Hi there! Let's break down your questions and walk through practical steps to get your legacy app updated smoothly to .NET 4.8. I’ve dealt with similar projects before, so I know how tedious these repetitive changes can feel—let’s make this as painless as possible.

1. 先搞定Web.config的基础配置 #

First things first: update the targetFramework attribute in the <compilation> element to 4.8. Your current web.config has it set to 4.7.2, so changing this tells the ASP.NET runtime to use 4.8 framework features and resolve assemblies accordingly.

关于Web.config中引用的冗余性 #

You’re right that many of these references feel redundant—and for core framework assemblies like System.Web.Extensions, they often are. The runtime will automatically resolve the correct version based on your target framework.

However, for third-party assemblies like the old ReportViewer v8.0, these references do matter because they aren’t part of the core .NET Framework. But since v8.0 is extremely outdated (from SQL Server 2008), it’s likely incompatible with .NET 4.8—we’ll cover upgrading that later.

If removing these references didn’t break your build/startup, that’s a good sign the runtime was already resolving framework assemblies correctly. Stick with what works, but plan to update third-party references like ReportViewer.

2. 处理.aspx文件中的@Register指令 #

The explicit version numbers in your <%@ Register %> directives (like the 1.0.61025.0 for System.Web.Extensions) force the runtime to use that old 2.0/3.5-era version, which conflicts with .NET 4.8.

不需要手动逐个修改（高效方法在此） #

You don’t have to edit every .aspx file one by one. Use your IDE’s Find and Replace in Files feature (in Visual Studio: Edit > Find and Replace > Quick Replace, set "Look in" to your project folder).

For System.Web.Extensions, the correct version for .NET 4.x is 4.0.0.0. So you can search for:

<%@ Register Assembly="System.Web.Extensions, Version=1.0.61025.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35" Namespace="System.Web.UI" TagPrefix="asp" %>

And replace it with:

<%@ Register Assembly="System.Web.Extensions, Version=4.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35" Namespace="System.Web.UI" TagPrefix="asp" %>

Even better: since System.Web.Extensions is part of the core .NET 4.8 framework, you can omit the version, culture, and public key token entirely. The runtime will auto-pick the correct version for your target framework. The simplified directive becomes:

<%@ Register Namespace="System.Web.UI" TagPrefix="asp" Assembly="System.Web.Extensions" %>

This saves you from version updates if you retarget again in the future.

3. 升级ReportViewer（关键！旧版本v8.0不兼容.NET 4.8） #

Your current ReportViewer v8.0 references are way too old for .NET 4.8—you’ll almost certainly hit runtime errors with this combination. Here’s how to fix it:

1. 获取兼容的ReportViewer版本: For .NET 4.8, use the latest available version (usually v150.1400.0 or later). In your controlled environment:
   • Download the Microsoft Report Viewer 2022 Runtime MSI (compatible with .NET 4.7.2+), or
   • Manually copy Microsoft.ReportViewer.WebForms.dll, Microsoft.ReportViewer.Common.dll, and Microsoft.ReportViewer.ProcessingObjectModel.dll to your project’s Bin folder.
2. 更新Web.config引用: Replace all v8.0 references with the new version. For example:

   <compilation debug="true" strict="false" explicit="true" targetFramework="4.8">
     <buildProviders>
       <add extension=".rdlc" type="Microsoft.Reporting.RdlBuildProvider, Microsoft.ReportViewer.Common, Version=150.1400.0, Culture=neutral, PublicKeyToken=89845dcd8080cc91"/>
     </buildProviders>
     <assemblies>
       <add assembly="Microsoft.ReportViewer.ProcessingObjectModel, Version=150.1400.0, Culture=neutral, PublicKeyToken=89845dcd8080cc91" />
       <add assembly="Microsoft.ReportViewer.Common, Version=150.1400.0, Culture=neutral, PublicKeyToken=89845dcd8080cc91" />
       <add assembly="Microsoft.ReportViewer.WebForms, Version=150.1400.0, Culture=neutral, PublicKeyToken=89845dcd8080cc91" />
     </assemblies>
   </compilation>
   

3. 更新.aspx/ascx中的ReportViewer指令: If you have explicit @Register for ReportViewer controls, update their version numbers to match the new one.

4. 解决添加引用时的错误 #

When you tried adding System.Web.Extensions via Property Pages and got the "already referencing" error, that’s because:

• System.Web.Extensions is part of the core .NET Framework, so it’s already included in your target framework—you don’t need to add it manually.
• For third-party assemblies like ReportViewer, first copy the DLLs to your Bin folder, then add the reference via Property Pages. This ensures the IDE recognizes the correct version.

最后一步：测试与诊断 #

After making these changes:

1. Clean your project (Build > Clean Solution) and rebuild it.
2. Run the app and test key features, especially those using System.Web.UI controls and ReportViewer.
3. If you hit assembly binding errors, use the Fusion Log Viewer (fuslogvw.exe) to see which version the runtime is trying to load and why it’s failing. This is a lifesaver for debugging legacy assembly issues.

Let me know if you hit any specific snags—legacy ASP.NET projects can have weird quirks, but these steps should cover most dependency-related issues!