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Difference between revisions of "EvalExpression"

m (Added section on usage in Xamarin)
m (#2161 - Added XAML Category)
 
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}}
 
}}
  
−
== Usage ==
+
== 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 [[EvalExpression#Parameters|EvalExpressionParameter]] items.
+
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 [[EvalExpression#Parameters|EvalExpressionParameter]] items.
 +
 
 +
=== 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.
  
 
=== Examples ===
 
=== Examples ===
−
{{Attention|The namespace used in the following examples is for the UBIK.UWP client. If you are applying them to the UBIK.Xamarin clients, please check the table above for the corresponding namespace.}}
+
 
 
{{Hint|When writing expressions in XAML code, you have to avoid (escape) special characters. There are useful [https://www.freeformatter.com/xml-escape.html online tools] for this.}}
 
{{Hint|When writing expressions in XAML code, you have to avoid (escape) special characters. There are useful [https://www.freeformatter.com/xml-escape.html online tools] for this.}}
 +
{{Hint|When writing expressions, it's always better (even necessary in some cases) to write them in their full forms. For example, while <nowiki>Bool_A || Bool_B</nowiki> is a valid expression, you should still write <nowiki>Bool_A==true || Bool_B==true</nowiki> instead.}}
 +
{{Hint|If you want to use bindings in EvalExpressionParameters, a lot of the times you need to add <nowiki>Context="{Binding}"</nowiki> to the EvalExpression. This is because child UI elements (parameters in this case) do not inherit the binding context of the parent by default. Manually setting the context that way ensures that the same binding expressions that work outside the EvalExpression also work inside. Although this is not necessary if the binding you use explicitly refers to a named UI element.}}
  
 
==== Evaluation without parameters ====
 
==== Evaluation without parameters ====
 
The following example shows how to evaluate a simple expression without using any parameters and then use the result for visibility binding.
 
The following example shows how to evaluate a simple expression without using any parameters and then use the result for visibility binding.
−
 
+
<tabs>
 +
<tab name="UWP">
 
<source lang = "xml">
 
<source lang = "xml">
 
<StackPanel xmlns:ctrls="using:UBIK.WinX.Controls">
 
<StackPanel xmlns:ctrls="using:UBIK.WinX.Controls">
−
<ctrls:EvalExpression
+
    <ctrls:EvalExpression
−
                x:Name="ExpressionEvaluator"
+
        x:Name="Evaluator"
−
                Context="{Binding Self}"
+
        Context="{Binding}"
−
                Expression="Context.Values[&quot;LK_OFFLINE&quot;]!=null || Context.Values[&quot;GUIDREF&quot;]!=null" />
+
        Expression="Context.Values[&quot;LK_OFFLINE&quot;]!=null || Context.Values[&quot;GUIDREF&quot;]!=null" />
−
<TextBlock Foreground="White" Visibility="{Binding ElementName=ExpressionEvaluator, Path=Result, Converter={StaticResource BoolToVisConverter}}" />
+
    <TextBlock
 +
        Foreground="White"
 +
        Text="Some Text"
 +
        Visibility="{Binding ElementName=Evaluator, Path=Result, Converter={StaticResource BoolToVisConverter}}" />
 
</StackPanel>
 
</StackPanel>
 
</source>
 
</source>
 +
The TextBlock should be visible as long as at least one of the context object's two named properties has a value.
 +
</tab>
  
−
The TextBlock should be visibile as long as at least one of the context object's two named properties has a value.
+
<tab name="Xamarin">
−
 
+
<source lang = "xml">
−
==== Simple Calculation ====
+
<StackLayout xmlns:ctrls="clr-namespace:UBIK.CPL.Controls;assembly=UBIK.CPL">
−
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.
+
    <ctrls:EvalExpression
 +
        x:Name="Evaluator"
 +
        Context="{Binding}"
 +
        Expression="Context.Values[&quot;LK_EXAMPLE&quot;]!=null || Context.Values[&quot;GUIDREF&quot;]!=null" />
 +
    <Label
 +
        TextColor="#00000"
 +
        IsVisible="{Binding Path=Result, Source={x:Reference Evaluator}, Converter={StaticResource BoolToBool}}" />
 +
</StackLayout>
 +
</source>
 +
The Label should be visible as long as at least one of the context object's two named properties has a value.
 +
</tab>
 +
</tabs>
  
 +
==== Simple calculation ====
 +
<tabs>
 +
<tab name="UWP">
 +
The following example shows how to use an expression with two parameters (''Param0'' and ''Param1''). The evaluated ''Result'' is then bound to a TextBlock for output in the UI.
 
