.Empty );
  }

  private static List<string> GetMissingImports(CompositionContainer container )
  {
    var brokenExports = GetBrokenExports(container);

    return container.Catalog
                    .SelectMany( part => part.ImportDefinitions )
                    .Where( impDef => IsInvalidImport(container, impDef) )
                    .Select( impDef => impDef.ContractName )
                    .Distinct()
                    .Where( contract => !contract.Contains( "ExportFactory" ) )
                    .Where( contract => !brokenExports.Contains( contract ) )
                    .OrderBy( contract => contract )
                    .ToList();
  }

  private static List<string> GetBrokenExports(CompositionContainer container)
  {
    return container.Catalog
                    .Where( part => part.ImportDefinitions.Any( impDef => IsInvalidImport(container, impDef) ) )
                    .SelectMany( part => part.ExportDefinitions )
                    .Select( exDef => exDef.ContractName )
                    .Distinct()
                    .OrderBy( c => c )
                    .ToList();
  }

  private static bool IsInvalidImport(CompositionContainer container, ImportDefinition impDef )
  {
    try
    {
      container.GetExports( impDef );
    }
    catch
    {
      return true;
    }

    return false;
  }
}

Thank you for that !
How/Where would you call PrintMissingError to display the error behind a failed viewmodel instanciation ?

In the bootstrapper after the MEF container has been built.