Question
Answer and Explanation
When using 'NSubstitute' with asynchronous methods in C#, you might encounter situations where the substitute returns null unexpectedly. This often occurs due to incorrect configuration of the substitute to handle async operations.
One common reason is that you are not using ReturnsAsync() correctly. When dealing with async methods that return a value (e.g., Task or Task), you need to specify what the Task should resolve to using ReturnsAsync() instead of Returns().
Here’s an example to illustrate the problem and the correct solution:
Incorrect Approach:
Assuming you have an interface:
public interface IMyService {
Task GetNameAsync();
}
And you attempt to substitute like this:
var myService = Substitute.For
myService.GetNameAsync().Returns(null); // Incorrect
In this case, 'NSubstitute' might return null because you're effectively setting the Task to return a null Task, not a Task that resolves to null string.
Correct Approach:
You should use ReturnsAsync() to specify the value that the Task should resolve to:
var myService = Substitute.For();
myService.GetNameAsync().ReturnsAsync((string)null); // Correct: Returns a Task that resolves to null
Another possible reason is that the async method is not awaited correctly in the test. Ensure that you await the method call within your test to properly handle the asynchronous nature of the operation.
Example:
[Fact]
public async Task TestMethod()
{
var myService = Substitute.For();
myService.GetNameAsync().ReturnsAsync("TestName");
var result = await myService.GetNameAsync();
Assert.Equal("TestName", result);
}
Additionally, if you are using a custom return type, make sure you are providing the correct type when using ReturnsAsync(). Type mismatches can also lead to null results.
In summary, ensure that you are using ReturnsAsync(), awaiting the async method in your test, and verifying that your return types are correctly aligned. These steps should help prevent 'NSubstitute' from returning null for your asynchronous method calls.