<source lang = "xml">
 
<source lang = "xml">
−
<StackPanel xmlns:ctrls="using:UBIK.WinX.Controls"
+
<StackPanel xmlns:ctrls="using:UBIK.WinX.Controls">
−
HorizontalAlignment="Center"
+
    <TextBox x:Name="Expression" Width="200" />
−
Orientation="Vertical">
+
    <TextBox x:Name="Param0" Width="200" />
−
<TextBox x:Name="Expression" Width="200" />
+
    <TextBox x:Name="Param1" Width="200" />
−
<TextBox x:Name="Param0" Width="200" />
+
    <ctrls:EvalExpression x:Name="Evaluator" Expression="{Binding ElementName=Expression, Path=Text}" Context="{Binding}">
−
<TextBox x:Name="Param1" Width="200" />
+
        <ctrls:EvalExpressionParameter Name="P0" Value="{Binding ElementName=Param0, Path=Text, Converter={StaticResource ToType}, ConverterParameter='System.Int32'}" />
−
<ctrls:EvalExpression x:Name="Evaluator" Expression="{Binding ElementName=Expression, Path=Text}">
+
        <ctrls:EvalExpressionParameter Name="P1" Value="{Binding ElementName=Param1, Path=Text, Converter={StaticResource ToType}, ConverterParameter='System.Int32'}" />
−
<ctrls:EvalExpressionParameter Name="P0" Value="{Binding ElementName=Param0, Path=Text, Converter={StaticResource ToType}, ConverterParameter='System.Int32'}" />
+
    </ctrls:EvalExpression>
−
<ctrls:EvalExpressionParameter Name="P1" Value="{Binding ElementName=Param1, Path=Text, Converter={StaticResource ToType}, ConverterParameter='System.Int32'}" />
+
    <TextBlock Foreground="White" Text="{Binding ElementName=Evaluator, Path=Result}" />
−
<ctrls:EvalExpressionParameter Name="P2" Value="{Binding IsLoggedIn}" />
+
−
</ctrls:EvalExpression>
+
−
<TextBlock Foreground="White" Text="{Binding ElementName=Evaluator, Path=Result}" />
+
 
</StackPanel>
 
</StackPanel>
 
</source>
 
</source>
 +
Lets assume that the Textbox ''Param0'' contains a text of 42 and ''Param1'' contains a text of 43. If ''Expression'' now contains <code>(P0 + P1) *2</code> then the result would display 170.
 +
</tab>
  
−
Lets assume that the Textbox ''Param0'' contains a text of 42 and ''Param1'' contains a text of 43. If ''Expression'' now contains  <code>(P0 + P1) *2</code> then the result would display 170.
+
<tab name="Xamarin">
 +
The following example shows how to use an expression with two parameters (''Param0'' and ''Param1''). The evaluated ''Result'' is then bound to a Label for output in the UI.
 +
<source lang = "xml">
 +
<StackLayout xmlns:ctrls="clr-namespace:UBIK.CPL.Controls;assembly=UBIK.CPL">
 +
    <Editor x:Name="Expression"/>
 +
    <Editor x:Name="Param0" WidthRequest="32"/>
 +
    <Editor x:Name="Param1" WidthRequest="43"/>
 +
    <ctrls:EvalExpression x:Name="Evaluator" Expression="{Binding Path=Text, Source={x:Reference Expression}}" Context="{Binding}">
 +
        <ctrls:EvalExpressionParameter Name="P0" Value="{Binding Path=WidthRequest, Source={x:Reference Param0}}" />
 +
        <ctrls:EvalExpressionParameter Name="P1" Value="{Binding Path=WidthRequest, Source={x:Reference Param1}}" />
 +
    </ctrls:EvalExpression>
 +
    <Label Text="{Binding Path=Result, Source={x:Reference Evaluator}}" />
 +
</StackLayout>
 +
</source>
 +
Since Textbox ''P0'' has a width of 32 and ''P1'' has a width of 43. If ''Expression'' now contains  <code>(P0 + P1)</code> then the result would display 75.
 +
</tab>
 +
</tabs>
  
−
==== Setting a calculated Property Value ====
+
==== Setting a calculated property value ====
 
