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Get If _propertyInfoCache Is Nothing Then SyncLock _cacheLock If _propertyInfoCache Is Nothing Then _propertyInfoCache = New Dictionary(Of Type, List(Of IPropertyInfo))() End If End SyncLock End If Return _propertyInfoCache End Get End Property The Private field _cacheLock is used to lock the region of code that creates the Dictionary if it doesn t already exist. Notice how the code checks the existence of the Dictionary both before and after the SyncLock statement. This avoids a race condition, where multiple threads could wait on the lock and run the code inside the lock, even though the first thread to reach that point would have already created the Dictionary. Similarly, the GetPropertyListCache() method protects both the use of the Dictionary and the creation of individual List objects for each business object type: Public Function GetPropertyListCache(ByVal objectType As Type) _ As List(Of IPropertyInfo) Dim cache = PropertyInfoCache Dim list As List(Of IPropertyInfo) = Nothing If Not (cache.TryGetValue(objectType, list)) Then SyncLock cache If Not (cache.TryGetValue(objectType, list)) Then list = New List(Of IPropertyInfo)() cache.Add(objectType, list) End If End SyncLock End If Return list End Function This method uses the PropertyInfoCache property to safely get a reference to the Dictionary. It then uses the TryGetValue() method to attempt to retrieve the specific List<IPropertyInfo> for the business object type. If that is unsuccessful, a SyncLock statement is used to ensure that only one thread can run the code that creates and adds the new List object to the Dictionary. Notice how the TryGetValue() is called inside the SyncLock statement to prevent multiple threads from getting that far and creating duplicate List objects. The RegisterProperty() method is used to register properties for a business object type by adding an IPropertyInfo object to the correct List. This method also employs locking to avoid threading issues. Public Function RegisterProperty(Of T)(ByVal objectType As Type, _ ByVal info As PropertyInfo(Of T)) As PropertyInfo(Of T) Dim list = GetPropertyListCache(objectType) SyncLock list list.Add(info) list.Sort() End SyncLock Return info End Function
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In this case, the GetPropertyListCache() method is used to safely get a reference to the List object; then a SyncLock statement is used to block access to that specific List object so only one property can be registered at a time. Notice that the list is sorted as each item is added. This ensures that the list is sorted when all properties have been registered and guarantees that the values are in the same order each time. Later in the chapter, I ll discuss how these values provide a numeric index into the list of managed field values for each business object. The order of the properties is very important. Of course, the RegisterProperty() methods are called when .NET does its initialization of the Shared fields on each class. You might expect that those method calls would occur in the same order all the time, thanks to .NET. Unfortunately, I don t trust that to be the case across C# and VB or between the 32- and 64-bit .NET runtimes. As you ll see later, these values must be in the same order in a client/server situation, even if the client is 32-bit .NET in VB and the server is 64-bit .NET in C#. Sorting the property objects ensures that they re in the same order in the list, regardless of the programming language or the .NET runtime version. Finally, the GetRegisteredProperties() method returns a list of properties registered for a business object type. Since this method is Public, there s no way to know what the calling code will do with the result, so this method doesn t return the actual List. Instead, it returns a copy of the data in a new List. Public Function GetRegisteredProperties(objectType As Type) _ As List(Of IPropertyInfo) Dim list = GetPropertyListCache(objectType) SyncLock list Return New List(Of IPropertyInfo)(list) End SyncLock End Function The original List object is locked to block any RegisterProperty() calls from changing the list while the items are being copied to the result. BusinessBase and ReadOnlyBase use the PropertyInfoManager to manage all the details around tracking the properties registered for each business type.
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