Setting Values in Visual Basic .NET

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Setting Values
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The core of the DataMapper class is the SetValue() method. This method is ultimately responsible for putting a value into a specified property of a target object: Private Sub SetValue( _ ByVal target As Object, ByVal propertyName As String, _ ByVal value As Object) Dim propertyInfo As PropertyInfo = _ target.GetType.GetProperty(propertyName) Dim pType As Type = Utilities.GetPropertyType(propertyInfo.PropertyType) If value Is Nothing Then propertyInfo.SetValue(target, value, Nothing) Else If pType.Equals(value.GetType) Then ' types match, just copy value propertyInfo.SetValue(target, value, Nothing) Else ' types don't match, try to coerce types If pType.Equals(GetType(Guid)) Then propertyInfo.SetValue(target, New Guid(value.ToString), Nothing) Else propertyInfo.SetValue(target, _ Convert.ChangeType(value, pType), Nothing) End If End If End If End Sub
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CHAPTER 5 s COMPLETING THE FRAMEWORK
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Reflection is used to retrieve a PropertyInfo object corresponding to the specified property on the target object. The specific type of the property s return value is retrieved using a GetPropertyType() helper method in the Utilities class. That helper method exists to deal with the possibility that the property could return a value of type Nullable(Of T). If that happens, the real underlying data type (behind the Nullable(Of T) type) must be returned. Here s the GetPropertyType() method: Public Function GetPropertyType(ByVal propertyType As Type) As Type Dim type As Type = propertyType If (type.IsGenericType AndAlso _ (type.GetGenericTypeDefinition Is GetType(Nullable))) Then Return type.GetGenericArguments(0) End If Return type End Function If Nullable(Of T) isn t involved, then the original type passed as a parameter is simply returned. But if Nullable(Of T) is involved, then the first generic argument (the value of T) is returned instead: Return type.GetGenericArguments(0) This ensures that the actual data type of the property is used rather than Nullable(Of T). Back in the SetValue() method, the PropertyInfo object has a SetValue() method that sets the value of the property, but it requires that the new value have the same data type as the property itself. Given that the values from an IDictionary collection or a DTO may not exactly match the property types on a business object, DataMapper.SetValue() attempts to coerce the original type to the property type before setting the property on the target object. To do this, it retrieves the type of the target property. If the new value is not Nothing, then the type of the new value is compared to the type of the property to see if they match: If pType.Equals(value.GetType) Then ' types match, just copy value propertyInfo.SetValue(target, value, Nothing) If they do match, then the property is set to the new value. If they don t match, then there s an attempt to coerce the new value to the same type as the property: ' types don't match, try to coerce types If pType.Equals(GetType(Guid)) Then propertyInfo.SetValue(target, New Guid(value.ToString), Nothing) Else propertyInfo.SetValue(target, _ Convert.ChangeType(value, pType), Nothing) End If For most common data types, the Convert.ChangeType() method will work fine. It handles string, date, and primitive data types in most cases. But Guid values won t convert using that technique (because Guid doesn t implement IConvertible), so they are handled as a special case, by using ToString() to get a string representation of the value, and using that to create a new instance of a Guid object. If the coercion fails, Convert.ChangeType() will throw an exception. In such a case, the business developer will have to manually set that particular property; adding that property name to the list of properties ignored by DataMapper.
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CHAPTER 5 s COMPLETING THE FRAMEWORK
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Mapping from IDictionary
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A collection that implements IDictionary is effectively a name/value list. The DataMapper.Map() method assumes that the names in the list correspond directly to the names of properties on the business object to be loaded with data. It simply loops through all the keys in the dictionary, attempting to set the value of each entry into the target object: Public Sub Map( _ ByVal source As System.Collections.IDictionary, _ ByVal target As Object, _ ByVal suppressExceptions As Boolean, _ ByVal ParamArray ignoreList() As String) Dim ignore As New List(Of String)(ignoreList) For Each propertyName As String In source.Keys If Not ignore.Contains(propertyName) Then Try SetValue(target, propertyName, source.Item(propertyName)) Catch ex As Exception If Not suppressExceptions Then Throw New ArgumentException( _ String.Format("{0} ({1})", _ My.Resources.PropertyCopyFailed, propertyName), ex) End If End Try End If Next End Sub While looping through the key values in the dictionary, the ignoreList is checked on each entry. If the key from the dictionary is in the ignore list, then that value is ignored. Otherwise, the SetValue() method is called to assign the new value to the specified property of the target object. If an exception occurs while a property is being set, it is caught. If suppressExceptions is True, then the exception is ignored; otherwise it is wrapped in an ArgumentException. The reason for wrapping it in a new exception object is so the property name can be included in the message returned to the calling code. That bit of information is invaluable when using the Map() method.
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