Error when getAsync after adding column EF core - asp.net

Question

I init a User info table only not have FacebookID column, and there were 2 record. Now I add a FacebookID column to practice register by Facebook using migration EF core Framework. In first called, a record added and 2 FacebookID under is null. And I want to second called, It have to check that FacebookID is exist, If it is exist, please return message that FacebookID is exist. If it is not exist, add new User
I wrote a function to check that, because 2 Facebookid in image is null so It not working
RuleFor(model => model).MustAsync((x, cancellationToken) => FacebookIdMustUnique(x.FacebookId, cancellationToken))
.WithMessage(string.Format(Resource.Validation_Existed, Resource.FacebookId));
async Task<bool> FacebookIdMustUnique(string facebookId, CancellationToken cancellationToken = default(CancellationToken))
{
var user = await userService.GetAsync(x =>
x.FacebookId.Equals(facebookId, StringComparison.InvariantCultureIgnoreCase),
cancellationToken
);
return user == null;
}
image description here

Answer 1

I wrote a function to check that, because 2 Facebookid in image is null so It not working
Your following code will throw in this line when the Facebookid is null :
x.FacebookId.Equals(facebookId, StringComparison.InvariantCultureIgnoreCase)
To fix it, you need take consideration whether it is null before invoke its .Equals() method：
async Task FacebookIdMustUnique(string facebookId, CancellationToken cancellationToken = default(CancellationToken))
{
if(facebookId ==null){ return true; }
var user = await userService.GetAsync(
x => x.FacebookId.Equals(facebookId, StringComparison.InvariantCultureIgnoreCase),
cancellationToken
);
return user == null;
}
(In your case, it likely be fine if the facebook id is null, so I just return true)
As a reminder: it' always a good practice to check null for Nullable types.