The multi-targeting feature of Visual Studio 2010 allows web developers to develop web applications targeting ASP.NET 2.0, 3.0, 3.5 and 4.0. Our earlier posts here and here describe some of the multi-targeting capabilities of Visual Studio 2010.

Targeting frameworks 2.0, 3.0 or 3.5 using VS 2010 requires 3.5 Sp1 to be installed

With VS 2010, only the latest 4.0 version of the .NET framework will be installed on your machine as part of the Visual Studio installation. For VS 2010 to be able to target any of the earlier frameworks 2.0, 3.0 or 3.5, the .NET Framework 3.5 Sp1 must be installed on the machine.

When 3.5 Sp1 is not present

If 3.5 Sp1 was not already present on your machine prior to installation of VS 2010, you will see the following behavior when you start VS 2010:

• The New Website dialog will show 4.0 as the only available option (as shown in Figure 1)

Figure 1

• If you open the Property pages of the website, you will see that 4.0 is the only available option (as shown in Figure 2)

Figure 2

• Conversion of projects created using Visual Studio 2005 or Visual Studio 2008 to VS 2010 -
  During conversion of a web project created using VS 2005 or VS 2008, instead of the upgrade prompt (shown in figure 6), you will see a dialog (shown in figure 3) that allows you to either upgrade your project to 4.0 or take you to the installation page for .NET 3.5 Sp1.

Figure 3

To then be able to target 2.0, 3.0 or 3.5 using VS 2010, you should install the 3.5 Sp1 framework from the installation page. To quickly navigate to the installation page, you can use the 'More Frameworks' link in figure 1 or select the option 'Take me to the Framework Download Web Site' in figure 3.

When 3.5 Sp1 is installed

Once 3.5 Sp1 is installed, then simply restart VS 2010 and the earlier frameworks such as 2.0, 3.0 and 3.5 will now light-up as available target frameworks.

• The New Website dialog will show 2.0, 3.0, 3.5 and 4.0 (as shown in Figure 4)

Figure 4

• If you open the Property pages of the website, you will see that you can re-target your website to 2.0, 3.0, 3.5 or 4.0 (as shown in Figure 5)

Figure 5

• Conversion of projects created using Visual Studio 2005 or Visual Studio 2008 to VS 2010 -
  During conversion of a web project created using VS 2005 or VS 2008, you will see the more familiar prompt (shown in figure 6) allowing you to continue targeting the lower .NET framework version such as ASP.NET 2.0, 3.0 or 3.5, or optionally, upgrade the project to target the latest .NET framework version ASP.NET 4.0

Figure 6

Bala Chirtsabesan | SDET | Visual Studio Web Developer