Value cannot be null. (Parameter 'connectionString') Xunit test - unit-testing

Question

I am trying to write unit test by Xunit:
public class UserTest
{
private readonly TestServer _server;
private readonly HttpClient _client;
public UserTest()
{
_server = new TestServer(new WebHostBuilder().UseStartup<Startup>());
_client = _server.CreateClient();
}
[Fact]
public async Task GetAllUserTest()
{
var request = new HttpRequestMessage(new HttpMethod("Get"), "/Api/Users");
var response =await _client.SendAsync(request);
Assert.Equal(HttpStatusCode.OK, response.StatusCode);
}
}
request validation is in my Solution.
After the test runs, the test fails and message error is:
Value cannot be null. (Parameter 'connectionString')
If I run api, it runs smoothly and has no CONNECTION problem.
startup.cs:
services.AddDbContext<BlogProjectContext>(option =>
option.UseSqlServer(Configuration.GetConnectionString("BlogProjectConnection"))
);
appsetings.json:
{
"AppSettings": {
"Secret": "This is the secret key and its very important"
},
"Logging": {
"LogLevel": {
"Default": "Information",
"Microsoft": "Warning",
"Microsoft.Hosting.Lifetime": "Information"
}
},
"AllowedHosts": "*",
"ConnectionStrings": {
"BlogProjectConnection": "Data Source=.;Initial Catalog=BlogProject_DB;Integrated Security=True"
}
}

Answer 1

I assume your unit tests are in a different project to your API? Your unit test projects run in a different directory to your API, so they can't 'see' the appsettings.json file that your API can, causing you to see this error.
You really don't want your unit tests to be issuing real HTTP requests though. The remedy to this is to mock the HttpClient. Here's a useful blog post on mocking HttpClient.