c# – ToListAsync抛出’Value不能为null.’例外

发布时间：2020-12-15 21:05:59 所属栏目：百科来源：网络整理

导读：拥有IQueryable var query = _db .ApplicationPaidServices .Include(it = it.Application) .Include(it = it.Application.User) .Where(it = Enum1.Value1 == it.Application.Enum1) .Skip(0); 执行1 var result1 = await (from it in query select it) .To

拥有IQueryable

var query = _db
    .ApplicationPaidServices
    .Include(it => it.Application)
    .Include(it => it.Application.User)
    .Where(it => Enum1.Value1 == it.Application.Enum1)
    .Skip(0);

执行1

var result1 = await (from it in query
                      select it)
            .ToListAsync();

作品.

执行2

var result2 = await (from it in query
                      select
                      new
                      { it })
                     .ToListAsync();

使用消息抛出ArgumentNullException：Value不能为null.参数名称：参数[2]

堆栈跟踪

System.Dynamic.Utils.ContractUtils.RequiresNotNull(object value,string paramName,int index)
System.Dynamic.Utils.ExpressionUtils.RequiresCanRead(Expression expression,int idx)
System.Dynamic.Utils.ExpressionUtils.ValidateOneArgument(MethodBase method,ExpressionType nodeKind,Expression arguments,ParameterInfo pi,string methodParamName,string argumentParamName,int index)
System.Dynamic.Utils.ExpressionUtils.ValidateArgumentTypes(MethodBase method,ref ReadOnlyCollection arguments,string methodParamName)
System.Linq.Expressions.Expression.Call(Expression instance,MethodInfo method,IEnumerable arguments)
Microsoft.EntityFrameworkCore.Query.ExpressionVisitors.Internal.IncludeExpressionVisitor.VisitMethodCall(MethodCallExpression methodCallExpression)
System.Linq.Expressions.MethodCallExpression.Accept(ExpressionVisitor visitor)

在初始查询中注释Where或Skip会修复错误.

不知道是什么原因.

使用Npgsql.EntityFrameworkCore.PostgreSQL和.NET Core 1.1

解决方法

今天被这个咬了.一种可能的解决方法是在Select()之前等待ToListAsync：

var paged = await queryable
            .OrderBy(m => m.Name)
            .Skip(0).Take(30)
            .ToListAsync();

        var result = paged.Select(ViewModel.Map).ToList();

（编辑：李大同）

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