Empty JSON Response from Controller in C# Web API

Question

I am learning about building web APIs in C#.

I am testing my controller, however I am getting an empty JSON response.

This is my controller

public class ClassInfoController : ApiController
{
    private ClassRepository classRepo = new ClassRepository();

    public IList<string> getClassListByTerm(string termID)
    {
        List<string> classList = classRepo.getClassListByTerm(termID);

        return classList;
    }

This is my repository class which queries my Entity model

public class ClassRepository : IClass
{

    private cet_databaseEntities db = new cet_databaseEntities();

    public List<string> getClassListByTerm(string termID)
    {
        List<string> result = (from t in db.classCodeLookup
                where t.termID == termID
                orderby t.@class ascending
                select t.@class).ToList<string>();

        return result;
    }
}

I have debugged my controller and when the API call is made, this repository class returns no data to the controller i.e. an empty list. However, the reason I am confused, is that when I use the following test code on the repository class

public class ClassRepositoryTests
{
    ClassRepository testRepo = new ClassRepository();

    [TestMethod()]
    public void getClassListByTermTest()
    {
        List<string> output = testRepo.getClassListByTerm("316a");
        foreach (string className in output)
        {
            Console.WriteLine(className);
        }
    }
}

I get the result from the repository class I expect.

Why am I having this issue?

Answer 1

My issue was the automatic API documentation provided by a Web API project in Visual Studio told me that I should call my controller from the browser with the following: http://localhost:51520/api/ClassInfo?termID={316a}. Unbeknownst to me, this meant the {} stayed with the 316a - I thought they were just like the parentheses used in a method signature for receiving a parameter, but I presume they are used for passing list/json to a method (not relevant in this case). This meant that when my repository query ran, it returned nothing because I was not looking for the string {316a}.

If I call the API with the following address http://localhost:51520/api/ClassInfo?termID=316a it works because the string 316a gets passed to the query.

Thank you all.

Answer 2

Try to add attribute to your API method

[HttpGet]
public List<string> getClassListByTerm([FromUri]string termID)

or you can use another class object for request data. I mean

 public class TermData
 {
     //use if nessassry [JsonProperty("TermId")]
     public string TermId {get;set;}
 }

 [HttpGet]
 public List<string> getClassListByTerm([FromUri]TermData data)

Answer 3

I had a class inheriting from DynamicObject, and that would only return empty JSON (while XML seemed to return properly).

In my case, I was able to remove the DynamicObject inheritance and JSON serialization started working.

//didn't serialize to json, but xml worked
public MyClass : DynamicObject, IMyClass { //... }

//works for json and xml
public MyClass : IMyClass { //... }

Unfortunately, I didn't have a chance to look into why DynamicObject breaks JSON serialization.