Web.Config Debug/Release

It is possible using ConfigTransform build target available as a Nuget package - https://www.nuget.org/packages/CodeAssassin.ConfigTransform/

All "web.*.config" transform files will be transformed and output as a series of "web.*.config.transformed" files in the build output directory regardless of the chosen build configuration.

The same applies to "app.*.config" transform files in non-web projects.

and then adding the following target to your *.csproj.

<Target Name="TransformActiveConfiguration" Condition="Exists('$(ProjectDir)/Web.$(Configuration).config')" BeforeTargets="Compile" >
    <TransformXml Source="$(ProjectDir)/Web.Config" Transform="$(ProjectDir)/Web.$(Configuration).config" Destination="$(TargetDir)/Web.config" />
</Target>

Posting an answer as this is the first Stackoverflow post that appears in Google on the subject.

If your are going to replace all of the connection strings with news ones for production environment, you can simply replace all connection strings with production ones using this syntax:

<configuration xmlns:xdt="http://schemas.microsoft.com/XML-Document-Transform">

<connectionStrings xdt:Transform="Replace">
    <!-- production environment config --->
    <add name="ApplicationServices" connectionString="data source=.\SQLEXPRESS;Integrated Security=SSPI;AttachDBFilename=|DataDirectory|\aspnetdb.mdf;User Instance=true"
      providerName="System.Data.SqlClient" />
    <add name="Testing1" connectionString="Data Source=test;Initial Catalog=TestDatabase;Integrated Security=True"
      providerName="System.Data.SqlClient" />
</connectionStrings>
....

Information for this answer are brought from this answer and this blog post.

notice: As others explained already, this setting will apply only when application publishes not when running/debugging it (by hitting F5).

To make the transform work in development (using F5 or CTRL + F5) I drop ctt.exe (https://ctt.codeplex.com/) in the packages folder (packages\ConfigTransform\ctt.exe).

Then I register a pre- or post-build event in Visual Studio...

$(SolutionDir)packages\ConfigTransform\ctt.exe source:"$(ProjectDir)connectionStrings.config" transform:"$(ProjectDir)connectionStrings.$(ConfigurationName).config" destination:"$(ProjectDir)connectionStrings.config"
$(SolutionDir)packages\ConfigTransform\ctt.exe source:"$(ProjectDir)web.config" transform:"$(ProjectDir)web.$(ConfigurationName).config" destination:"$(ProjectDir)web.config"

For the transforms I use SlowCheeta VS extension (https://visualstudiogallery.msdn.microsoft.com/69023d00-a4f9-4a34-a6cd-7e854ba318b5).

The web.config transforms that are part of Visual Studio 2010 use XSLT in order to "transform" the current web.config file into its .Debug or .Release version.

In your .Debug/.Release files, you need to add the following parameter in your connection string fields:

xdt:Transform="SetAttributes" xdt:Locator="Match(name)"

This will cause each connection string line to find the matching name and update the attributes accordingly.

Note: You won't have to worry about updating your providerName parameter in the transform files, since they don't change.

Here's an example from one of my apps. Here's the web.config file section:

<connectionStrings>
      <add name="EAF" connectionString="[Test Connection String]" />
</connectionString>

And here's the web.config.release section doing the proper transform:

<connectionStrings>
      <add name="EAF" connectionString="[Prod Connection String]"
           xdt:Transform="SetAttributes"
           xdt:Locator="Match(name)" />
</connectionStrings>

One added note: Transforms only occur when you publish the site, not when you simply run it with F5 or CTRL+F5. If you need to run an update against a given config locally, you will have to manually change your Web.config file for this.

For more details you can see the MSDN documentation

https://msdn.microsoft.com/en-us/library/dd465326(VS.100).aspx