Hey Sharath, I’ve run into this exact issue before—let’s get it sorted out quickly!

Why This Happens #

The $ string interpolation syntax you’re using is a C# 6.0 feature, which requires .NET Framework 4.6 or later to work properly. Even though your local setup supports this (since it runs fine), your Azure App Service is likely configured to use an older .NET Framework version or an outdated compiler that doesn’t recognize the $ character. The version info you shared shows .NET Framework Version:4.0.30319—that’s the root of the problem.

Step-by-Step Fixes #

1. Verify Your Project’s Target Framework

First, make sure your local project is set to target a compatible .NET Framework version:

• In Visual Studio, right-click your project → Properties → Application tab.
• Under Target framework, select .NET Framework 4.6 or higher (I recommend matching your local ASP.NET version, 4.7.2, for consistency).
• Save the changes, rebuild your project, and republish to Azure.

2. Configure Azure App Service’s .NET Version

Next, update your Azure App Service to use the correct framework version:

• Log into the Azure Portal, navigate to your App Service resource.
• On the left sidebar, go to Settings → Configuration → General settings.
• Find the .NET Framework version dropdown and select the same version you set in your project (e.g., v4.7).
• Click Save, then restart your App Service to apply the changes.

3. Ensure Roslyn Compiler is Configured (If Needed)

If you still see the error after the first two steps, your project might be using the old legacy compiler instead of Roslyn (which supports C# 6+ features):

• Install the Microsoft.CodeDom.Providers.DotNetCompilerPlatform NuGet package in your project (this adds Roslyn support).
• Open your project’s web.config file and add or update the following section to enforce C# 6 syntax:

  <system.codedom>
    <compilers>
      <compiler language="c#;cs;csharp" extension=".cs"
        type="Microsoft.CodeDom.Providers.DotNetCompilerPlatform.CSharpCodeProvider, Microsoft.CodeDom.Providers.DotNetCompilerPlatform, Version=2.0.1.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35"
        warningLevel="4" compilerOptions="/langversion:6 /nowarn:1659;1699;1701"/>
    </compilers>
  </system.codedom>
  

• Republish your project to Azure after making this change.

Final Notes #

Most of the time, just updating the project’s target framework and Azure’s .NET version will fix the issue. The Roslyn compiler step is only needed if your project was originally created with an older .NET version and hasn’t been migrated to use modern tooling.

内容的提问来源于stack exchange，提问作者Sharath