Warning CS8600 Converting null literal or possible null value to non-nullable type

Question

However I try I get the following warning.

Severity Code Description Project File Line Suppression State Warning CS8600 Converting null literal or possible null value to non-nullable type.

The code is as follows.

HttpResponseMessage response = await _httpClient.PutAsync(url, requestContent);
string? userResponse = await response.Content.ReadAsStringAsync();
JsonSerializerOptions? options = new JsonSerializerOptions
{
  PropertyNamingPolicy = JsonNamingPolicy.CamelCase,
};

//if (userResponse.Length > 0)
//{
  user = JsonSerializer.Deserialize<GetUserById>(userResponse, options);
//}

Looked at some posts like this and this, but could not figure out how to resolve.

Update.

I removed the if condition altogether, but still getting the warning.

Update 2

Added the null check as suggested. But still getting that warning.

You should check if userResponse is null before you check the length — TheTanic, Feb 18 '22 at 08:03
Check my answer below. I added the null check you are missing in my code example. — mathis1337, Feb 18 '22 at 08:20
Do you have to use your string as "string?" ? "string userResponse" can still be null, you dont have to specify it by using "string?" — Jan Macháček, Feb 18 '22 at 08:29

score 9 · Accepted Answer · answered Feb 18 '22 at 09:25

The JsonSerializer.Deserialize returns nullable GetUserById. You are assigning the nullable value to a nonnullable field/property.

You have two options:

Disable nullable reference types in your application. Set Nullable node to disable in your csproj, see https://learn.microsoft.com/en-us/dotnet/csharp/whats-new/tutorials/nullable-reference-types
or assign the result of JsonSerializer.Deserialize to a nullable property/field. (GetUserById? user = null; instead of GetUserById user = null;)

Jatin Sanghvi · Answer 2 · 2023-05-09T04:30:04.013

This can be solved by throwing an exception for null values. By setting the type of user as GetUserById (not GetUserById?), you are stating that it cannot be null, so throwing an exception at the earliest possible point will be the correct choice.

GetUserById user =
    JsonSerializer.Deserialize<GetUserById>(userResponse, options) ??
    throw new InvalidOperationException();

Note that JsonSerializer.Deserialize returns null in a special case when the string equals "null". See https://stackoverflow.com/a/72279451.

You can also use null-forgiving operator starting with C# 8. This is generally not recommended as it makes the program to fail at a later point if the value happens be null.

GetUserById user = JsonSerializer.Deserialize<GetUserById>(userResponse, options)!;

Imo it's a design flaw that Deserialize even returns a nullable reference though, as that method throws exceptions in any case of parsing errors anyway instead of returning null. — Vinz, Feb 02 '23 at 22:50

mathis1337 · Answer 3 · 2022-02-18T08:24:54.077

1

I believe it is warning you that userResponse is null and that it needs to be checked for a null state. If you do a null check that warning should go away.

Do this for you do the userResponse.Length Try this:

HttpResponseMessage response = await _httpClient.PutAsync(url, requestContent);
var userResponse = (string?)null;
userResponse = await response.Content.ReadAsStringAsync();
JsonSerializerOptions? options = new JsonSerializerOptions
{
  PropertyNamingPolicy = JsonNamingPolicy.CamelCase,
};

if (userResponse != null)
{
  user = JsonSerializer.Deserialize<GetUserById>(userResponse, options);
}

edited Feb 18 '22 at 08:24

answered Feb 18 '22 at 08:04

mathis1337

1,426
8
13

Tried the following(added ! to the parms). But still getting the warning. user = JsonSerializer.Deserialize(userResponse!, options!); – VivekDev Feb 18 '22 at 08:13
You need to do an actual NULL check. not say it can be null. I'll update code. – mathis1337 Feb 18 '22 at 08:15
Still getting the warning. See my update – VivekDev Feb 18 '22 at 08:28
What is the line number it's complaining about and can you point that line out in your example ? – mathis1337 Feb 18 '22 at 08:47

score 1 · Answer 4 · answered May 20 '23 at 10:43

JsonConvert.DeserializeObject can return null so the receiving variable should be nullable eg:

if (userResponse != null)
{
  var user? = JsonSerializer.Deserialize<GetUserById>(userResponse, options);
  if (user != null)
  {
    // Do stuff with non null user here...
  }
}

The simplest way for me to check for the null returned object in one go is just:

if (JsonSerializer.Deserialize<GetUserById>(userResponse, options) is GetUserById user)
{
  // Do stuff with non null user here...
}

Warning CS8600 Converting null literal or possible null value to non-nullable type

Update.

Update 2

4 Answers4