vb.net 中的“字符串类型的值不能转换为 system.data.datatable”

Question

这是我的代码。我一直有一个错误“字符串类型的值无法转换为 system.data.datatable”

Function GetTable() As DataTable
        Dim SQLConnection As New SqlConnection(ConfigurationManager.ConnectionStrings("Zeinchatconnection").ToString())
        Dim CommSQL As New SqlClient.SqlCommand
        Dim ChatDataAdapter As SqlDataAdapter
        Dim paramSQL As SqlClient.SqlParameter
        Dim DStable As DataSet
        Dim table As New DataTable
        Dim szName As String = ""
        Dim szNumber As String = ""
        Try
            If SQLConnection.State = ConnectionState.Closed Then
                SQLConnection.Open()
            End If
            CommSQL.Connection = SQLConnection
            CommSQL.CommandType = CommandType.StoredProcedure
            CommSQL.CommandText = "spc_newselect"



        CommSQL.ExecuteNonQuery()

        ChatDataAdapter = New SqlDataAdapter(CommSQL)
        ChatDataAdapter.Fill(DSTable)

        table.Rows.Clear()
        table.Clear()
        table = DStable.Tables(0)

        Dim i As Integer = 0

        For i = 0 To table.Rows.Count - 1
            szName = szName & "  " & table.Rows(i).Item(0) & table.Rows(i).Item(1)
            szNumber = szNumber & "  " & table.Rows(i).Item(0) & table.Rows(i).Item(1)
        Next

        GetTable = "1"
    Catch ex As System.Data.SqlClient.SqlException
        GetTable = "0"
    Catch ex As Exception
        GetTable = "0"

        If (IsNothing(ChatDataAdapter) = False) Then ChatDataAdapter.Dispose()
        If (IsNothing(CommSQL) = False) Then CommSQL.Dispose()
        SQLConnection.Close()
    End Try
    Return table


End Function

出错的部分是gettable="1" 及以下。

Answer 1

GetTable = "1" 表示要设置函数的returnValue。由于您的函数定义为Function GetTable() As DataTable，您的编译器会显示错误！

下面几行有一个正确的 return stmt (Return table) 所以我不太确定你对GetTable = "1" 的目标是什么？

我假设您希望有一个额外的 returnValue 来指示您的函数调用是否成功。但事实上函数可能只有一个 returnValue。

您可以选择将表变量设置为空，或使用 ref 参数...

' possible solution 1 - using nothing value
Function GetTable() As DataTable
    Try
        ' your code goes here
        ' remove GetTable = "1" 
    Catch ex as Exception
        ' change GetTable = "0" to 
        table = nothing
    End Try
    ' other code ...
End Function

' possible solution 2 - ref param
Function GetTable(ByRef status as integer) as DataTable
   Try
        ' your code goes here
        ' remove GetTable = "1" 
        status = 1
    Catch ex as Exception
        ' change GetTable = "0" to 
        status = 0
    End Try
    ' other code ...
End Function

在解决方案 2 中，您还可以选择一个 布尔参数，指示您的调用是否成功。

Answer 2

您的函数GetTable 返回一个DataTable

您无法将"1" 转换为DataTable，因为消息表明GetTable = "1" 线上正在发生的事情

如果你想返回一个DataTable 并设置一个标志来查看状态；改变你的函数定义：

Function GetTable(byref result as Integer) As DataTable

然后将 GetTable = "1" 改为 result = 1，而不是 result = 1。这样您就可以检查结果值并返回一个 DataTable：

Dim res as Integer
Dim dt as DataTable = GetTable(res)
If res = 1 then
    'It worked!
End If

旁注：Turn Option Strict On