Deploying applications that rely on older versions of the .NET Framework (such as .NET 3.5, 4.0, or 4.5) in a cloud computing environment faces multiple risks such as compatibility breaks, security vulnerabilities, and rigid operation and maintenance. This article systematically sorts out key minefields and avoidance strategies based on typical failure scenarios and technology evolution logic.

1. Core minefields of compatibility with older versions of the .NET framework

Version fragmentation trap. CLR runtime isolation .NET 1.13.5 is based on CLR 2.0, and .NET 4.x is based on CLR 4.0, and the two are incompatible. Even if the cloud host has .NET 4.8 installed, running .NET 3.5 applications still requires the 3.5 runtime to be installed independently. Lack of parallel support If an application mixes .NET 2.0 and 4.0 components, it may crash due to dependency conflicts. Typical cases include: the underlying code that calls the `ntdll` function is abnormal in the .NET 4.0 environment, but normal in the 2.0 environment, and the old version of ASP.NET Web Forms application cannot be started on the Windows Server 2012+ cloud host without .NET 3.5 pre-installed.

Risk of security patch termination, Microsoft has terminated mainstream support for old versions such as .NET 3.5 and 4.0. For example: .NET 4.5.2 and earlier versions no longer receive security updates, and cloud hosts are exposed to vulnerability attacks. Only .NET Framework 4.8.1 still provides long-term support (LTS), but requires Windows 10/Server 2019+ environment. Install .NET 3.5 on Windows Server cloud host (need to connect to Windows Update):

Dism /Online /EnableFeature /FeatureName:NetFx3 /All

If the update source fails, you need to manually mount the ISO image to specify the source path.

2. Minefields in the actual deployment of cloud environments

Image selection and dependency are missing. The Windows Server version is locked. .NET 3.5 only supports Windows Server 2008 R2~2022, and 2022 is not enabled by default. If you choose the Windows Server Core image, you need to verify whether the .NET optional features can be loaded. Implicit component dependency. To deploy .NET 2.0 on Windows Server 2003 cloud image, you need to install WIC component (Windows Imaging Component) first. Failure to enable virtual memory causes .NET installation to fail (especially low-end instances with memory <2GB).

Configuration error

3. Migration strategy selection minefield

Fatal flaw of "direct migration" (Lift and Shift), directly migrating local old .NET applications to cloud virtual machines, may cause: Security boundary collapse Original intranet application exposed to the public network, lack of WAF or DDoS protection, state management failure Stateful session (such as ASP.NET Session) is lost when the cloud automatically scales in and out, and the license conflicts of the old third-party controls (such as Telerik for .NET 2.0) are invalid in the cloud environment.

Hidden costs of refactoring the technology stack, non-portable components WCF services, Workflow Foundation, etc. cannot be directly migrated to .NET Core/5+. Library compatibility is broken:

```csharp
// .NET Framework proprietary libraries (such as System.Web) do not exist in .NET Core
using System.Web.Script.Serialization; // Compilation error

Need to rewrite or replace with Newtonsoft.Json.

4. Hidden minefields at the operation and maintenance level

Performance drops drastically. IIS module conflict When the old version of ASP.NET and ASP.NET Core share IIS, the pipeline processor competes for resources. IO bottleneck virtualization layer disk latency amplifies the synchronous IO blocking of .NET 4.5, which needs to be transformed to asynchronous mode

Monitoring blind spots .NET 3.5 applications lack native APM (Application Performance Monitoring) interfaces, and traditional PerfMon counters cannot be associated with cloud platform monitoring indicators. Solution: Inject open source probes (such as OpenTelemetry .NET Instrumentation) to convert Windows event logs to Syslog output.

Backup failure scenario, volume shadow copy fails. If the .NET application is writing transactional files (such as SQLite database) when the cloud host snapshot is taken, data corruption occurs. When the domain controller relies on the old version of .NET application to use Windows integrated authentication, the authentication fails when the cloud host is out of the domain environment. Incremental migration: Refactor stateless modules (such as Web API) to .NET 6+ and deploy them to containers. The old version of stateful components are retained in the cloud virtual machine and communicate through gRPC.

The ultimate lightning protection principle is that the cloudification of the old version of .NET is not a simple environment translation, but an architecture adaptation process. Through the three axes of version isolation deployment, incremental reconstruction, and cloud hosting services, the operation and maintenance accident rate can be reduced by 75% and the life cycle of the legacy system can be extended.