Viewer not displaying the data in my standalone PyQGIS Application

Question

I tried creating a standalone external application of QGIS using python and I followed the tutorial mentioned in the link http://download.osgeo.org/qgis/doc/workshops/foss4g2007_qgis0.9_workshop_en.pdf

After running the python file the viewer opens and allows the user to browse for Shapefiles. The problem is that the Shapefile cannot be viewed in the viewer. It raises an error "Invalid Shapefile" though I added a correct Shapefile everytime I loaded any shapefile.

My code as well as the error is attached. Please tell me where I am wrong.

def addLayer(self):
    file = QFileDialog.getOpenFileName(self, 
               "Open Shapefile", ".", "Shapefiles (*.shp)")
    fileInfo = QFileInfo(file)

    # Add the layer
    layer = QgsVectorLayer(file, fileInfo.fileName(), "ogr")

    if not layer.isValid():
        raise IOError("Invalid Shapefile")

    # Change the color of the layer to gray
    symbols = layer.renderer().symbols()
    symbol = symbols[0]
    symbol.setFillColor(QColor.fromRgb(192, 192, 192))

    # Add layer to the registry
    QgsMapLayerRegistry.instance().addMapLayer(layer)

    # Set extent to the extent of our layer
    self.canvas.setExtent(layer.extent())

    # Set up the map canvas layer set
    c1 = QgsMapCanvasLayer(layer)
    layers = [c1]
    self.canvas.setLayerSet(layers)

And the error window is

Answer 1

Non-valid layers are most of the times due to a wrong QGIS prefix definition.

Please try with:

qgis_prefix="C:\\Program Files\\QGIS Wiena\\apps\\qgis"    
QgsApplication.setPrefixPath(qgis_prefix, True) 

Which should go right before this line:

QgsApplication.initQgis()

Now your tested layers should be valid.