Side by Side execution

Side-by-side execution is the ability to run multiple versions of an application
or component on the same computer. You can have multiple versions of the
common language runtime, and multiple versions of applications and
components that use a version of the runtime, on the same computer at the
same time. Since versioning is only applied to shared assemblies, and not to
private assemblies, two application one using private assembly and one using
shared assembly cannot be stated as side-by-side
executables.

As meaning of side by side means that same assembly name with different version to able to access in same computer.
Example-
DotNetFunda.dll - version 1.0
DotnetFunda.dll - version 2.0 

Side-by-Side execution is the ability to run multiple versions of an application or component on the same computer. You can have multiple versions of the common language runtime, and multiple versions of applications and components that use a version of the runtime, on the same computer at the same time.

Strong-named assemblies:-
----------------------------
Side-by-side execution uses strong-named assemblies to bind type information to a specific version of an assembly. This prevents an application or component from binding to an invalid version of an assembly. Strong-named assemblies also allow multiple versions of a file to exist on the same computer and to be used by applications. 

You can install multiple versions of the .NET Framework on the same computer. In ASP.NET, Web applications are said to be running side-by-side when they are installed on the same computer but use different versions of the .NET Framework. This topic describes side-by-side support in ASP.NET.

You have to run multiple versions of the .NET Framework side-by-side when the following conditions are true:

• You want to upgrade to a new version of the .NET Framework but you cannot upgrade all existing Web applications at the same time. For example, you might want to test each application as it is upgraded and your test resources are limited.
• You do not want to maintain a separate server for each version of the .NET Framework that a Web application runs under. You might not have enough servers to support that scenario, or you might want to minimize the number of servers in order to reduce expenses.
  
  
   Side-by-Side Features
  
  ASP.NET side-by-side execution enables you to perform the following tasks:
  

  • Deploy new Web applications that run under the latest version of the .NET Framework on the same server as existing applications that run under an older version.
  • Consolidate existing Web applications that require different versions of the .NET Framework onto one server.
  • Specify the version of the .NET Framework that each Web application will run under.

  You can run .NET Framework 1.1 applications, .NET Framework 2.0 applications, .NET Framework 3.0 applications, .NET Framework 3.5 applications, and .NET Framework 4 applications all on the same server.
    
  Installing Multiple Versions of the .NET Framework
  

  To add a version of ASP.NET to a computer, run the .NET Framework redistributable package Setup program (Dotnetfx.exe) for the version that you want to install.
  Each ASP.NET application is configured to run under a specific version of the common language runtime (CLR). Some versions of the .NET Framework use the CLR that was introduced with an earlier version of the .NET Framework. The .NET Framework 1.1, the .NET Framework 2.0, and the .NET Framework 4 include different versions of the CLR. Because .NET Framework 3.0 and the .NET Framework 3.5 use the .NET Framework 2.0 CLR, ASP.NET applications developed for these versions of the .NET Framework run under the .NET Framework 2.0 CLR.

 The way that the common language runtime manages side-by-side execution depends on the type of application you are running. This section applies to all managed executable applications, such as Windows Forms, and executable applications installed locally through an Internet browser. ASP.NET applications and XML Web services, as well as other hosted applications, have their own method of side-by-side execution. For information about ASP.NET and side-by-side execution, see ASP.NET Side-by-Side Overview.