如何修复 vb 中 dbcontext 上的空引用错误？

Question

我的代码构建并运行，但是当我在运行时引用 dbcontext 时，我得到“System.NullReferenceException: 'Object reference not set to an instance of an object.'”。我在解决方案中的很多其他地方使用了上下文并且它可以工作，但其他地方是 c#。这是VB。

Imports DataServices
Imports Previdence.Domain.Model
Imports Previdence.Business.Model.Report

Namespace UserControls    
Partial Class RemissionControl
        Inherits UserControl
        Private previdenceContext As PrevidenceContext
        Private patient As Subject
        Private remissionButtonStatus As Boolean?
        Private Sub Page_Load(ByVal sender As Object, ByVal e As EventArgs) Handles MyBase.Load

            Dim snapshotId As Guid = Utility.StringToGuid(Request.QueryString("snapshotId"))
            patient = (From su In previdenceContext.Subjects
                       Join ep In previdenceContext.Episodes On su.SubjectId Equals ep.SubjectId
                       Join sn In previdenceContext.Snapshots On ep.SubjectId Equals sn.SnapshotId
                       Where sn.SnapshotId = snapshotId
                       Select su).FirstOrDefault()
            remissionButtonStatus = patient.RemissionButtonOn
            If remissionButtonStatus = True Then
                remissionRButtonYes.Checked = True
                remissionRButtonNo.Checked = False
            Else
                remissionRButtonYes.Checked = False
                remissionRButtonNo.Checked = True
            End If
        End Sub

        'TODO: getting null reference error on the dbcontext
        Private Sub remissionRButtonYes_click() Handles remissionRButtonYes.CheckedChanged
            If remissionRButtonYes.Checked = True Then
                patient.RemissionButtonOn = True
            Else patient.RemissionButtonOn = False
            End If
            previdenceContext.SaveChanges()
        End Sub

        Private Sub remissionRButtonNo_click() Handles remissionRButtonNo.CheckedChanged
            If remissionRButtonNo.Checked = True Then
                patient.RemissionButtonOn = False
            Else patient.RemissionButtonOn = True
            End If
            previdenceContext.SaveChanges()
        End Sub
    End Class
End Namespace

Answer 1

因为您在类的多个成员中使用了previdenceContext，所以您可以在类级别声明它，并在Sub New() 中实例化它。必须释放上下文，因此您的类需要实现IDisposable 并释放previdenceContext。

Partial Class RemissionControl
    Implements IDisposable
    Inherits UserControl

    Private previdenceContext As PrevidenceContext

    Sub New()
        ' This is the parameterless constructor. If you have a constructor with
        ' parameter such as passing the connection string name, use it. Do what you
        ' you do in your C# code.
        previdenceContext New PrevidenceContext()
    End Sub

    ' Your class implementation ...

#Region "IDisposable Support"
    Private disposedValue As Boolean ' To detect redundant calls

    ' IDisposable
    Protected Overridable Sub Dispose(disposing As Boolean)
        If Not disposedValue Then
            If disposing Then
                ' TODO: dispose managed state (managed objects).
                If previdenceContext IsNot Nothing Then previdenceContext.Dispose()
            End If

            ' TODO: free unmanaged resources (unmanaged objects) and override Finalize() below.
            ' TODO: set large fields to null.
        End If
        disposedValue = True
    End Sub

    ' TODO: override Finalize() only if Dispose(disposing As Boolean) above has code to free unmanaged resources.
    'Protected Overrides Sub Finalize()
    '    ' Do not change this code.  Put cleanup code in Dispose(disposing As Boolean) above.
    '    Dispose(False)
    '    MyBase.Finalize()
    'End Sub

    ' This code added by Visual Basic to correctly implement the disposable pattern.
    Public Sub Dispose() Implements IDisposable.Dispose
        ' Do not change this code.  Put cleanup code in Dispose(disposing As Boolean) above.
        Dispose(True)
        ' TODO: uncomment the following line if Finalize() is overridden above.
        ' GC.SuppressFinalize(Me)
    End Sub
#End Region

End Class

Answer 2

这应该是 IntelliSense 捕获的错误类型。

无论如何，如果您替换声明，您将避免错误： Private previdenceContext As PrevidenceContext 和 Private previdenceContext As New PrevidenceContext，但是您的代码仍然无法工作，因为 previdenceContext 没有数据。

您可能缺少一行代码来填充previdenceContext。