Building Custom Forms - ASP.NET Dynamic Data - page 336

LISTING 10.16 Continued
TypeDescriptor.GetConverter(this.Column.ColumnType);
return converter.ConvertFrom(dictionary[this.Column.Name]);
}
}
}
FieldValue Property
The built-in FieldTemplateUserControl base class already provides a property called
FieldValue . However, it is available only during the data binding stage of the page life-
cycle, and it throws an InvalidOperationException if you try accessing it during post-
back. This limitation also affects the FieldValueString and FieldValueEditString
properties that return a formatted version of the FieldValue . Luckily, the FieldValue
property is declared as virtual, and the UnleashedFieldTemplate overrides it to remove
this limitation.
The overridden FieldValue property getter calls the inherited getter, which invokes the
GetColumnValue method to do the actual work. This method is overridden in the
UnleashedFieldTemplate class to catch the InvalidOpertationException and call the
ExtractFieldValue method, which is discussed shortly, to retrieve the current field value
from the template's data control. The overridden setter of the FieldValue property checks
if the new field value is different from the old one and calls the OnFieldValueChanged
method to raise the FieldValueChanged event if that is the case.
ExtractFieldValue Method
The ExtractFieldValue method calls the ExtractValues method, which is defined as
virtual in the FieldTemplateUserControl class specifically for extracting the current field
value from the template's data control. The ExtractValues method is already imple-
mented in all appropriate Edit mode field templates that come with Dynamic Data
projects, and you can simply call it here instead of reinventing the wheel. It takes an
OrderedDictionary as a parameter and populates it with column names and values.
Because a particular data control might store a field value in a different format, such as a
TextBox storing a numeric value as a string, the ExtractFieldValue method uses a
TypeConverter to convert the extracted field value to the actual type of the column.
The ExtractFieldValue method has a special logic for handling foreign key columns. At
the time of data binding, a foreign key column is a navigation property, so the FieldValue
property returns an actual target entity instance, such as a Customer instance. At post-back
time, however, there are no entity instances, and you can only get the values of the
underlying primitive columns, such as a CustomerID. If the foreign key consists of multi-
ple primitive columns, the ExtractFieldValue method returns a coma-separated list of
their values, which is also how the built-in ForeignKey_Edit field template stores them in
the ListItem instances of its DropDownList control.