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

C# and .NET: Unveiling Hidden Surprises

Software development often reveals unexpected behaviors. C# and .NET, while powerful, are no exception. This article explores some intriguing corner cases that can challenge even experienced developers.

String Creation: A Counterintuitive Result

Consider this seemingly simple code snippet:

string x = new string(new char[0]);
string y = new string(new char[0]);
Console.WriteLine(object.ReferenceEquals(x, y));

The output is True, contradicting the expectation that new creates distinct objects for reference types. The Common Language Runtime (CLR) optimizes this specific scenario, reusing the same empty string instance.

Generic Types and Nullable: A NullReferenceException Mystery

The following code demonstrates another unexpected behavior:

static void Foo<T>() where T : new()
{
    T t = new T();
    Console.WriteLine(t.ToString()); // Works fine
    Console.WriteLine(t.GetHashCode()); // Works fine
    Console.WriteLine(t.Equals(t)); // Works fine

    // This throws a NullReferenceException...
    Console.WriteLine(t.GetType());
}

When T is Nullable<t></t> (e.g., int?), a NullReferenceException occurs when calling GetType(). This is because Nullable<t></t> overrides most methods, but not GetType(). The boxing process during the call to the non-overridden GetType() results in a null value.

Proxy Attributes and the new() Constraint: Defying Expectations

Ayende Rahien highlighted a similar, yet more sophisticated, scenario:

private static void Main() {
    CanThisHappen<MyFunnyType>();
}

public static void CanThisHappen<T>() where T : class, new() {
    var instance = new T(); // new() on a ref-type; should be non-null, then
    Debug.Assert(instance != null, "How did we break the CLR?");
}

This code, surprisingly, can fail the assertion. By using a proxy attribute (like MyFunnyProxyAttribute) that intercepts the new() call and returns null, the assertion can be violated. This demonstrates the potential for unexpected interactions between runtime behavior and custom attributes. These examples highlight the importance of thorough testing and a deep understanding of the CLR's inner workings to avoid unexpected pitfalls in C# and .NET development.

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