Calling a property or method that does not exist on the specified type.
Occurs when calling a method or property that is not defined on the target class or struct type.
Spelling typos, calling a method on an interface that hasn't declared it, or forgetting to import the LINQ extension methods namespace (`using System.Linq;`).
1using System;2using System.Collections.Generic;3 4public class Program {5 public static void Main() {6 List<int> numbers = new List<int> { 1, 2, 3 };7 // Missing: using System.Linq;8 var evens = numbers.Where(n => n % 2 == 0); // Error CS1061: 'List<int>' does not contain a definition for 'Where'9 }10}1using System;2using System.Collections.Generic;3using System.Linq; // Added System.Linq extension methods4 5public class Program {6 public static void Main() {7 List<int> numbers = new List<int> { 1, 2, 3, 4 };8 var evens = numbers.Where(n => n % 2 == 0);9 Console.WriteLine(string.Join(", ", evens));10 }11}Simulate standard system builds to trigger compiler trace records and track memory crashes locally.
The `.Where()` extension method is defined in `System.Linq`. Without `using System.Linq;`, `List<int>` does not expose `.Where()`.