Compatibility version for ASP.NET Core MVC

By Rick Anderson

The SetCompatibilityVersion method allows an app to opt-in or opt-out of potentially breaking behavior changes introduced in ASP.NET Core MVC 2.1 or later. These potentially breaking behavior changes are generally in how the MVC subsystem behaves and how your code is called by the runtime. By opting in, you get the latest behavior, and the long-term behavior of ASP.NET Core.

The following code sets the compatibility mode to ASP.NET Core 2.1:

public void ConfigureServices(IServiceCollection services)
{
    services.AddMvc()
        .SetCompatibilityVersion(CompatibilityVersion.Version_2_1);
}

We recommend you test your app using the latest version (CompatibilityVersion.Version_2_1). We anticipate that most apps won't have breaking behavior changes using the latest version.

Apps that call SetCompatibilityVersion(CompatibilityVersion.Version_2_0) are protected from potentially breaking behavior changes introduced in the ASP.NET Core 2.1 MVC and later 2.x versions. This protection:

  • Does not apply to all 2.1 and later changes, it's targeted to potentially breaking ASP.NET Core runtime behavior changes in the MVC subsystem.
  • Does not extend to the next major version.

The default compatibility for ASP.NET Core 2.1 and later 2.x apps that do not call SetCompatibilityVersion is 2.0 compatibility. That is, not calling SetCompatibilityVersion is the same as calling SetCompatibilityVersion(CompatibilityVersion.Version_2_0).

The following code sets the compatibility mode to ASP.NET Core 2.1, except for the following behaviors:

public void ConfigureServices(IServiceCollection services)
{
    services.AddMvc()
        // Include the 2.1 behaviors
        .SetCompatibilityVersion(CompatibilityVersion.Version_2_1)
        // Except for the following.
        .AddMvcOptions(options =>
        {
            // Don't combine authorize filters (keep 2.0 behavior).
            options.AllowCombiningAuthorizeFilters = false;
            // All exceptions thrown by an IInputFormatter are treated
            // as model state errors (keep 2.0 behavior).
            options.InputFormatterExceptionPolicy =
                InputFormatterExceptionPolicy.AllExceptions;
        });
}

For apps that encounter breaking behavior changes, using the appropriate compatibility switches:

  • Allows you to use the latest release and opt out of specific breaking behavior changes.
  • Gives you time to update your app so it works with the latest changes.

The MvcOptions class source comments have a good explanation of what changed and why the changes are an improvement for most users.

At some future date, there will be an ASP.NET Core 3.0 version. Old behaviors supported by compatibility switches will be removed in the 3.0 version. We feel these are positive changes benefitting nearly all users. By introducing these changes now, most apps can benefit now, and the others will have time to update their apps.