Question

我当前的方法在MVC控制器中看起来像这样：

public async Task<IActionResult> AddTests(int? batchId, int? batchVersion)
{
    var btu_DatabaseContext = _context.Test;
    var tests = from info in btu_DatabaseContext select info;
    var btu_DatabaseContext2 = _context.BatchTest;
    var batchTests = from info in btu_DatabaseContext2 select info;
    batchTests = batchTests.Where(s => (s.BatchId == batchId && s.BatchVersion == batchVersion));

    foreach(var batchTest in batchTests)
    {
        tests = tests.Where(s => s.BatchTest != batchTest);
    }



    ViewData["BatchId"] = batchId;
    ViewData["BatchVersion"] = batchVersion;
    if(tests != null && tests.Any())
    {
        return View(tests);
    }

    return RedirectToAction(nameof(Edit), new { id = batchId, version = batchVersion, message = "No unused tests." });
}

如果我在foreach循环删除所有测试元素的情况下调用它，我总会收到以下错误：

处理请求时发生未处理的异常。

ArgumentNullException：值不能为null。

参数名称：querySource

在我添加if语句之前，代码会在返回时中断。我在最后的if语句中添加了尝试检查测试是否为空或空，但这似乎不起作用;当test.Any（）被调用时，代码现在就会中断。

