C#•2y ago

✅ Default operator

Can somebody give me ELI5 explanation for the necessity of the default operator? I'm under the impression everything that's declared but not initialized is automatically given a default value, typically 0 or null depending on the type. When would something not get an implicit default value? (Assuming that's why the default operator exists)

11 Replies

Aaron•2y ago

locals are never given a value implicitly

MODiX•2y ago

Windows10CE#8553

REPL Result: Failure

void Main() {
int i;
Console.WriteLine(i);
}
Main();

void Main() {
int i;
Console.WriteLine(i);
}
Main();

Exception: CompilationErrorException

- Use of unassigned local variable 'i'

- Use of unassigned local variable 'i'

Compile: 507.910ms | Execution: 0.000ms | React with ❌ to remove this embed.

Aaron•2y ago

you can use = default; to say you want the default value which might matter if you have say, a generic method, because you won't know what the default is ahead of time

MODiX•2y ago

Windows10CE#8553

REPL Result: Success

T GetDefaultValue<T>()
{
    // can't use = null here, because T might be a struct like int or long.
    // also can't use = 0, because it could be a class like string.
    T val = default;
    return val;
}
GetDefaultValue<int>()

T GetDefaultValue<T>()
{
    // can't use = null here, because T might be a struct like int or long.
    // also can't use = 0, because it could be a class like string.
    T val = default;
    return val;
}
GetDefaultValue<int>()

Result: int

Compile: 459.078ms | Execution: 33.766ms | React with ❌ to remove this embed.

swilli•2y ago

So for locals that makes sense, but I was under the impression fields always got a default value. I'm seeing this in the language extension library though:

swilli•2y ago

wouldn't bottom always get a default value upon instantiation? Regardless of whether A was a class, struct, or primitive type?

Aaron•2y ago

yes they're just being explicit

cathei•2y ago

While it would have default value, analyzer could yell at you for not assigning anything. Especially for static readonly it’s better to be explicit

333fred•2y ago

For the specific reason of why default exists, and we don't just tell people to use null or new MyStructType(), consider generics:

void M<T>()
{
    T t1 = null; // Illegal: if T is a value type, null isn't valid.
    T t2 = new T(); // Illegal: If T is a reference type, it may not have a parameterless ctor, and that's not a default instance anyway.
    T t3 = default(T); // Null if T is a reference type, all 0s if T is a value type.
}

void M<T>()
{
    T t1 = null; // Illegal: if T is a value type, null isn't valid.
    T t2 = new T(); // Illegal: If T is a reference type, it may not have a parameterless ctor, and that's not a default instance anyway.
    T t3 = default(T); // Null if T is a reference type, all 0s if T is a value type.
}

Accord•2y ago

Was this issue resolved? If so, run /close - otherwise I will mark this as stale and this post will be archived until there is new activity.

swilli•2y ago

That generic local makes things clear. Thanks everyone.

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✅ Default operator