What is NInjects equivalent of TryAdd

Question

In other DI containers, I have observed TryAddScoped, TryAddTransient, TryAddSingleton etc.

The idea behind Try is to avoid registering multiple times. If a service is already registered, then using Try will not attempt to register again I guess.

With inject

Kernel.Bind<IHttpContextAccessor>().To<HttpContextAccessor>().InSingletonScope();

So is there any Try equivalent in Ninject?

Answer 1

There's no simple equivalent.

Performing

Kernel.Bind<IHttpContextAccessor>().To<HttpContextAccessor>().InSingletonScope();
Kernel.Bind<IHttpContextAccessor>().To<HttpContextAccessor>().InSingletonScope();

will result in in an exception when resolving a IHttpContextAccessor, and when resolving IEnumerable<IHttpContextAccessor> it will return two HttpContextAccessor instances.

However, you can write your own "Try":

Checking whether Binding already exists

if(!Kernel.GetBindings(typeof(IHttpContextAccessor)).Any())
{
    Kernel.Bind<IHttpContextAccessor>().To<HttpContextAccessor>().InSingletonScope();
}

Of course you can also write your own extension method for that:

public static class NinjectBindingExtensions
{
    public static void TryBind<T>(
        this IKernel kernel,
        Action<IBindingToSyntax<T>> configureBinding)
    {
        if (!kernel.GetBindings(typeof(T)).Any())
        {
            configureBinding(kernel.Bind<T>());
        }
    }
}

Rebind

One way to work around the issue is to use .Rebind instead of .Bind. If there's no pre-existing binding it will work just like .Bind. And if there's a pre-existing binding, it will replace it. Thus:

Kernel.Rebind<IHttpContextAccessor>().To<HttpContextAccessor>().InSingletonScope();
Kernel.Rebind<IHttpContextAccessor>().To<HttpContextAccessor>().InSingletonScope();

Resolving IHttpContextAccessor will result in one instance of HttpContextAccessor.

Preventing Duplicate Module Loading

In case the problem is not with multiple components / NinjectModules creating bindings for the same type, but rather with loading the same NinjectModule twice, you can can prevent duplicate loading by:

if(!Kernel.HasModule(typeof(MyModule)))
{
    Kernel.Load<MyModule>();
}