System.NullReferenceException: Object reference not set to an instance of an object

You’re accessing a member of an object that’s null.

Fix 1: Null check

// ❌ user might be null
var name = user.Name;

// ✅ Null check
if (user != null)
{
    var name = user.Name;
}

// ✅ Null-conditional operator (C# 6+)
var name = user?.Name;
var name = user?.Name ?? "Unknown";

Fix 2: Initialize objects

// ❌ Not initialized
List<string> items;
items.Add("hello");  // NullReferenceException!

// ✅ Initialize
List<string> items = new List<string>();
items.Add("hello");

Fix 3: Enable nullable reference types

// In .csproj
<Nullable>enable</Nullable>

The compiler will warn you about potential null references.

Related: C# Cannot Implicitly Convert Type — How to Fix It Related: C# StackOverflowException — How to Fix It