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:
 
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:
−
 
+
<tabs>
 +
<tab name="UWP">
 
<source lang = "xml">
 
<source lang = "xml">
 
<Grid xmlns:ctrls="using:UBIK.WinX.Controls">
 
<Grid xmlns:ctrls="using:UBIK.WinX.Controls">
−
     <ctrls:EvalExpression x:Name="Evaluator" Expression="&quot;MP_EXAMPLE|&quot; + (P0 + 5)">
+
     <ctrls:EvalExpression x:Name="Evaluator" Expression="&quot;MP_EXAMPLE|&quot; + (P0 + 5)" Context="{Binding}">
−
         <ctrls:EvalExpressionParameter Name="P0" Value="{Binding Values[MP_EXAMPLE], Converter={StaticResource ToType}, ConverterParameter='System.Int32'}" />
+
         <ctrls:EvalExpressionParameter Name="P0" Value="{Binding Values[MP_EXAMPLE]}" />
 
     </ctrls:EvalExpression>
 
     </ctrls:EvalExpression>
−
     <Button Content="Tap for 5 more" Command="{Binding SetPropertyValueAndValidateCommand}" CommandParameter="{Binding ElementName=Evaluator, Path=Result}"/>
+
     <Button
 +
        Content="Tap for 5 more"
 +
        Command="{Binding SetPropertyValueAndValidateCommand}"
 +
        CommandParameter="{Binding ElementName=Evaluator, Path=Result}"/>
 
</Grid>
 
</Grid>
 
</source>
 
</source>
 +
</tab>
  
−
== Conditional Statement ==
+
<tab name="Xamarin">
−
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.
+
−
 
+
 
<source lang = "xml">
 
<source lang = "xml">
−
        <Controls:EvalExpression x:Name="InitialiseParameterWhenNull" Expression="(P0==null) ? P1 : P2">
+
<Grid xmlns:ctrls="clr-namespace:UBIK.CPL.Controls;assembly=UBIK.CPL">
−
            <Controls:EvalExpressionParameter Name="P0" Value="{Binding StoredProfileParameters[WPType1]}" />
+
    <ctrls:EvalExpression x:Name="Evaluator" Expression="&quot;MP_EXAMPLE|&quot; + (P0 + 5)" Context="{Binding}">
−
<Controls:EvalExpressionParameter Name="P1" Value="WPType1=1" />
+
        <ctrls:EvalExpressionParameter Name="P0" Value="{Binding Values[MP_EXAMPLE]}" />
−
<Controls:EvalExpressionParameter Name="P2" Value="{Binding ElementName=InitialiseParameter, Path=Result}" />
+
    </ctrls:EvalExpression>
−
        </Controls:EvalExpression>
+
    <Button
 +
        Text="Tap to add 5 more"
 +
        Command="{Binding SetPropertyValueAndValidateCommand}"
 +
        CommandParameter="{Binding Path=Result, Source={x:Reference Evaluator}}"/>
 +
</Grid>
 
</source>
 
</source>
 +
</tab>
 +
</tabs>
  
 +
==== Conditional statement ====
  
−
== Parameters ==
+
If / Or statements can also be evaluated using C# syntax.
−
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.
+
In this case, if the result of the "Condi" parameter is true, namely if the context object's MP_STATUS property value equals to 0, P0's value will be displayed in the Label, otherwise P1's value will be displayed.
−
 
+
−
== Usage in Xamarin ==
+
−
EvalExpressions are also available for usage with Xamarin customisations.
+
−
 
+
−
{{Attention|Note that extra syntax is required to get the EvalExpression to work in Xamarin!}}
+
  
 +
<tabs>
 +
<tab name="UWP">
 
<source lang = "xml">
 
<source lang = "xml">
−
xmlns:controls="clr-namespace:UBIK.CPL.Controls;assembly=UBIK.CPL"
+
<Grid xmlns:ctrls="using:UBIK.WinX.Controls">
−
...
+
    <ctrls:EvalExpression x:Name="Evaluator" Expression="Condi == true ? P0 : P1" Context="{Binding}">
−
<controls:EvalExpression x:Name="UppercaseTitle" Expression="(P1.ToUpper())" Context="{Binding}">
+
        <ctrls:EvalExpressionParameter Name="Condi" Value="{Binding Values[MP_STATUS], Converter={StaticResource EqualToTrueConverter}, ConverterParameter=0}" />
−
<controls:EvalExpressionParameter Name="P1" Value="{Binding PropertyItem.MetaProperty.Description}" />
+
        <ctrls:EvalExpressionParameter Name="P0" Value="State1" />
−
</controls:EvalExpression>
+
        <ctrls:EvalExpressionParameter Name="P1" Value="State2" />
 +
    </ctrls:EvalExpression>
 +
    <TextBlock Text="{Binding ElementName=Evaluator, Path=Result}" />
 +
</Grid>
 