为了避免此错误，检查测试是否为空/空的适当方法是什么？

更新

这是完整的堆栈跟踪：

System.ArgumentNullException: Value cannot be null.
Parameter name: querySource
at Microsoft.EntityFrameworkCore.Utilities.Check.NotNull[T](T value, String parameterName)
at Microsoft.EntityFrameworkCore.Query.QueryCompilationContext.FindEntityType(IQuerySource querySource)
at Microsoft.EntityFrameworkCore.Query.ExpressionVisitors.RelationalEntityQueryableExpressionVisitor.VisitEntityQueryable(Type elementType)
at Microsoft.EntityFrameworkCore.Query.ExpressionVisitors.EntityQueryableExpressionVisitor.VisitConstant(ConstantExpression constantExpression)
at System.Linq.Expressions.ConstantExpression.Accept(ExpressionVisitor visitor)
at System.Linq.Expressions.ExpressionVisitor.Visit(Expression node)
at System.Linq.Expressions.ExpressionVisitor.VisitUnary(UnaryExpression node)
at System.Linq.Expressions.UnaryExpression.Accept(ExpressionVisitor visitor)
at System.Linq.Expressions.ExpressionVisitor.Visit(Expression node)
at System.Linq.Expressions.ExpressionVisitor.Visit(ReadOnlyCollection`1 nodes)
at System.Linq.Expressions.ExpressionVisitor.VisitNewArray(NewArrayExpression node)
at System.Linq.Expressions.NewArrayExpression.Accept(ExpressionVisitor visitor)
at System.Linq.Expressions.ExpressionVisitor.Visit(Expression node)
at System.Linq.Expressions.ExpressionVisitor.VisitAndConvert[T](ReadOnlyCollection`1 nodes, String callerName)
at Remotion.Linq.Parsing.RelinqExpressionVisitor.VisitNew(NewExpression expression)
at System.Linq.Expressions.NewExpression.Accept(ExpressionVisitor visitor)
at System.Linq.Expressions.ExpressionVisitor.Visit(Expression node)
at System.Linq.Expressions.ExpressionVisitor.VisitBinary(BinaryExpression node)
at System.Linq.Expressions.BinaryExpression.Accept(ExpressionVisitor visitor)
at System.Linq.Expressions.ExpressionVisitor.Visit(Expression node)
at Microsoft.EntityFrameworkCore.Query.EntityQueryModelVisitor.ReplaceClauseReferences(Expression expression, IQuerySource querySource, Boolean inProjection)
at Microsoft.EntityFrameworkCore.Query.EntityQueryModelVisitor.VisitWhereClause(WhereClause whereClause, QueryModel queryModel, Int32 index)
at Microsoft.EntityFrameworkCore.Query.RelationalQueryModelVisitor.VisitWhereClause(WhereClause whereClause, QueryModel queryModel, Int32 index)
at Remotion.Linq.Clauses.WhereClause.Accept(IQueryModelVisitor visitor, QueryModel queryModel, Int32 index)
at Remotion.Linq.QueryModelVisitorBase.VisitBodyClauses(ObservableCollection`1 bodyClauses, QueryModel queryModel)
at Remotion.Linq.QueryModelVisitorBase.VisitQueryModel(QueryModel queryModel)
at Microsoft.EntityFrameworkCore.Query.EntityQueryModelVisitor.VisitQueryModel(QueryModel queryModel)
at Microsoft.EntityFrameworkCore.Query.RelationalQueryModelVisitor.VisitQueryModel(QueryModel queryModel)
at Microsoft.EntityFrameworkCore.Query.EntityQueryModelVisitor.CreateQueryExecutor[TResult](QueryModel queryModel)
at Microsoft.EntityFrameworkCore.Query.Internal.QueryCompiler.CompileQueryCore[TResult](Expression query, INodeTypeProvider nodeTypeProvider, IDatabase database, IDiagnosticsLogger`1 logger, Type contextType)
at Microsoft.EntityFrameworkCore.Query.Internal.QueryCompiler.<>c__DisplayClass15_0`1.<Execute>b__0()
at Microsoft.EntityFrameworkCore.Query.Internal.CompiledQueryCache.GetOrAddQueryCore[TFunc](Object cacheKey, Func`1 compiler)
at Microsoft.EntityFrameworkCore.Query.Internal.QueryCompiler.Execute[TResult](Expression query)
at System.Linq.Queryable.Any[TSource](IQueryable`1 source)
at WebAppCore.Controllers.BatchesController.<AddTests>d__14.MoveNext() in C:\Users\Rex\Documents\School\Capstone\WebAp2\WebApp\Btu Database\WebAppCore\Controllers\BatchesController.cs:line 315
--- End of stack trace from previous location where exception was thrown ---
at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
at System.Runtime.CompilerServices.TaskAwaiter`1.GetResult()
at Microsoft.AspNetCore.Mvc.Internal.ControllerActionInvoker.<InvokeActionMethodAsync>d__12.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
at Microsoft.AspNetCore.Mvc.Internal.ControllerActionInvoker.<InvokeNextActionFilterAsync>d__10.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
at Microsoft.AspNetCore.Mvc.Internal.ControllerActionInvoker.Rethrow(ActionExecutedContext context)
at Microsoft.AspNetCore.Mvc.Internal.ControllerActionInvoker.Next(State& next, Scope& scope, Object& state, Boolean& isCompleted)
at Microsoft.AspNetCore.Mvc.Internal.ControllerActionInvoker.<InvokeInnerFilterAsync>d__14.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
at Microsoft.AspNetCore.Mvc.Internal.ResourceInvoker.<InvokeNextResourceFilter>d__22.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
at Microsoft.AspNetCore.Mvc.Internal.ResourceInvoker.Rethrow(ResourceExecutedContext context)
at Microsoft.AspNetCore.Mvc.Internal.ResourceInvoker.Next(State& next, Scope& scope, Object& state, Boolean& isCompleted)
at Microsoft.AspNetCore.Mvc.Internal.ResourceInvoker.<InvokeFilterPipelineAsync>d__17.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
at Microsoft.AspNetCore.Mvc.Internal.ResourceInvoker.<InvokeAsync>d__15.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
at Microsoft.AspNetCore.Builder.RouterMiddleware.<Invoke>d__4.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
at Microsoft.AspNetCore.Diagnostics.DeveloperExceptionPageMiddleware.<Invoke>d__7.MoveNext()

我正在使用Microsoft.AspNetCore.All v2.0.6和Microsoft.EntityFrameworkCore v2.0.2。

Answer 1

我喜欢将this扩展方法与null object pattern结合使用，以确保如果从LINQ查询中获取null，我不会得到任何意外的异常。

var items = listOfItems.FirstOrDefault().IfDefaultGiveMe(new NullItem());

在linq查询后检查实体是否为空

1 个答案: