Why Does My .NET Core MVC Application Fail to Inject a Repository Dependency?

.NET Core MVC app dependency injection error: Unable to resolve dependencies for interface

Problem Description

A .NET Core MVC app encountered an error while trying to inject a repository dependency into a controller. The error message is as follows:

<code>InvalidOperationException: Unable to resolve service for type 'WebApplication1.Data.BloggerRepository' while attempting to activate 'WebApplication1.Controllers.BlogController'.</code>

Analysis

This error indicates that the application cannot create an instance of the BloggerRepository class to pass to the BlogController constructor. Although BloggerRepository is registered in the startup class:

<code class="language-csharp">services.AddScoped<IBloggerRepository, BloggerRepository>();</code>

Solution

The problem is that the controller class directly requests the concrete BloggerRepository class. However, the dependency injection container only knows how to create instances of the IBloggerRepository interface. To resolve this error, the controller should be adjusted to accept the interface:

<code class="language-csharp">public BlogController(IBloggerRepository repository)
{
    _repository = repository;
}</code>

Other instructions

For external NuGet packages, be sure to check their documentation as they may have specific registration requirements. For example, to resolve an error message about IHttpContextAccessor:

<code>Unable to resolve service for type 'Microsoft.AspNetCore.Http.IHttpContextAccessor' ...</code>

The solution is to use a custom extension method:

<code class="language-csharp">services.AddHttpContextAccessor();</code>

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