VS2019中WinForms Designer无法打开新建窗体求助

我有一个基于.NET Framework 4.8的C# SDK风格项目，右键添加新WinForm后，设计器无法打开，仅显示错误：

Object reference not set to an instance of an object.

对应的调用栈如下：

at System.Resources.Tools.StronglyTypedResourceBuilder.Create(IDictionary resourceList, String baseName, String generatedCodeNamespace, String resourcesNamespace, CodeDomProvider codeProvider, Boolean internalClass, String[]& unmatchable)
at System.Resources.Tools.StronglyTypedResourceBuilder.Create(IDictionary resourceList, String baseName, String generatedCodeNamespace, CodeDomProvider codeProvider, Boolean internalClass, String[]& unmatchable)
at Microsoft.VisualStudio.Design.Serialization.ResXGlobalObject.BuildType()
at Microsoft.VisualStudio.Design.Serialization.ResXGlobalObject.GetObjectType()
at Microsoft.VisualStudio.Shell.Design.GlobalType.get_ObjectType()
at Microsoft.VisualStudio.Design.Serialization.ResXGlobalObject.get_Children()
at Microsoft.VisualStudio.Design.Serialization.ResXGlobalObjectProvider.CreateGlobalObjectsForItem(ProjectItem item, GlobalObjectCollection oldObjects, GlobalObjectCollection newObjects, ITypeResolutionService typeResolver)
at Microsoft.VisualStudio.Design.Serialization.ResXGlobalObjectProvider.CreateGlobalObjectsForItem(ProjectItem item, GlobalObjectCollection oldObjects, GlobalObjectCollection newObjects, ITypeResolutionService typeResolver)
at Microsoft.VisualStudio.Design.Serialization.ResXGlobalObjectProvider.CreateGlobalObjectsForItem(ProjectItem item, GlobalObjectCollection oldObjects, GlobalObjectCollection newObjects, ITypeResolutionService typeResolver)
at Microsoft.VisualStudio.Design.Serialization.ResXGlobalObjectProvider.CreateGlobalObjects(Project project)
at Microsoft.VisualStudio.Design.Serialization.ResXGlobalObjectProvider.GetGlobalObjectsCore(Project project, Type baseType)
at Microsoft.VisualStudio.Shell.Design.GlobalObjectProvider.GetGlobalObjects(Project project, Type baseType)
at Microsoft.VisualStudio.Shell.Design.GlobalObjectService.GetGlobalObjects(Type baseType)
at Microsoft.VisualStudio.Design.Serialization.CodeDom.AggregateTypeResolutionService.GetTypeFromGlobalObjects(String name, Boolean throwOnError, Boolean ignoreCase)
at Microsoft.VisualStudio.Design.Serialization.CodeDom.AggregateTypeResolutionService.GetType(String name, Boolean throwOnError, Boolean ignoreCase)
at Microsoft.VisualStudio.Design.Serialization.CodeDom.AggregateTypeResolutionService.GetType(String name)
at System.ComponentModel.Design.Serialization.DesignerSerializationManager.GetRuntimeType(String typeName)
at System.ComponentModel.Design.Serialization.DesignerSerializationManager.GetType(String typeName)
at System.ComponentModel.Design.Serialization.DesignerSerializationManager.System.ComponentModel.Design.Serialization.IDesignerSerializationManager.GetType(String typeName)
at System.ComponentModel.Design.Serialization.CodeDomSerializerBase.GetType(IDesignerSerializationManager manager, String name, Dictionary`2 names)
at System.ComponentModel.Design.Serialization.CodeDomSerializerBase.FillStatementTable(IDesignerSerializationManager manager, IDictionary table, Dictionary`2 names, CodeStatementCollection statements, String className)
at System.ComponentModel.Design.Serialization.TypeCodeDomSerializer.Deserialize(IDesignerSerializationManager manager, CodeTypeDeclaration declaration)
at System.ComponentModel.Design.Serialization.CodeDomDesignerLoader.PerformLoad(IDesignerSerializationManager manager)
at Microsoft.VisualStudio.Design.Serialization.CodeDom.VSCodeDomDesignerLoader.PerformLoad(IDesignerSerializationManager serializationManager)
--- End of stack trace from previous location where exception was thrown ---
at Microsoft.VisualStudio.Design.Serialization.CodeDom.VSCodeDomDesignerLoader.PerformLoad(IDesignerSerializationManager serializationManager)
at Microsoft.VisualStudio.Design.Serialization.CodeDom.VSCodeDomDesignerLoader.DeferredLoadHandler.Microsoft.VisualStudio.TextManager.Interop.IVsTextBufferDataEvents.OnLoadCompleted(Int32 fReload)  

求解决办法让设计器正常加载WinForm。

可行的解决方案 #

• 修复强类型资源文件
  从调用栈看问题出在资源生成环节，检查项目中Properties/Resources.resx及对应的Resources.Designer.cs：

  1. 删除自动生成的Resources.Designer.cs文件
  2. 右键Resources.resx文件，选择「运行自定义工具」重新生成代码
  3. 检查资源文件中是否存在无效的引用或命名空间不一致的情况

• 切换为传统.NET Framework项目格式
  SDK风格项目对WinForm设计器的兼容存在问题，编辑.csproj文件，替换为传统项目结构：

  <Project ToolsVersion="15.0" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
    <Import Project="$(MSBuildExtensionsPath)\$(MSBuildToolsVersion)\Microsoft.Common.props" Condition="Exists('$(MSBuildExtensionsPath)\$(MSBuildToolsVersion)\Microsoft.Common.props')" />
    <PropertyGroup>
      <Configuration Condition=" '$(Configuration)' == '' ">Debug</Configuration>
      <Platform Condition=" '$(Platform)' == '' ">AnyCPU</Platform>
      <ProjectGuid>{你的项目GUID}</ProjectGuid>
      <OutputType>Library</OutputType>
      <RootNamespace>你的命名空间</RootNamespace>
      <AssemblyName>你的程序集名称</AssemblyName>
      <TargetFrameworkVersion>v4.8</TargetFrameworkVersion>
    </PropertyGroup>
    <ItemGroup>
      <Reference Include="System" />
      <Reference Include="System.Drawing" />
      <Reference Include="System.Windows.Forms" />
    </ItemGroup>
    <ItemGroup>
      <Compile Include="Form1.cs">
        <SubType>Form</SubType>
      </Compile>
      <Compile Include="Form1.Designer.cs">
        <DependentUpon>Form1.cs</DependentUpon>
      </Compile>
      <EmbeddedResource Include="Form1.resx">
        <DependentUpon>Form1.cs</DependentUpon>
        <SubType>Designer</SubType>
      </EmbeddedResource>
    </ItemGroup>
    <Import Project="$(MSBuildToolsPath)\Microsoft.CSharp.targets" />
  </Project>
  

  替换后重新加载项目，再尝试打开设计器。

• 清理重建项目

  1. 执行「项目 -> 清理」操作
  2. 删除项目目录下的bin和obj文件夹
  3. 重启Visual Studio
  4. 重新生成项目后打开设计器

• 检查Visual Studio组件
  确保已安装.NET Framework 4.8开发工具和Windows Forms设计器组件，可通过Visual Studio安装程序的「单个组件」选项卡检查并补充安装。

内容的提问来源于stack exchange，提问作者Boris