The 30-Day .NET Challenge, Day 9: Null Safety

Introduction

The article demonstrates the use of null-state analysis to remove compiler warnings of “check code for null safety”.

Learning Objectives

Discover how to set up the nullable context in your C# project or codebase.

Prerequisites for Developers

Familiarity with introductory-level C# programming
Know how to use Visual Studio Code or Visual Studio
.NET SDK version 6.0 or newer

Getting Started

.NET developers often face the System.NullReferenceException, which happens when a null is referenced at runtime and results in the most common exception in the .NET application.

As the creator of null, Sir Tony Hoare, refers to null as the "billion-dollar mistake."

Example

The variable records is set to null and then immediately referenced which results in System.NullReferenceException

    TestRecord records = null;
    
    _ = records.ToString();
    
    record TestRecord(int Id, string Name);

As the applications grow in the number of lines of code and become more complex, spotting such issues as a developer can be challenging.

This is where C# compiler steps in.

Define null safety

In the previous example, a developer can avoid the System.NullReferenceExceptionby checking if the records variable was null as shown below.

    TestRecord records = null;
    
    // Check for null
    if (records is not null)
    {
        _ = records.ToString();
    }
    
    record TestRecord(int Id, string Name);

Nullable Types

The default value for all reference types is null.

    string first;                  // first is null
    string second = string.Empty   // second is not null, instead it's an empty string ""
    int third;                     // third is 0 because int is a value type
    DateTime date;                 // date is DateTime.MinValue

In the example mentioned earlier:

The variable “first” is null because a reference type “string” was declared but not assigned any value.
The variable “second” is assigned the value “string.Empty” during declaration.
The variable “third” has a value of 0 even though it was not explicitly assigned.
The variable “date” is uninitialized, but its default value is “System.DateTime.MinValue.”

Post C# 2.0 version, we can define nullable values using Nullable<T>. This allowed value types to be assigned with a value of null.

    int? first;            // first is implicitly null (uninitialized)
    int? second = null;    // second is explicitly null
    int? third = default;  // third is null as the default value for Nullable<Int32> is null
    int? fourth = new();    // fourth is 0, since new calls the nullable constructor

Nullable context

As per my experience, this is a must-have feature that should be enabled in every .Net application as it enables control for how the compiler understands reference type variables.

There are four types of nullable contexts

disable
enable
warnings
annotations

Enable nullable context

it can be enabled by adding <Nullable> item to the <PropertyGroup> inside the application .csproj file as shown below

    <Project Sdk="Microsoft.NET.Sdk">
    
        <PropertyGroup>
            <OutputType>Exe</OutputType>
            <TargetFramework>net6.0</TargetFramework>
            <Nullable>enable</Nullable>
        </PropertyGroup>
    
        <!-- Omitted for brevity -->
    
    </Project>

Alternatively, developers can also add scope nullable context which means the nullable context will be applicable only in the defined scope.

    #nullable enable

Complete Code on GitHub

GitHub — ssukhpinder/30DayChallenge.Net

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