What Are Some Unexpected Behaviors and Corner Cases in C# and .NET?

Explore the strange corners of C# and the .NET framework

In software development, we often encounter some puzzling edge cases. This article will explore some of the most bizarre edge cases in C# and the .NET framework, revealing their anomalous behavior.

String resident exception

Consider the following code snippet:

<code class="language-csharp">string x = new string(new char[0]);
string y = new string(new char[0]);
Console.WriteLine(object.ReferenceEquals(x, y));</code>

Intuitively, we would expect the output to be False since the "new" keyword usually allocates space for a new object. Surprisingly, this code outputs True in all framework versions tested. Although the specification states that new objects should always be created, this becomes a special case.

Nullable type exception

This code further illustrates the complexity of C#:

<code class="language-csharp">static void Foo<T>() where T : new()
{
    T t = new T();
    // ... (其他操作)

    // 此行引发NullReferenceException
    Console.WriteLine(t.GetType());
}</code>

The mystery lies in type T. To uncover it, we have to examine the code carefully. The Nullable<T> structure represents a nullable value type, which overrides most member methods, but does not override GetType(). So when GetType() is called, the nullable value is cast to object (resulting in null), resulting in NullReferenceException.

Class instantiation trick

Another puzzling edge case occurs in the following code, where T is restricted to a reference type:

<code class="language-csharp">private static void Main() {
    CanThisHappen<MyFunnyType>();
}

public static void CanThisHappen<T>() where T : class, new() {
    var instance = new T();
    Debug.Assert(instance != null, "我们是如何破坏CLR的？");
}</code>

Through excellent engineering design, this code can be broken using indirect methods similar to remote calls:

<code class="language-csharp">class MyFunnyProxyAttribute : ProxyAttribute {
    // ... (重写)
}

[MyFunnyProxy]
class MyFunnyType : ContextBoundObject { }</code>

By defining a custom proxy attribute and redirecting the new() call to return null, the assertion in CanThisHappen is broken, demonstrating the tremendous flexibility and potential pitfalls in the C# language and .NET runtime.

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