Question

我想创建一个列表（MyList），其中每个IDNumber只有一行，SQL表中只有PersonName（TABLE01）。这是如何用LINQ编写的。使用下面的代码我得到＆＃34;不支持指定的方法＆＃34; -

var MyList = (from a in TABLE01
              where a.IDNumber != " "
              select new
              {
                  Num = a.IDNumber.Distinct(),
                  Name = a.PersonName
              }).ToList();

** Jon完全堆积的痕迹：

    at     Devart.Data.Linq.LinqCommandExecutionException.CanThrowLinqCommandExecutionException(String message, Exception e) at Devart.Data.Linq.Provider.DataProvider.ExecuteQuery(CompiledQuery compiledQuery, Object[] parentArgs, Object[] userArgs, Object lastResult)    
at Devart.Data.Linq.Provider.DataProvider.ExecuteAllQueries(CompiledQuery compiledQuery, Object[] userArguments)
   at Devart.Data.Linq.Provider.DataProvider.CompiledQuery.Devart.Data.Linq.Provider.ICompiledQuery.Execute(IProvider provider, Object[] userArgs)
  at Devart.Data.Linq.DataQuery`1.i()
at System.Collections.Generic.List`1..ctor(IEnumerable`1 collection)
at System.Linq.Enumerable.ToList[TSource](IEnumerable`1 source)
at Form1.Create_Modelist() in D:\projects\Form1.cs:line 488
at Form1.Form1_Load(Object sender, EventArgs e) in D:\projects\Form1.cs:line 565
at System.Windows.Forms.Form.OnLoad(EventArgs e)
at System.Windows.Forms.Form.OnCreateControl()
at System.Windows.Forms.Control.CreateControl(Boolean fIgnoreVisible)
at System.Windows.Forms.Control.CreateControl()
at System.Windows.Forms.Control.WmShowWindow(Message& m)
at System.Windows.Forms.Control.WndProc(Message& m)
at System.Windows.Forms.ScrollableControl.WndProc(Message& m)
at System.Windows.Forms.Form.WmShowWindow(Message& m)
at System.Windows.Forms.Form.WndProc(Message& m)
at System.Windows.Forms.Control.ControlNativeWindow.OnMessage(Message& m)
at System.Windows.Forms.Control.ControlNativeWindow.WndProc(Message& m)
at System.Windows.Forms.NativeWindow.DebuggableCallback(IntPtr hWnd, Int32 msg, IntPtr wparam, IntPtr lparam)
at System.Windows.Forms.SafeNativeMethods.ShowWindow(HandleRef hWnd, Int32 nCmdShow)
at System.Windows.Forms.Control.SetVisibleCore(Boolean value)
at System.Windows.Forms.Form.SetVisibleCore(Boolean value)
at System.Windows.Forms.Control.set_Visible(Boolean value)
at System.Windows.Forms.Application.ThreadContext.RunMessageLoopInner(Int32 reason, ApplicationContext context)
at System.Windows.Forms.Application.ThreadContext.RunMessageLoop(Int32 reason, ApplicationContext context)
at System.Windows.Forms.Application.Run(Form mainForm)
at .Program.Main() in D:\projects\Program.cs:line 18
at System.AppDomain._nExecuteAssembly(RuntimeAssembly assembly, String[] args)
at System.AppDomain.ExecuteAssembly(String assemblyFile, Evidence assemblySecurity, String[] args)
at Microsoft.VisualStudio.HostingProcess.HostProc.RunUsersAssembly()
at System.Threading.ThreadHelper.ThreadStart_Context(Object state)
at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean ignoreSyncCtx)
at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state)
at System.Threading.ThreadHelper.ThreadStart()

Answer 1

我怀疑问题在于它将IDNumber视为IEnumerable<char>（因为string确实是一系列字符）。你可能想要更像的东西：

var list = (from a in TABLE01
            where a.IDNumber != " "
            group a.Name by a.IDNumber into g
            select new { Num = g.Key, Name = g.First() }).ToList();

这基本上由IDNumber分组（这样你每个号码只能获得一个组），然后选择＆＃34;第一个＆＃34; （即一些任意的）来自每个组的名称。

Answer 2

如果您希望每个ID都有单个名称，请使用Jon的答案。如果您希望将不同的ID /名称放在一起，那么请在列表中添加distinct而不是单个属性。

var MyList = (from a in TABLE01
                    where a.IDNumber != " "
                    select new
                               {
                                   Num = a.IDNumber,
                                   Name = a.PersonName
                               }).Distinct().ToList();

在LINQ中使用SQL-distinct函数

2 个答案: