C# : 'is' keyword and checking for Not

This is a silly question, but you can use this code to check if something is a particular type...

if (child is IContainer) { //....

Is there a more elegant way to check for the "NOT" instance?

if (!(child is IContainer)) { //A little ugly... silly, yes I know...

//these don't work :)
if (child !is IContainer) {
if (child isnt IContainer) { 
if (child aint IContainer) { 
if (child isnotafreaking IContainer) {

Yes, yes... silly question....

Because there is some question on what the code looks like, it's just a simple return at the start of a method.

public void Update(DocumentPart part) {
    part.Update();
    if (!(DocumentPart is IContainer)) { return; }
    foreach(DocumentPart child in ((IContainer)part).Children) {
       //...etc...

Solution 1:

if(!(child is IContainer))

is the only operator to go (there's no IsNot operator).

You can build an extension method that does it:

public static bool IsA<T>(this object obj) {
    return obj is T;
}

and then use it to:

if (!child.IsA<IContainer>())

And you could follow on your theme:

public static bool IsNotAFreaking<T>(this object obj) {
    return !(obj is T);
}

if (child.IsNotAFreaking<IContainer>()) { // ...

Update (considering the OP's code snippet):

Since you're actually casting the value afterward, you could just use as instead:

public void Update(DocumentPart part) {
    part.Update();
    IContainer containerPart = part as IContainer;
    if(containerPart == null) return;
    foreach(DocumentPart child in containerPart.Children) { // omit the cast.
       //...etc...

Solution 2:

You can do it this way:

object a = new StreamWriter("c:\\temp\\test.txt");

if (a is TextReader == false)
{
   Console.WriteLine("failed");
}

Solution 3:

New In C# 9.0

https://devblogs.microsoft.com/dotnet/welcome-to-c-9-0/#logical-patterns

if (part is not IContainer)
{
    return;
}

Original Answer

This hasn't been mentioned yet. It works and I think it looks better than using !(child is IContainer)

if (part is IContainer is false)
{
    return;
}

Solution 4:

C# 9 (released with .NET 5) includes the logical patterns and, or and not, which allows us to write this more elegantly:

if (child is not IContainer) { ... }

Likewise, this pattern can be used to check for null:

if (child is not null) { ... }