C# 8通過(guò)引入可空引用類型減少空引用異常。 1. 可在項(xiàng)目文件中全局啟用，2. 也可在單個(gè)文件中使用#nullable指令啟用，3. 或通過(guò)編譯器參數(shù)啟用。啟用後引用類型默認(rèn)非空，需顯式添加?以允許為空。處理警告時(shí)應(yīng)優(yōu)先修復(fù)實(shí)際空值問(wèn)題，使用null-forgiving操作符或模式匹配進(jìn)行空值檢查。

C# 8 introduced nullable reference types as a way to help developers catch potential null reference exceptions at compile time, rather than at runtime. In earlier versions of C# (and many other languages), reference types could always be null by default, which often led to NullReferenceException errors in real-world applications.

With nullable reference types enabled, reference types are treated as non-nullable by default , and you have to explicitly declare when a reference type can be null. This adds a layer of safety and clarity to your code.

How to enable nullable reference types

There are a few ways to enable nullable reference types depending on your project and preference.

1. Enable globally in the project file

The most common and recommended way is to enable nullable reference types for the entire project by editing the .csproj file:

 <PropertyGroup>
    <TargetFramework>net6.0</TargetFramework>
    <Nullable>enable</Nullable>
</PropertyGroup>

This enables nullable reference types across all files in the project. It's simple and effective for new or existing projects.

2. Enable per file using a directive

If you want to control it on a per-file basis, you can use the #nullable directive at the top of a .cs file:

 #nullable enable

This is useful during migration or if you want to gradually adopt the feature in legacy code.

3. Enable via compiler options

You can also enable the feature via compiler options when building from the command line:

 csc /nullable Program.cs

This is less common unless you're compiling manually or in specific CI/CD scenarios.

Understanding the basics of nullable reference types

Once enabled, the compiler treats all reference types like string , object , or custom classes as non-nullable by default . If you want a reference type to allow nulls, you must explicitly add a ? after the type:

 string name = null; // Warning: Cannot assign null to non-nullable reference
string? name = null; // OK: This string can be null

This distinction helps you write safer code by making nullability part of the type system.

Key behaviors:

• Assigning null to a non-nullable reference type generates a compiler warning.
• You can suppress warnings using null-forgiving operator ( ! ), like name!.Length , but use this sparingly.
• You can annotate APIs to help tools and analyzers understand null expectations better.

How to deal with warnings and common patterns

Enabling nullable reference types might cause a lot of warnings in existing codebases. Here's how to handle them effectively.

Fix actual issues

Start by reviewing the warnings and fixing actual null violations. For example:

 string user = GetUser(); // Warning: May return null
int length = user.Length; // Warning: Possible null reference

If GetUser() can return null, update it to return string? , and handle the null case properly:

 string? user = GetUser();
if (user != null)
{
    int length = user.Length; // Safe access
}

Use null-forgiving operator when appropriate

Sometimes the compiler can't infer that a value isn't null, but you know it isn't. In those cases, you can use the null-forgiving operator:

 string user = GetUser()!;

Use this only when you're certain the value won't be null, or you risk reintroducing runtime errors.

Use pattern matching and null checks

Take advantage of pattern matching and built-in null checks to write cleaner and safer code:

 if (GetUser() is string { } user)
{
    Console.WriteLine(user.Length);
}

This combines null checking and assignment in a single line.

Summary

Enabling nullable reference types in C# 8 helps reduce null-related bugs by making non-null the default for reference types. You can enable it globally, per file, or via compiler flags. Once enabled, the compiler will warn you when you try to assign null to non-nullable references, and you can use ? to indicate nullable ones.

Fixing warnings often means updating method return types, using null checks, or applying the null-forgiving operator where appropriate.

基本上就這些。

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