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CHAPTER 5 s COMPLETING THE FRAMEWORK
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Each of the PropertyDescriptor methods is checked to see if the corresponding property is marked as [Browsable(false)], in which case the IsBrowsable property will return false. Those properties are ignored. All other properties are used to create an instance of an ObjectFieldInfo object, which represents that property s schema information: foreach (PropertyDescriptor item in props) if (item.IsBrowsable) result.Add(new ObjectFieldInfo(item)); In the end, an array of these ObjectFieldInfo objects is returned as a result: return result.ToArray(); Each element in this array contains schema information about a business object property that is available for data binding.
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The ObjectFieldInfo class maintains schema information about a single property on a business object. One of these objects is created for each bindable property on a business object, and is returned to ASP.NET through the GetFields() method of ObjectViewSchema. The ObjectFieldInfo class implements the System.Web.UI.Design.IDataSourceFieldSchema interface as required by ASP.NET. This is a lengthy interface that defines a number of properties which provide information about the column/property. Many of these properties are designed specifically for columns from a DataTable, and have little meaning when applied to elements of a business object s interface, while others are valid in both cases. The ObjectFieldInfo class maintains a set of instance fields to hold information about the business object property: public class ObjectFieldInfo : IDataSourceFieldSchema { private PropertyDescriptor _field; private bool _primaryKey; private bool _isIdentity; private bool _isNullable; private int _length; public ObjectFieldInfo(PropertyDescriptor field) { _field = field; GetDataObjectAttributes(); } } Most of these values come from a [DataObjectField()] attribute that the business object developer can apply to properties of a business object.
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DataObjectField Attribute
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The [DataObjectField()] attribute comes from the System.ComponentModel namespace and can be used on a business object s property like this: [System.ComponentModel.DataObjectField(true, true, false)] public Guid Id { get
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CHAPTER 5 s COMPLETING THE FRAMEWORK
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{ CanReadProperty(true); return _id; } } In this example, the attribute specifies that the Id property is a primary key, represents the identity of the object, and is not nullable. Table 5-9 lists the possible parameters to the [DataObjectField()] attribute. Table 5-9. Parameters for the [DataObjectField()] Attribute
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primaryKey isIdentity isNullable length
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Description
Indicates whether the property acts as a primary key value for the object Indicates whether the property uniquely identifies the object within a parent collection Indicates whether the property is nullable Provides the maximum length of the property in bytes
The GetDataObjectAttributes() helper method retrieves this attribute from the business object s property and sets the corresponding instance fields: private void GetDataObjectAttributes() { DataObjectFieldAttribute attribute = (DataObjectFieldAttribute) _field.Attributes[typeof(DataObjectFieldAttribute)]; if (attribute != null) { _primaryKey = attribute.PrimaryKey; _isIdentity = attribute.IsIdentity; _isNullable = attribute.IsNullable; _length = attribute.Length; } } Each of these values is exposed as a property through the IDataSourceFieldSchema interface. For instance, here s the PrimaryKey property: public bool PrimaryKey { get { return _primaryKey; } } Visual Studio can use these properties to assist the UI developer in building an appropriate interface for the business object s properties. The Nullable property is a bit more complex, however, because it is sometimes possible to detect that a property is nullable even without the use of the [DataObjectField()] attribute. This is due to the nullable support built into .NET 2.0: public bool Nullable { get { Type t = _field.PropertyType;
CHAPTER 5 s COMPLETING THE FRAMEWORK
if (!t.IsValueType || _isNullable) return true; if (t.IsGenericType) return (t.GetGenericTypeDefinition() == typeof(Nullable)); return false; } } A property can be nullable if it is a reference type or if _isNullable is true. It can also be nullable if the property is declared using the Nullable<T> generic type.
Other Property Information
The IDataSourceFieldSchema interface defines other properties as well. Some of these properties have little meaning for a business object. For instance, there s no way to determine meaningful values for Scale and Precision based on a business object s property, so these just return -1: public int Scale { get { return -1; } } Other properties can be determined based on information from the PropertyDescriptor object passed into the ObjectFieldInfo constructor. That PropertyDescriptor object provides information about the specific business object property. The property s name, for instance, can be directly retrieved: public string Name { get { return _field.Name; } }
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