C#: The type or namespace name could not be found

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Root Cause

This compile-time error in C# indicates that the compiler doesn't recognize a class, struct, or namespace you are trying to use. The most frequent causes are forgetting to add the required `using` directive at the top of the file, misspelling the type name, or failing to add a reference to the external project or NuGet package that contains the type. It can also happen if there is a mismatch in Target Frameworks between a library project and the main application consuming it.

Fix / Solution

Check the spelling of the class name. Use your IDE's quick-fix feature (Ctrl+. in Visual Studio) to automatically add the missing `using` directive. If the class belongs to an external library, ensure you have successfully installed the NuGet package. If referencing another project in your solution, ensure the project reference is added and both projects target compatible .NET versions.

Code Snippet

// ❌ Error: 'List' could not be found
List<string> names = new List<string>();

// ✅ Fix: Add the missing using directive
using System.Collections.Generic;

List<string> names = new List<string>();