The comparison-based filter makes basic comparisons, generally based on a Boolean argument. For this type of filter, you provide the name of a capability and the value that you want the filter to compare. At run time, the evaluator succeeds if the capability value and the value that you provide are equal. The compared Boolean properties are case-insensitive, so true and True are equivalent. Other compared properties are case-sensitive.

For more complex evaluation, you can specify an evaluator-delegate-based filter by providing the class and method name of a method. At run time, the method that you provide is called to test the evaluator. You must write and compile your own method to test the evaluator. Alternatively, you can define a method in your page or user control, and then reference it directly from the filter attribute, as described previously.

When specifying device filters in the Web.config file, you add them to the <system.web> section. To view the syntax, see <deviceFilters>. The syntax applies to both types of filters. In the following example, the first filter shows the comparison-based filter, and the second filter shows the evaluator-delegate-based filter:

<system.web>
  <deviceFilters>
    <filter
      name="capability"
      compare="capabilityName"
      argument="argument" />
    <filter
      name="capability"
      type="className"
      method="methodName" />
  </deviceFilters>
</system.web>

The MobileCapabilities object evaluates these filters at run time. Each device filter specifies an evaluation condition based on capabilities of the device. The sets of devices targeted by device filters are usually not discrete; for example, many devices can match both the IsColor and IsPDAfilter attributes.

Note:
Device filter names are case-sensitive.

Within the <DeviceSpecific><Choice> construct, you specify the values for these filters. For example, the following code accesses the IsColor filter attribute defined in the Web.config file.

<DeviceSpecific>
    <Choice Filter="IsColor" ImageUrl="colorImg.gif" />
</DeviceSpecific>

Choices are placed inside a <DeviceSpecific> element, and represent device characteristic/value pairs, where the device characteristic is drawn from a number of sources. Choices are evaluated in the order that they appear in the DeviceSpecific/Choice construct.

Although a control can contain only one <DeviceSpecific> element, you can add any number of <Choice> elements within a <DeviceSpecific> element. Each <Choice> element can contain the following:

• A filter name, which specifies the device filter to evaluate. If the filter name is omitted, the choice is selected by default.

• Additional properties that override properties of the parent control.

• Template definitions for the control.

ASP.NET selects the <Choice> element to use by going through each choice in order and evaluating the filter specified by the filter name. If the filter matches the current target device (by evaluating to true), that choice is selected. The control then applies any property overrides specified in the choice, and can use any templates defined in rendering.