从“VisitLambda”调用时，重写“System.Linq.Expressions.ParameterExpression”类型的节点必须返回相同类型的非空值

我将数据层类库项目从 .NET Core 2.2 升级到 .NETCore3.1，并将 EntityFrameWork Core 等 Nuget 包从 2.2.0 升级到 EFCore 3.1.0。

然后我在验证以下代码时收到如下错误：

Code:

private async Task<List<CountryDTO>> GetCountriesDataAsync(int languageId)
{
    int pageNo = 1, pageSize = 100;
    var images = await _cacheService.GetAllAsync("imagesCacheKey");
    return await _dbContext.Countries
                    .Where(cc => cc.IsPublished.Equals(true) 
                                     && cc.LanguageId.Equals(languageId))
                    .Select(co => new CountryDTO
                            {  Uuid = co.CountryId, 
                               PNGImagePath = images.FirstOrDefault(im => im.ImageId.Equals(co.PNGImageId))
                                                    .FilePath, 
                                SVGImagePath = images.FirstOrDefault(im => im.ImageId.Equals(co.SVGImageId))
                                                    .FilePath, 
                          DisplayName = co.DisplayName, 
                          DisplayNameShort = co.DisplayName, 
                          Name = Helper.ReplaceChars(co.DisplayName), 
                          Path = Helper.ReplaceChars(co.DisplayName), 
                          CompleteResponse = true})
                   .Skip((pageNo - 1) * 100)
                   .Take(pageSize)
                   .ToListAsync();
}

Error:

When called from 'VisitLambda', rewriting a node of type 'System.Linq.Expressions.ParameterExpression' must return a non-null value of the same type. Alternatively, override 'VisitLambda' and change it to not visit children of this type.
 ---> System.InvalidOperationException: When called from 'VisitLambda', rewriting a node of type 'System.Linq.Expressions.ParameterExpression' must return a non-null value of the same type. Alternatively, override 'VisitLambda' and change it to not visit children of this type.
   at System.Linq.Expressions.ExpressionVisitor.VisitAndConvert[T](T node, String callerName)
   at System.Dynamic.Utils.ExpressionVisitorUtils.VisitParameters(ExpressionVisitor visitor, IParameterProvider nodes, String callerName)
   at System.Linq.Expressions.ExpressionVisitor.VisitLambda[T](Expression`1 node)
   at Microsoft.EntityFrameworkCore.Cosmos.Query.Internal.CosmosProjectionBindingExpressionVisitor.Visit(Expression expression)
   at System.Dynamic.Utils.ExpressionVisitorUtils.VisitArguments(ExpressionVisitor visitor, IArgumentProvider nodes)
   at System.Linq.Expressions.ExpressionVisitor.VisitMethodCall(MethodCallExpression node)
   at Microsoft.EntityFrameworkCore.Cosmos.Query.Internal.CosmosProjectionBindingExpressionVisitor.VisitMethodCall(MethodCallExpression methodCallExpression)
   at Microsoft.EntityFrameworkCore.Cosmos.Query.Internal.CosmosProjectionBindingExpressionVisitor.Visit(Expression expression)
   at Microsoft.EntityFrameworkCore.Cosmos.Query.Internal.CosmosProjectionBindingExpressionVisitor.VisitMember(MemberExpression memberExpression)
   at Microsoft.EntityFrameworkCore.Cosmos.Query.Internal.CosmosProjectionBindingExpressionVisitor.Visit(Expression expression)
   at Microsoft.EntityFrameworkCore.Cosmos.Query.Internal.CosmosProjectionBindingExpressionVisitor.VisitMemberAssignment(MemberAssignment memberAssignment)
   at Microsoft.EntityFrameworkCore.Cosmos.Query.Internal.CosmosProjectionBindingExpressionVisitor.VisitMemberInit(MemberInitExpression memberInitExpression)
   at System.Linq.Expressions.MemberInitExpression.Accept(ExpressionVisitor visitor)
   at Microsoft.EntityFrameworkCore.Cosmos.Query.Internal.CosmosProjectionBindingExpressionVisitor.Visit(Expression expression)
   at Microsoft.EntityFrameworkCore.Cosmos.Query.Internal.CosmosProjectionBindingExpressionVisitor.Translate(SelectExpression selectExpression, Expression expression)
   at Microsoft.EntityFrameworkCore.Cosmos.Query.Internal.CosmosQueryableMethodTranslatingExpressionVisitor.TranslateSelect(ShapedQueryExpression source, LambdaExpression selector)
   at Microsoft.EntityFrameworkCore.Query.QueryableMethodTranslatingExpressionVisitor.VisitMethodCall(MethodCallExpression methodCallExpression)
   at Microsoft.EntityFrameworkCore.Query.QueryableMethodTranslatingExpressionVisitor.VisitMethodCall(MethodCallExpression methodCallExpression)
   at Microsoft.EntityFrameworkCore.Query.QueryableMethodTranslatingExpressionVisitor.VisitMethodCall(MethodCallExpression methodCallExpression)
   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)
   at Author.Query.Persistence.CountryService.GetCountriesDataAsync(Int32 languageId) in /src/QueryStack/Author.Query.Persistence/CountryService.cs:line 243
  at Author.Query.Persistence.CountryService.GetCountriesAsync(Int32 dftLanguageId, Int32 localeLangId) in /src/QueryStack/Author.Query.Persistence/CountryService.cs:line 228
  at Author.Query.Persistence.CountryService.GetAllCountriesAsync(LanguageDTO language) in /src/QueryStack/Author.Query.Persistence/CountryService.cs:line 164
  at GraphQL.DataLoader.DataLoaderBase`1.DispatchAsync(CancellationToken cancellationToken)
   at Author.Query.New.API.GraphQL.Resolvers.CountriesResolver.<>c__DisplayClass5_1.<<Resolve>b__3>d.MoveNext() in /src/QueryStack/Author.Query.New.API/GraphQL/Resolvers/CountriesResolver.cs:line 40
--- End of stack trace from previous location where exception was thrown ---
   at GraphQL.Types.ResolveFieldContext`1.TryAsyncResolve[TResult](Func`2 resolve, Func`2 error)
   --- End of inner exception stack trace ---

