Unit test ASP.NET, Open a file in web api

Question

I have a Web api with a function like this:

    [HttpPost]
    public IHttpActionResult PostData([FromBody] JObject new_data)
    {
        //Some code here
        var fullPath = System.Web.Hosting.HostingEnvironment.MapPath(@"~/App_Data/data.txt");

        System.IO.File.WriteAllText(fullPath, new_data.ToString());

        return Ok();
    }

Then I am testing this api like this:

    [TestMethod]
    public void TestPostData()
    {
        // Arrange
        JObject data = getSampleData();
        var controller = new MyController();
        //Act
        IHttpActionResult result = controller.PostData(data);

        //Assert
        Assert.IsInstanceOfType(result, typeof(OkNegotiatedContentResult<JObject>));      
    }

Here I get error at the line below:

    System.IO.File.WriteAllText(fullPath, new_data.ToString());

And the error says: Message: Test method NetWorth.Tests.UnitTest1.TestPostData threw exception: System.ArgumentNullException: Value cannot be null. Parameter name: path

Can someone help please? I'm very new to ASP and c# and unit testing! Any suggestions would help a lot.

I should say that the api works perfect when I run it and this problem is just at the time of testing.

Answer 1

for the test, use the full path to the file instead of var fullPath = System.Web.Hosting.HostingEnvironment.MapPath(@"~/App_Data/data.txt"); for example, let the method take a file path that, if not passed, will be equal to your constant