Type Casting

h5 provides robust support for C# type casting operations in JavaScript.

is and as Operators

You can use the is and as operators to check type compatibility and cast between types at runtime.

public void Process(object obj)
{
    if (obj is string s)
    {
        Console.WriteLine("String: " + s);
    }
    else if (obj is int i)
    {
        Console.WriteLine("Int: " + i);
    }

    var list = obj as IList<string>;
    if (list != null)
    {
        Console.WriteLine("List count: " + list.Count);
    }
}

h5 generates JavaScript code that checks the constructor and prototype chain of the object to determine compatibility.

[Cast] and [IgnoreCast] Attributes

You can control how casting operations are translated using attributes.

[Cast] Attribute

The [Cast] attribute allows you to specify a custom casting implementation.

[External]
[Cast("parseInt({0})")]
public static extern int ToInt(object value);

[IgnoreCast] Attribute

Sometimes you want to treat an object as a specific type without any runtime checks or conversion logic, purely for the compiler's benefit (compile-time casting). This is common when working with dynamic JavaScript objects or generic wrappers.

The [IgnoreCast] attribute tells the compiler to emit the casted expression directly, without wrapping it in a type check or conversion function.

[IgnoreCast]
public T As<T>()
{
    return (T)(object)this;
}

// Usage
var result = myObj.As<int>();

When compiled, myObj.As<int>() simply becomes myObj in JavaScript.

Runtime Checks

By default, explicit casts (T)obj will throw an InvalidCastException if the cast is invalid at runtime, just like in .NET.

object o = "hello";
int i = (int)o; // Throws InvalidCastException

To avoid runtime overhead in performance-critical code where you are certain of the types, you can disable these checks globally in h5.json using the ignoreCast option, or locally with [IgnoreCast].