Environment specific .NET configuration transforms with sensitive values

It is possible to combine the configuration features that you use in your deployments. One scenario where this is useful is if you need to provide environment specific configuration that includes sensitive values.

This can be achieved using both the Substitute Variables in Templates feature and the .NET Configuration Transforms features.

One transform and variable replacement

For example, let’s assume we have a web application that’s being deployed to Development, Staging, and Production environments, and you want to change your Web.Config file to reflect environment-specific values.

To achieve this you would have a single configuration transformation file in your project. If it’s named Web.Release.Config, the transformation will be applied to your Web.Config file automatically, however you can have your own filename and apply it to any config file you like.

This transform file can contain #{variable} values. Because your config will only get transformed on deployment, you can safely work with your Web.Config file during development, and you can keep sensitive variables like production passwords out of source control.

The process

It’s important to note that variable substitution occurs before the configuration transformation. This means you need to target your transform files for variable substitution by adding them to the Target files setting.

For example, let’s assume our Web.Config file has a MyDatabaseConnection connection string and a special MyCustomSettingsSection element. Something like this:

<?xml version="1.0"?>
<configuration>
  <connectionStrings>
    <add name="MyDatabaseConnection" connectionString="Server=(local)\SQLExpress;Database=OctoFX;Trusted_connection=SSPI"/>
  </connectionStrings>
  <MyCustomSettingsSection>
    <TestMode>True</TestMode>
  </MyCustomSettingsSection>
</configuration>

We also have a Web.Release.Config transform file with the following contents:

<?xml version="1.0"?>
<configuration xmlns:xdt="http://schemas.microsoft.com/XML-Document-Transform">
  <connectionStrings>
    <add name="MyDatabaseConnection" connectionString="#{OctoFXDatabase}" xdt:Transform="SetAttributes" xdt:Locator="Match(name)"/>
  </connectionStrings>
  <MyCustomSettingsSection>
    <TestMode xdt:Transform="Replace">#{RunTestMode}</TestMode>
  </MyCustomSettingsSection>
</configuration>

Finally, we have the following variables configured in Octopus:

On deployment to your Staging environment, your process would go like this:

1. Your package, complete with your original Web.Config and your Web.Release.Config transform file, will be extracted to the target.
2. Variable Substitution will run against your Web.Release.Config file (assuming it’s been listed in the Target files setting). This will change the #{OctoFXDatabase} string to the Staging connection string, and will insert False into the TestMode element.
3. Then, the .NET configuration transformation feature will run and apply this new transform file to your Web.Config.

The end result is a correctly transformed configuration for your staging environment. All without a specific Staging transform file, and while keeping your Web.Config file clean for development.

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Page updated on Sunday, January 1, 2023