任何人都可以通过提供解决此问题的指导来帮助我吗？

这里有几个问题很容易修复，将查询分成两半，第一个查询数据库并返回部分填充的 CountryDTO 列表，第二个迭代列表填充缺失的项目：

private async Task<List<CountryDTO>> GetCountriesDataAsync(int languageId)
{
            int pageNo = 1, pageSize = 100;

            var results = _dbContext.Countries // <- do not use await on db context
                    .Where(cc => cc.IsPublished.Equals(true)
                      && cc.LanguageId.Equals(languageId))
                    .Select(co => new {
                        co.CountryId,
                        co.DisplayName,
                        co.PNGImageId,
                        co.SVGImageId
                    })
                     .Skip((pageNo - 1) * pageSize) // <- use page size no 100
                     .Take(pageSize)
                     .ToListAsync();

            var images = await _cacheService.GetAllAsync("imagesCacheKey");

            return Task.FromResult( results.Select(co => new CountryDTO
            {
                Uuid = co.CountryId,
                PNGImagePath = images.FirstOrDefault(im => im.ImageId.Equals(co.PNGImageId))?.FilePath, // <- could be null so use ?.FilePath
                SVGImagePath = images.FirstOrDefault(im => im.ImageId.Equals(co.SVGImageId))?.FilePath,
                DisplayName = co.DisplayName,
                DisplayNameShort = co.DisplayName,
                Name = Helper.ReplaceChars(co.DisplayName),
                Path = Helper.ReplaceChars(co.DisplayName),
                CompleteResponse = true
            }).ToList());

}

我创建了一个匿名类型来保存数据库的返回值，而不是向 DTO 添加附加字段来获取图像 id 等。 如果您需要从数据库中获取更多信息，只需将字段添加到上下文 select 语句中的匿名类型即可。