Currently, the shape file reader can read the files only once when the ReadAsync() method is called. When new files are added in the SourceCollection and the method is called again, the reader doesn't read properly. Implement a mechanism that allows the new files to be read.

At this moment, to read additional files you can create a new reader and add the files in it. Then reset the Reader property of the layer.

var reader = new AsyncShapeFileReader();
reader.SourceCollection.Add(new AsyncReaderSource()
{
Source = new Uri("/WpfApplication16;component/DataSources/Alabama.shp", UriKind.RelativeOrAbsolute),
DataSource = new Uri("/WpfApplication16;component/DataSources/Alabama.dbf", UriKind.RelativeOrAbsolute)
});
this.layer.Reader = reader;
reader.ReadAsync();