Using Expressions in Configuration Files

Many attributes of the configuration schema accept expressions, which are dynamically evaluated. Expressions in the PostSharp configuration system work similarly to expressions in XSLT. Substrings enclosed by curly brackets, for instance {$property}, are interpreted as XPath expressions.

For instance, the following code contains two XPath expressions:

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<Project xmlns="http://schemas.postsharp.org/1.0/configuration">
  <Property Name="LoggingEnabled" Value="{has-plugin('PostSharp.Patterns.Diagnostics')}" Deferred="true" />
  <Multicast xmlns:d="clr-namespace:PostSharp.Patterns.Diagnostics;assembly:PostSharp.Patterns.Diagnostics">
    <When Condition="{$LoggingEnabled}">
      <d:Log  />
    </When>
  </Multicast>
</Project>

For general information about XPath, see the MSDN documentation.

Note

In the context of PostSharp configuration files, XPath expressions cannot refer to XML elements or attributes, but only to variables, functions, operators and constants.

Accessing properties

PostSharp properties are mapped to XPath variables.

For instance, the expression {$LoggingEnabled} evaluates to the value of the LoggingEnabled property.

Using operators and functions

You can use any XPath function or operator.

In addition to the standard XPath 1.0 functions, PostSharp defines the following functions:

Function Description

has-plugin(name) Evaluates to true if the given plug-in can be found in the project, otherwise false. The argument name is the filename without extension of the plug-in assembly without the .Weaver.dll suffix. It is typically identical to the package name. For example, for the PostSharp logging weaver, it would be PostSharp.Patterns.Diagnostics.
A plug-in can be found in the project if it is in the PostSharpSearchPath property or if it is imported in a NuGet package.

environment(variable) Returns the value of an environment variable.

Function	Description
`has-plugin(name)`	Evaluates to `true` if the given plug-in can be found in the project, otherwise `false`. The argument name is the filename without extension of the plug-in assembly without the .Weaver.dll suffix. It is typically identical to the package name. For example, for the PostSharp logging weaver, it would be PostSharp.Patterns.Diagnostics. A plug-in can be found in the project if it is in the `PostSharpSearchPath` property or if it is imported in a NuGet package.
`environment(variable)`	Returns the value of an environment variable.

Mixing expressions and literal strings

An attribute value can contain both text and expressions. This is illustrated in the following example:

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<Project xmlns="http://schemas.postsharp.org/1.0/configuration">
  <Property Name="A" Value="A" />            <!-- Evaluates to A -->
  <Property Name="B" Value="B;{$A}" />       <!-- Evaluates to B;A -->
  <Property Name="C" Value="C;{$B};{$A}" />  <!-- Evaluates to C;B;A;A -->
</Project>