</source>
 
</source>
 +
</tab>
  
−
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 on Xamarin.
+
<tab name="Xamarin">
−
 
+
−
To output the result of your expression, use the following syntax:
+
 
<source lang = "xml">
 
<source lang = "xml">
−
<Label Text="{Binding Path=Result, Source={x:Reference UppercaseTitle}}" />
+
<Grid xmlns:ctrls="clr-namespace:UBIK.CPL.Controls;assembly=UBIK.CPL">
 +
    <ctrls:EvalExpression x:Name="Evaluator" Expression="Condi == true ? P0 : P1" Context="{Binding}">
 +
        <ctrls:EvalExpressionParameter Name="Condi" Value="{Binding Values[MP_STATUS], Converter={StaticResource EqualityToBool}, ConverterParameter=0}" />
 +
        <ctrls:EvalExpressionParameter Name="P0" Value="State1" />
 +
        <ctrls:EvalExpressionParameter Name="P1" Value="State2" />
 +
    </ctrls:EvalExpression>
 +
    <Label Text="{Binding Path=Result, Source={x:Reference Evaluator}}" />
 +
</Grid>
 
</source>
 
</source>
−
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.
+
</tab>
 +
</tabs>
 +
 
 +
 
 +
 
 +
 
 +
 
  
 
==See also==
 
==See also==
Line 108: Line 170:
 
[[Category:WinX|EvalExpression]]
 
[[Category:WinX|EvalExpression]]
 
[[Category:Xamarin|EvalExpression]]
 
[[Category:Xamarin|EvalExpression]]
 +
[[Category:XAML|EvalExpression]]

Latest revision as of 12:39, 4 April 2024

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.

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.

Examples

IC Hint square.pngWhen writing expressions in XAML code, you have to avoid (escape) special characters. There are useful online tools for this.
IC Hint square.pngWhen writing expressions, it's always better (even necessary in some cases) to write them in their full forms. For example, while Bool_A || Bool_B is a valid expression, you should still write Bool_A==true || Bool_B==true instead.
IC Hint square.pngIf you want to use bindings in EvalExpressionParameters, a lot of the times you need to add Context="{Binding}" to the EvalExpression. This is because child UI elements (parameters in this case) do not inherit the binding context of the parent by default. Manually setting the context that way ensures that the same binding expressions that work outside the EvalExpression also work inside. Although this is not necessary if the binding you use explicitly refers to a named UI element.

Evaluation without parameters

The following example shows how to evaluate a simple expression without using any parameters and then use the result for visibility binding.

UWP

<StackPanel xmlns:ctrls="using:UBIK.WinX.Controls">
    <ctrls:EvalExpression
       x:Name="Evaluator"
       Context="{Binding}"
       Expression="Context.Values[&quot;LK_OFFLINE&quot;]!=null || Context.Values[&quot;GUIDREF&quot;]!=null" />
    <TextBlock
       Foreground="White"
       Text="Some Text"
       Visibility="{Binding ElementName=Evaluator, 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

<StackLayout xmlns:ctrls="clr-namespace:UBIK.CPL.Controls;assembly=UBIK.CPL">
    <ctrls:EvalExpression
       x:Name="Evaluator"
       Context="{Binding}"
       Expression="Context.Values[&quot;LK_EXAMPLE&quot;]!=null || Context.Values[&quot;GUIDREF&quot;]!=null" />
    <Label
       TextColor="#00000"
       IsVisible="{Binding Path=Result, Source={x:Reference Evaluator}, Converter={StaticResource BoolToBool}}" />
</StackLayout>

The Label should be visible as long as at least one of the context object's two named properties has a value.

Simple calculation

UWP

The following example shows how to use an expression with two parameters (Param0 and Param1). The evaluated Result is then bound to a TextBlock for output in the UI.

