透过自定义Function当作LINQ的Where条件(多重条件筛选)范例与动态教学

本文主要是介绍透过自定义Function当作LINQ的Where条件(多重条件筛选)范例与动态教学，希望对大家解决编程问题提供一定的参考价值，需要的开发者们随着小编来一起学习吧！

以往为了让用户筛选资料的时候可以灵活应用，会在画面上每个条件前面加个CheckBox，让使用者可以自由勾选要筛选的字段并设定条件，如下图：

这样的方式在以往数据源是SQL的时候，可以透过判断式、加上组合SQL字符串的方式达到，但是在Linq里面要怎么处理呢??

小喵再练习Linq的过程中，想到了这个问题...刚好康廷数位的吕高旭老师MSN过来与小喵聊到要写LINQ书籍的计划，小喵于是向吕老师请教了这方面的问题...

小喵做了一个教学动画，透过自己写的一个Function来当作筛选的条件，相关内容请点选以下的超链结：

透过自定义Function当作LINQ的Where条件

相关的画面程序代码如下：

文件夹： < asp:TextBox ID ="txtDir" runat ="server" ></ asp:TextBox >
< asp:Button ID ="btnGetFiles" runat ="server" Text ="取得档案信息" />
< br />
讯息： < asp:Label ID ="lblMsg" runat ="server" Text ="" ForeColor ="Red" Font-Bold ="True" ></ asp:Label >
< hr />
< table >
     < tr >
         < td valign ="top" >
            筛选条件： < br />
             < asp:CheckBox ID ="chkExten" runat ="server" Text ="扩展名：" />
             < asp:DropDownList ID ="ddlExten" runat ="server" >
             </ asp:DropDownList >< br />
             < asp:CheckBox ID ="chkFileName" runat ="server" Text ="檔名包含：" />
             < asp:TextBox ID ="txtFileName" runat ="server" ></ asp:TextBox >< br />
             < asp:Button ID ="btnQry" runat ="server" Text ="筛选" />< br />
             < asp:Button ID ="btnQryFunc" runat ="server" Text ="透过Function筛选" />

         </ td >
         < td >
             < asp:GridView ID ="GridView1" runat ="server" >
             </ asp:GridView >
         </ td >
     </ tr >
</ table >

CodeFile的内容如下：

     Dim myFiles As List( Of FileInfo)
     Protected Sub Page_Load() Sub Page_Load( ByVal sender As Object , ByVal e As System.EventArgs) Handles Me .Load
        myFiles = ViewState( " myFiles " )
         If myFiles Is Nothing Then
            myFiles = New List( Of FileInfo)
         End If
     End Sub
     Protected Sub btnGetFiles_Click() Sub btnGetFiles_Click( ByVal sender As Object , ByVal e As System.EventArgs) Handles btnGetFiles.Click
        myFiles.Clear()
              For Each FileName As String In My.Computer.FileSystem.GetFiles( Me .txtDir.Text)
             ' 取得FileInfo並放入myFiles
            myFiles.Add(My.Computer.FileSystem.GetFileInfo(FileName))
         Next
             ViewState( " myFiles " ) = myFiles
              Me .GridView1.DataSource = myFiles
         Me .GridView1.DataBind()
              Me .ddlExten.DataSource = myFiles
         Me .ddlExten.DataTextField = " Extension "
         Me .ddlExten.DataValueField = " Extension "
         Me .ddlExten.DataBind()
     End Sub
     Protected Sub btnQry_Click() Sub btnQry_Click( ByVal sender As Object , ByVal e As System.EventArgs) Handles btnQry.Click
         Dim o = From FI In myFiles _
                Where FI.Extension = Me .ddlExten.SelectedValue And FI.Name Like " * " & Me .txtFileName.Text & " * " _
                 Select FI
         Me .GridView1.DataSource = o
         Me .GridView1.DataBind()
     End Sub
     Protected Sub btnQryFunc_Click() Sub btnQryFunc_Click( ByVal sender As Object , ByVal e As System.EventArgs) Handles btnQryFunc.Click
         ' Dim o = From FI In myFiles _
         Try
             Dim o = From FI In myFiles _
                    Where ChkFile(FI) _
                     Select FI
             Me .GridView1.DataSource = o
             Me .GridView1.DataBind()

         Catch ex As Exception
             Me .lblMsg.Text = ex.Message
         End Try
     End Sub
     Private Function ChkFile() Function ChkFile( ByVal FI As FileInfo) As Boolean
         Dim rc As Boolean = True
         Try
             If Me .chkExten.Checked Then
                 Dim rcExten As Boolean = True
                 If Not FI.Extension = Me .ddlExten.SelectedValue Then
                    rcExten = False
                 End If
                rc = rc And rcExten
             End If
             If Me .chkFileName.Checked Then
                 Dim rcFN As Boolean = True
                 If InStr ( 1 , FI.Name, Me .txtFileName.Text, CompareMethod.Text) = 0 Then
                    rcFN = False
                 End If
                rc = rc And rcFN
             End If
             Return rc
              Catch ex As Exception
             Throw New Exception(ex.Message)
         End Try
     End Function

要记得两个Imports

Imports System.IO
Imports System.Linq

各位朋友可以自己实际的Step By Step试试看....透过这个过程可以了解