Quick Answer

Caused by missing references, incorrect usings, or build issues.

Understanding the Issue

This compilation error occurs when the compiler cannot locate a type, typically due to missing assembly references, incorrect namespaces, or project dependencies.

The Problem

This code demonstrates the issue:

Csharp Error 
// Problem: Missing reference
using External.Library;
var service = new DataService();  // Error if DLL not referenced

The Solution

Here's the corrected code:

Csharp Fixed 
// Solution 1: Add NuGet reference
// PM> Install-Package External.Library

// Solution 2: Verify namespace
using Correct.Namespace;
var service = new DataService();

Key Takeaways

Check references, namespaces, and build configurations.