System.ArgumentNullException: Value cannot be null. (Parameter 'source')
System.ArgumentNullException: Value cannot be null. (Parameter 'key')A method received null for a parameter that doesn’t accept it.
Fix 1: LINQ on Null Collection
// ❌ Calling LINQ on null
List<string> items = null;
var count = items.Where(x => x.Length > 3).Count(); // 💥
// ✅ Initialize or check
List<string> items = new List<string>();
// Or
var count = items?.Where(x => x.Length > 3).Count() ?? 0;Fix 2: Dictionary Null Key
// ❌ Null key
string key = null;
dict[key] = "value"; // 💥
// ✅ Check for null
if (key != null) dict[key] = "value";Fix 3: Dependency Injection Not Registered
// ❌ Service not registered in DI container
public class MyController {
public MyController(IMyService service) { } // 💥 null if not registered
}
// ✅ Register in Program.cs
builder.Services.AddScoped<IMyService, MyService>();Fix 4: Configuration Value Missing
// ❌ Config key doesn't exist
var connString = Configuration.GetConnectionString("Default");
new SqlConnection(connString); // 💥 If null
// ✅ Validate
var connString = Configuration.GetConnectionString("Default")
?? throw new InvalidOperationException("Connection string 'Default' not found");Fix 5: Guard Clauses
// ✅ Fail fast with clear messages
public void Process(string input) {
ArgumentNullException.ThrowIfNull(input); // C# 10+
// Or
if (input is null) throw new ArgumentNullException(nameof(input));
}