Null entry

Hello @ING Docteur Bazile "Pouchon" ,

Thanks for your question.

There are many ways to handle a null entry, you can refer to these following examples. For more information, you can refer to Nullable reference types.

1. Null Check (using if statement)

Console.Write("Enter your name: ");
string input = Console.ReadLine();

if (string.IsNullOrEmpty(input))
{
    Console.WriteLine("No name entered! Please provide a value.");
}
else
{
    Console.WriteLine($"Hello, {input}!");
}

2. Null-Coalescing Operator (??).

Returns a default value if the expression is null:

string name = null;
string result = name ?? "Default Name";
Console.WriteLine(result);

3. Null-Coalescing Assignment (??=)

Assigns a value only if the variable is null:

string name = null;
name ??= "Default Name";
Console.WriteLine(name);

4. Null-Conditional Operator (?.)

Safely accesses members/methods without throwing a NullReferenceException:

string name = null;
int? length = name?.Length;
Console.WriteLine(length);

string upper = name?.ToUpper()?.Trim();

5. Nullable Value Types (?) Value types (like int, bool) can't be null by default. Use ? to make them nullable:

int? age = null;

if (age.HasValue)
{
    Console.WriteLine($"Age: {age.Value}");
}
else
{
    Console.WriteLine("Age is not provided");
}

int actualAge = age ?? 0;

6. Guard Clause (Throwing on Null)

Useful for validating method parameters:

using System;

void PrintName(string name)
{
    ArgumentNullException.ThrowIfNull(name); // for C# 10+

    // Or for older versions:
    // if (name == null) throw new ArgumentNullException(nameof(name));

    Console.WriteLine(name);
}

I hope this addresses your question. If this response was helpful, please consider following the guidance to provide feedback.

In C#, null handling depends on what can be null: the variable itself, elements in a collection, or values inside a query.

For LINQ queries, explicitly filter out nulls before using members, to avoid NullReferenceException. For example, when querying collections that may contain null elements:

record Product(string Name, int CategoryID);
record Category(string Name, int ID);

IEnumerable<Category?> categories = GetCategories();
IEnumerable<Product?> products = GetProducts();

var query = from c in categories
            where c != null              // filter out null categories
            join p in products
                on c.ID equals p?.CategoryID
            select new
            {
                Category = c.Name,
                Name = p.Name
            };

Key points for handling nulls in C# code:

1. Check the source collection for null before querying:

      if (categories is null)
          return; // or handle appropriately
   

2. Filter out null elements in the sequence:

      var nonNullCategories = categories.Where(c => c != null);
   

3. Use null-conditional (?.) and null-coalescing (??) operators when accessing members:

      string name = product?.Name ?? "Unknown";
   

4. When joining on nullable keys, cast the non-nullable side to a nullable type:

      var query =
          from o in db.Orders
          join e in db.Employees
              on o.EmployeeID equals (int?)e.EmployeeID
          select new { o.OrderID, e.FirstName };
   

These patterns ensure that null entries are handled safely and prevent runtime null reference errors.

References:

• Write C# LINQ queries to query data