Blazor 6.0 await HttpClient.GetFromJsonAsync<List CS8601 Possible null reference asignment - entity-framework

Question

Hello Everyone im missing something fairly fundamental here and i could really appreciate some guidance .
I am fairly new to Blazor and the entity framework and am building one of my first Blazor apps
I have a fairly basic data class
using System.ComponentModel.DataAnnotations;
namespace BIDM2.Data
{
public class InvalidAddressMapping
{
[Key]
public int record_no { get; set; } = 0;
public string sales_org_pos { get; set; } = " ";
public string press_sales_mgr_code { get; set; } = " ";
public string press_sales_mgr_Name { get; set; } = " ";
public string forming_sales_rep_code { get; set; } = " ";
public string forming_sales_rep_Name { get; set; } = " ";
}
}
that i am using in my controller as follows
[HttpGet]
public async Task <ActionResult<List<InvalidAddressMapping>>> GetInvalidAdressMappings()
{
return await _db.GetInvalidAddressMappings.FromSqlRaw("EXEC BIDM_GetInvalidAddressMappings;").ToListAsync();
}
and im trying to use it in my razor page like this
#code {
List<InvalidAddressMapping> invalidMappings = new List<InvalidAddressMapping>();
protected override async Task OnInitializedAsync()
{
invalidMappings = await HttpClient.GetFromJsonAsync<List<InvalidAddressMapping>>(NavigationManager.BaseUri + "invalidmapping");
}
}
how ever when i try and run my razor page im getting an error CS8601 Possible null Reference asignment :(
Im not sure i totaly understand and i could use some guidance to point me in the right direction it has to be somethign fairly fundamental i am missing
interestingly this all came about because im trying to convert an existing non async method to an async method
when i use
//1. get invalid address mappings
public InvalidAddressMapping[] GetInvalidAddressMappings()
{
InvalidAddressMapping[] invMappings;
// exec BIDM_GetInvalidAddressMappings calls the stored procedure
invMappings = _dbcontext.GetInvalidAddressMappings.FromSqlRaw("EXEC BIDM_GetInvalidAddressMappings;").ToArray();
return invMappings;
}
}
it works beautifully and i can see a lovely list of JSON data
please help a struggling old git out and pint me in a direction where i can understand where i am going wrong :)
thank you every one

Answer 1

The GetFromJsonAsync extension method returns a nullable type. In your case, it is List<InvalidAddressMapping>? (note the extra ? on the end there). If it fails to deserialise properly, for example, it could return null. The code is telling you that you need to check for a null response to be safe.
So the safe version is to do something like this:
var result = await HttpClient.GetFromJsonAsync<List<InvalidAddressMapping>>(
NavigationManager.BaseUri + "invalidmapping");
if(result == null)
{
// Do something here to handle this error, for example:
throw new Exception("Can't get data from the server for some reason, help!");
}
invalidMappings = result;
Also note that the CS8601 message you see is a warning, not an error. So technically you could ignore it, though I would strongly advise against doing that.