I am attempting to mock a public async method in C#, using Moq, as part of the testing setup for the rendering of a Blazor Component, but the code within the method that is being mocked is being called. This seems like a very straight forward setup so I am unclear as to why the mocking of this method is not functioning as intended.

I have removed/edited the code to maintain confidentiality of this project but the error is occurring because _authenticationStateProvider.GetAuthenticationStateAsync() is being called in Service.MockedMethod(). The error is System.NullReferenceException: ‘Object reference not set to an instance of an object.’ because authState is set to null

MyComponentTests.cs File

internal class MyComponentTests : BunitTestContext
{
    private Mock<AuthenticationStateProvider> _mockServiceProvider;
    private Mock<IAuthenticationStateService> _mockAuthenticationStateService;

    [SetUp]
    public void Setup()
    {  
        _mockServiceProvider = new Mock<AuthenticationStateProvider>();
        _mockAuthenticationStateService = new Mock<IAuthenticationStateService>();

        Services.AddSingleton<IAuthenticationStateService>(new AuthenticationService(_mockServiceProvider.Object));
    }

    [Test]
    public void MyComponentRendersCorrectly()
    {
        _mockAuthenticationStateService.Setup(s => s.MockedMethod()).ReturnsAsync("user");

        var cut = RenderComponent<MyComponent>();

        cut.MarkupMatches("<h1>TEST</h1>");
    }
}

MyComponent.CS File

@inject AuthenticationStateService service

@code {
    protected override void OnInitialized()
    {
        UserName = service.MockedMethod().Result;
    }
}

Service.CS File

namespace Services;

public class AuthenticationStateService(AuthenticationStateProvider authenticationStateProvider) : IService
{
    private readonly AuthenticationStateProvider _authenticationStateProvider = authenticationStateProvider;

    public async Task<string> MockedMethod()
    {
        AuthenticationState authState = await _authenticationStateProvider.GetAuthenticationStateAsync();
        ClaimsPrincipal user = authState.User;

        if (user?.Identity is not null && user.Identity.IsAuthenticated && user?.Identity.Name is not null)
        {
            return user.Identity.Name.Split('@')[0].ToUpper();
        }
        else
        {
            return "UNKNOWN";
        }
    }

Blazor Test Project CS File

<Project Sdk="Microsoft.NET.Sdk.Razor">

    <PropertyGroup>
        <TargetFramework>net8.0</TargetFramework>
        <Nullable>enable</Nullable>
        <IsPackable>false</IsPackable>
    </PropertyGroup>

    <ItemGroup>
        <Using Include="Bunit" />
        <Using Include="Bunit.TestDoubles" />
        <Using Include="Microsoft.Extensions.DependencyInjection" />
        <Using Include="NUnit.Framework" />
    </ItemGroup>

    <ItemGroup>
        <PackageReference Include="Moq.EntityFrameworkCore" Version="8.0.1.2" />
        <PackageReference Include="bunit" Version="1.29.5" />
        <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.10.0" />
        <PackageReference Include="coverlet.collector" Version="6.0.0">
            <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
            <PrivateAssets>all</PrivateAssets>
        </PackageReference>
    </ItemGroup>


    <ItemGroup>
        <PackageReference Include="NUnit" Version="3.14.0" />
        <PackageReference Include="NUnit3TestAdapter" Version="4.5.0" />
    </ItemGroup>


</Project>