| EvalExpression | |
|---|---|
| 220px | |
| imagecaption | |
| Name | EvalExpression |
| Namespace | "using:UBIK.WinX.Controls" in UBIK.UWP "clr-namespace:UBIK.CPL.Controls;assembly=UBIK.CPL" in UBIK.Xamarin |
| Purpose | Evaluate a C# expression in XAML |
| Version | 3.2+ in UBIK.UWP 1.0+ in UBIK.Xamarin |
Concept
The EvalExpression control allows to evaluate a C# expression from within XAML markup. The Expression has to be a single-line, valid C# expression ("Lambda") and has to return a single value; expressions can also reference names of subordinate EvalExpressionParameter items.
Examples
 | When writing expressions in XAML code, you have to avoid (escape) special characters. There are useful online tools for this. |
Evaluation without parameters
UWP
The following example shows how to evaluate a simple expression without using any parameters and then use the result for visibility binding.
<ctrls:EvalExpression
x:Name="ExpressionEvaluator"
Context="{Binding Self}"
Expression="Context.Values["LK_OFFLINE"]!=null || Context.Values["GUIDREF"]!=null" />
<TextBlock Foreground="White" Visibility="{Binding ElementName=ExpressionEvaluator, Path=Result, Converter={StaticResource BoolToVisConverter}}" />
</StackPanel>
The TextBlock should be visible as long as at least one of the context object's two named properties has a value.
Xamarin
Simple Calculation
UWP
The following example shows how to use the control with three parameters, where the first two are user input (Textbox Param0 and Param1) and the third one is a property from its DataContext (ViewModel). The evaluated Result is then bound to a Textblock for output in the UI.
HorizontalAlignment="Center"
Orientation="Vertical">
<TextBox x:Name="Expression" Width="200" />
<TextBox x:Name="Param0" Width="200" />
<TextBox x:Name="Param1" Width="200" />
<ctrls:EvalExpression x:Name="Evaluator" Expression="{Binding ElementName=Expression, Path=Text}">
<ctrls:EvalExpressionParameter Name="P0" Value="{Binding ElementName=Param0, Path=Text, Converter={StaticResource ToType}, ConverterParameter='System.Int32'}" />
<ctrls:EvalExpressionParameter Name="P1" Value="{Binding ElementName=Param1, Path=Text, Converter={StaticResource ToType}, ConverterParameter='System.Int32'}" />
<ctrls:EvalExpressionParameter Name="P2" Value="{Binding IsLoggedIn}" />
</ctrls:EvalExpression>
<TextBlock Foreground="White" Text="{Binding ElementName=Evaluator, Path=Result}" />
</StackPanel>
Lets assume that the Textbox Param0 contains a text of 42 and Param1 contains a text of 43. If Expression now contains (P0 + P1) *2 then the result would display 170.
Xamarin
 | Note that extra syntax is required to get the EvalExpression to work in Xamarin! |
...
<controls:EvalExpression x:Name="UppercaseTitle" Expression="(P1.ToUpper())" Context="{Binding}">
<controls:EvalExpressionParameter Name="P1" Value="{Binding PropertyItem.MetaProperty.Description}" />
</controls:EvalExpression>
The above example converts a text (in this situation, the Description text of a property) to a capitalised version. Note that the text Context="{Binding}" is required for the expression to function in Xamarin.
To output the result of your expression, use the following syntax:
Note that the correct way to reference elements in Xamarin is x:Reference (ie. the UWP equivalent of ElementName=), followed by the name given to your EvalExpression.
Setting a calculated Property Value
UWP
The following example shows how to create a button that adds and stores +5 to the value of a a numeric property named MP_EXAMPLE, every time it is pressed:
<ctrls:EvalExpression x:Name="Evaluator" Expression=""MP_EXAMPLE|" + (P0 + 5)">
<ctrls:EvalExpressionParameter Name="P0" Value="{Binding Values[MP_EXAMPLE], Converter={StaticResource ToType}, ConverterParameter='System.Int32'}" />
</ctrls:EvalExpression>
<Button Content="Tap for 5 more" Command="{Binding SetPropertyValueAndValidateCommand}" CommandParameter="{Binding ElementName=Evaluator, Path=Result}"/>
</Grid>
Xamarin
Conditional Statement
If / Or statements can be evaluated using the C# syntax P0 ? P1 : P2. In this case, if the result of the P0 expression is True, P1 will be effected. Otherwise, P2 will be.
UWP
<Controls:EvalExpressionParameter Name="P0" Value="{Binding StoredProfileParameters[WPType1]}" />
<Controls:EvalExpressionParameter Name="P1" Value="WPType1=1" />
<Controls:EvalExpressionParameter Name="P2" Value="{Binding ElementName=InitialiseParameter, Path=Result}" />
</Controls:EvalExpression>
Xamarin
Parameters
EvalExpressionParameters can be added as child objects to an EvalExpression control. Each parameter object needs a unique Name and a Value, where the latter can be either a constant or dynamic value supplied through a binding.
UWP
Xamarin
EvalExpressions are also available for usage with Xamarin customisations.