已引用System.Data.SqlClient，C#仍无法识别SqlConnection求助

Hey there, let's work through this SqlConnection error you're seeing—even with the using System.Data.SqlClient; directive in place, there are a few common gotchas in Visual Studio that might be causing this. Here's what to check step by step:

• Verify you've added the correct reference/NuGet package

  • For .NET Framework projects: SqlConnection lives in the System.Data assembly. Right-click your project in Solution Explorer → select Add → Reference → go to the Assemblies tab, find System.Data, check the box next to it, and click OK.
  • For .NET Core/.NET 5+ projects: The old System.Data.SqlClient namespace has been moved to a NuGet package. Right-click your project → select Manage NuGet Packages → go to the Browse tab, search for Microsoft.Data.SqlClient, install the latest stable version. After installing, you can either use using Microsoft.Data.SqlClient; (the modern recommended namespace) or keep using System.Data.SqlClient; (most versions still support this for compatibility).

• Check your project's target framework
  Make sure your project is targeting a framework version that supports SqlConnection. Right-click your project → Properties → go to the Application tab, check the Target framework dropdown. For .NET Framework, aim for 4.6.1 or newer; for .NET Core/.NET, 3.1 or newer works best.

• Clean and rebuild your solution
  Visual Studio's build cache can sometimes cause false errors. Go to the top menu → Build → Clean Solution, then Build → Rebuild Solution. If that doesn't help, try restarting Visual Studio entirely to clear any cached issues.

• Quick note on your code
  Also, a small side issue: your entry point method should be Main (capital M) instead of main—C# is case-sensitive, so public static void Main() is required for the program to run correctly once you fix the SqlConnection error.

内容的提问来源于stack exchange，提问作者user9758771