<StackPanel xmlns:ctrls="using:UBIK.WinX.Controls">
    <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}" Context="{Binding}">
        <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: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

The following example shows how to use an expression with two parameters (Param0 and Param1). The evaluated Result is then bound to a Label for output in the UI.

<StackLayout xmlns:ctrls="clr-namespace:UBIK.CPL.Controls;assembly=UBIK.CPL">
    <Editor x:Name="Expression"/>
    <Editor x:Name="Param0" WidthRequest="32"/>
    <Editor x:Name="Param1" WidthRequest="43"/>
    <ctrls:EvalExpression x:Name="Evaluator" Expression="{Binding Path=Text, Source={x:Reference Expression}}" Context="{Binding}">
        <ctrls:EvalExpressionParameter Name="P0" Value="{Binding Path=WidthRequest, Source={x:Reference Param0}}" />
        <ctrls:EvalExpressionParameter Name="P1" Value="{Binding Path=WidthRequest, Source={x:Reference Param1}}" />
    </ctrls:EvalExpression>
    <Label Text="{Binding Path=Result, Source={x:Reference Evaluator}}" />
</StackLayout>

Since Textbox P0 has a width of 32 and P1 has a width of 43. If Expression now contains (P0 + P1) then the result would display 75.

Setting a calculated property value

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:

UWP

<Grid xmlns:ctrls="using:UBIK.WinX.Controls">
    <ctrls:EvalExpression x:Name="Evaluator" Expression="&quot;MP_EXAMPLE|&quot; + (P0 + 5)" Context="{Binding}">
        <ctrls:EvalExpressionParameter Name="P0" Value="{Binding Values[MP_EXAMPLE]}" />
    </ctrls:EvalExpression>
    <Button
       Content="Tap for 5 more"
       Command="{Binding SetPropertyValueAndValidateCommand}"
       CommandParameter="{Binding ElementName=Evaluator, Path=Result}"/>
</Grid>

Xamarin

<Grid xmlns:ctrls="clr-namespace:UBIK.CPL.Controls;assembly=UBIK.CPL">
    <ctrls:EvalExpression x:Name="Evaluator" Expression="&quot;MP_EXAMPLE|&quot; + (P0 + 5)" Context="{Binding}">
        <ctrls:EvalExpressionParameter Name="P0" Value="{Binding Values[MP_EXAMPLE]}" />
    </ctrls:EvalExpression>
    <Button
       Text="Tap to add 5 more"
       Command="{Binding SetPropertyValueAndValidateCommand}"
       CommandParameter="{Binding Path=Result, Source={x:Reference Evaluator}}"/>
</Grid>

Conditional statement

If / Or statements can also be evaluated using C# syntax. In this case, if the result of the "Condi" parameter is true, namely if the context object's MP_STATUS property value equals to 0, P0's value will be displayed in the Label, otherwise P1's value will be displayed.

UWP

<Grid xmlns:ctrls="using:UBIK.WinX.Controls">
    <ctrls:EvalExpression x:Name="Evaluator" Expression="Condi == true ? P0 : P1" Context="{Binding}">
        <ctrls:EvalExpressionParameter Name="Condi" Value="{Binding Values[MP_STATUS], Converter={StaticResource EqualToTrueConverter}, ConverterParameter=0}" />
        <ctrls:EvalExpressionParameter Name="P0" Value="State1" />
        <ctrls:EvalExpressionParameter Name="P1" Value="State2" />
    </ctrls:EvalExpression>
    <TextBlock Text="{Binding ElementName=Evaluator, Path=Result}" />
</Grid>

Xamarin

<Grid xmlns:ctrls="clr-namespace:UBIK.CPL.Controls;assembly=UBIK.CPL">
    <ctrls:EvalExpression x:Name="Evaluator" Expression="Condi == true ? P0 : P1" Context="{Binding}">
        <ctrls:EvalExpressionParameter Name="Condi" Value="{Binding Values[MP_STATUS], Converter={StaticResource EqualityToBool}, ConverterParameter=0}" />
        <ctrls:EvalExpressionParameter Name="P0" Value="State1" />
        <ctrls:EvalExpressionParameter Name="P1" Value="State2" />
    </ctrls:EvalExpression>            
    <Label Text="{Binding Path=Result, Source={x:Reference Evaluator}}" />
</Grid>




See also

Retrieved from "https://wiki.augmensys.com/index.php?title=EvalExpression&oldid=26175"