Null ref when calling .ToList() within a Select function

问题

in my .NET Core 3.1 application, I am getting a null reference exception when I include .ToList() on the following emailAddress =... line:

var customersTask = eshop.TieredCustomer
                                .Where(customer => customer != null)
                                .Where(customer => customer.SendPriceListAutomatically.HasValue 
                                                    && customer.SendPriceListAutomatically.Value 
                                                    && customer.DistributionList.HasValue
                                                    && !string.IsNullOrWhiteSpace(customer.Emails))
                                .Select(customer => new Customer
                                {
                                    distributionList = customer.DistributionList.Value,
                                    emailAddresses = customer.Emails.Split('|', StringSplitOptions.RemoveEmptyEntries).ToList()// ?? new List<string>()
                                })
                                .ToListAsync(token);

I have replaced the code with emailAddresses = "a|b".Split('|', StringSplitOptions.RemoveEmptyEntries).ToList() and I still get the error.

Here is the exception output:

System.NullReferenceException: Object reference not set to an instance of an object.
   at Microsoft.EntityFrameworkCore.Query.SqlExpressions.SelectExpression.AddCollectionProjection(ShapedQueryExpression shapedQueryExpression, INavigation navigation, Type elementType)
   at Microsoft.EntityFrameworkCore.Query.Internal.RelationalProjectionBindingExpressionVisitor.Visit(Expression expression)
   at Microsoft.EntityFrameworkCore.Query.Internal.RelationalProjectionBindingExpressionVisitor.VisitMemberAssignment(MemberAssignment memberAssignment)
   at System.Linq.Expressions.ExpressionVisitor.VisitMemberBinding(MemberBinding node)
   at Microsoft.EntityFrameworkCore.Query.Internal.RelationalProjectionBindingExpressionVisitor.VisitMemberInit(MemberInitExpression memberInitExpression)
   at System.Linq.Expressions.MemberInitExpression.Accept(ExpressionVisitor visitor)
   at System.Linq.Expressions.ExpressionVisitor.Visit(Expression node)
   at Microsoft.EntityFrameworkCore.Query.Internal.RelationalProjectionBindingExpressionVisitor.Visit(Expression expression)
   at Microsoft.EntityFrameworkCore.Query.Internal.RelationalProjectionBindingExpressionVisitor.Translate(SelectExpression selectExpression, Expression expression)
   at Microsoft.EntityFrameworkCore.Query.RelationalQueryableMethodTranslatingExpressionVisitor.TranslateSelect(ShapedQueryExpression source, LambdaExpression selector)
   at Microsoft.EntityFrameworkCore.Query.QueryableMethodTranslatingExpressionVisitor.VisitMethodCall(MethodCallExpression methodCallExpression)
   at Microsoft.EntityFrameworkCore.Query.RelationalQueryableMethodTranslatingExpressionVisitor.VisitMethodCall(MethodCallExpression methodCallExpression)
   at System.Linq.Expressions.MethodCallExpression.Accept(ExpressionVisitor visitor)
   at System.Linq.Expressions.ExpressionVisitor.Visit(Expression node)
   at Microsoft.EntityFrameworkCore.Query.QueryCompilationContext.CreateQueryExecutor[TResult](Expression query)
   at Microsoft.EntityFrameworkCore.Storage.Database.CompileQuery[TResult](Expression query, Boolean async)
   at Microsoft.EntityFrameworkCore.Query.Internal.QueryCompiler.CompileQueryCore[TResult](IDatabase database, Expression query, IModel model, Boolean async)
   at Microsoft.EntityFrameworkCore.Query.Internal.QueryCompiler.<>c__DisplayClass12_0`1.<ExecuteAsync>b__0()
   at Microsoft.EntityFrameworkCore.Query.Internal.CompiledQueryCache.GetOrAddQueryCore[TFunc](Object cacheKey, Func`1 compiler)
   at Microsoft.EntityFrameworkCore.Query.Internal.CompiledQueryCache.GetOrAddQuery[TResult](Object cacheKey, Func`1 compiler)
   at Microsoft.EntityFrameworkCore.Query.Internal.QueryCompiler.ExecuteAsync[TResult](Expression query, CancellationToken cancellationToken)
   at Microsoft.EntityFrameworkCore.Query.Internal.EntityQueryProvider.ExecuteAsync[TResult](Expression expression, CancellationToken cancellationToken)
   at Microsoft.EntityFrameworkCore.Query.Internal.EntityQueryable`1.GetAsyncEnumerator(CancellationToken cancellationToken)
   at System.Runtime.CompilerServices.ConfiguredCancelableAsyncEnumerable`1.GetAsyncEnumerator()
   at Microsoft.EntityFrameworkCore.EntityFrameworkQueryableExtensions.ToListAsync[TSource](IQueryable`1 source, CancellationToken cancellationToken)
   --- End of inner exception stack trace ---
   at System.Threading.Tasks.Task`1.GetResultCore(Boolean waitCompletionNotification)
   at System.Threading.Tasks.Task`1.get_Result()

If I do emailAddresses = new List<string>(), I do not get the error. eShop is a DbContext.

A friend and I have looked this over for awhile and it is a head scratcher. Any ideas?

来源：https://stackoverflow.com/questions/60766229/null-ref-when-calling-tolist-within-a-select-function