There are a few cases in the .NET Framework object model where a property is purposely typed to be vague. For example, the System.Windows.Forms.DataGridView.DataSource property is typed as object. The reason for this is that this property can accept several types of input. Unfortunately, this provides no common place to add metadata to describe the characteristics of the property. Each DataSource property throughout the .NET Framework needs to have identical metadata for type converters, UI type editors, and other services that require metadata. The System.ComponentModel.AttributeProviderAttribute remedies this situation.

Once this attribute is placed on a property, the rules for obtaining attributes for the property descriptor's MemberDescriptor.Attributes collection differ. Typically, the property descriptor gathers local attributes, and then merges these with attributes from the property type. In this case, the attributes are taken from the type returned from the System.ComponentModel.AttributeProviderAttribute, not from the actual property type. This attribute is used on System.Windows.Forms.DataGridView.DataSource to point the System.Windows.Forms.DataGridView.DataSource object's specific type to System.ComponentModel.IListSource, and the appropriate metadata is placed on System.ComponentModel.IListSource to enable data binding. In so doing, external parties can easily add metadata to all data sources.

Attributes obtained from a type declared in the System.ComponentModel.AttributeProviderAttribute have a priority in between the attributes of the property’s type and the attributes on the property. The following list, in priority order, shows the full set of available merged attributes:

• Property Attributes

• Attribute Provider Attributes

• Property Type Attributes